## 1. Introduction



There are three questions raised by the title of this entry. The first
is whether it is anachronistic. Is empiricism not a modern view? Can
we attribute it to ancient and medieval thinkers? The charge of
anachronism might, perhaps, be warranted if empiricism referred to a
single set of beliefs. As we shall see, however, the term covers a
variety of views regarding knowledge
 (sect. 2).
 Even among modern thinkers, the empiricism of John Locke
(1632-1704) is not identical with that of Francis Bacon
(1561-1626), and neither bears much resemblance to the
empiricism of Rudolf Carnap (1891-1970), A. J. Ayer
(1910-1989), or Hans Reichenbach (1891-1953). Yet all
these thinkers have an attitude in common: they adopt a certain
"stance" towards knowledge (van Fraassen 2002:
47-48). They all desire to keep factual claims (in the sense of
claims regarding contingent matters) closely related to the
deliverances of the senses. That attitude has ancient roots. Modern
empiricism was not a *creatio e nihilo*. It revived views
already found in the ancient world, particularly among Hellenistic
philosophers (Scott 1995: 238), and the particular questions it was
seeking to answer arose from late medieval developments.



A second question has to do with the article's association of
ancient and medieval views. The philosophers discussed here are
separated by a thousand years. Why bring them together? This question
is more easily answered. The intellectual renaissance of the twelfth
and thirteenth centuries involved a revival of the work of the
ancients. In the field of philosophy this took the form of a
rediscovery of the works of Aristotle (384-322 BCE), by way of
Latin translations from Arabic sources and Greek texts from the
Byzantine world. Indeed much of the philosophical writing of this
period was done by way of commentaries on Aristotle. Medieval
philosophers were also influenced by writers from a later age, such as
Plotinus (205-270 CE), Dionysius the Areopagite (5th-6th
century CE), and St Augustine (354-430 CE). But since these
writers were themselves influenced by Aristotle's predecessor,
Plato (427-347 BCE), this aspect of medieval philosophy also has
ancient roots.



A third question has to do with the scope of the discussion. The
tradition of philosophy being discussed here is that which began in
the Greek city states of the sixth and fifth centuries BCE. But
empiricist views are also found outside of what is (somewhat
misleadingly) called the 'Western' philosophical
tradition. The closest parallels are to be found among the
Lokayata (Carvaka), one of the *nastika*
(non-orthodox) schools of Indian philosophy that existed in the sixth
and seventh centuries CE. These thinkers are reported to have held
that sensory perception is the only reliable means of knowledge and to
have rejected the validity of inferences to unobservable entities
(Chatterjee & Datta 2004: 54-59). The Buddha (ca.
485-405 BCE) could be said to be an empiricist, insofar as he
based his *dharma* (teaching) on experience rather than
authority or reason. But the Buddhist idea of experience is broader
than that held by most empiricists (Jayatilleke 1963: 463-64
[para. 793]). The twelve *ayatanas* (sensory domains) of
Buddhist tradition include the mind as well as the bodily senses and
meditation is thought capable of producing higher forms of knowledge
(the *abhijna*s) that are independent of sensory
perception. Finally, we can find discussions of sense perception
within classical Chinese philosophy. Mozi (Mo Tzu, ca. 480-392
BCE), for instance, ranked everyday experience among the three
criteria of reliable knowledge (Mozi 35.3), while the more sceptical
Xunzi (Hsun Tzu, ca. 312-210 BCE) pointed out how easily
the senses are deceived (Xunzi 21.8). But while it would be
interesting to explore these traditions, empiricism does not play as
central a role in any of them as it does in the history of Western
philosophy.



The question of scope also arises when it comes to the subject matter
of the article, which is empiricism as a doctrine rather than as a
practice. From the very beginning of what the Greeks called "the
investigation of nature" (*he peri phuseos
historia*), there have been those who studied the natural world by
means of careful observation. There were medical writers who observed
and recorded symptoms, astronomers who observed the movements of the
heavens, and students of music who placed their ears "close [to
their instruments] as though trying to hear what their neighbours were
saying" (Plato *Rep.* 531a6-7). These people
believed it was important to learn from experience (*empeiria*,
in Aristotle's Greek). But they did not formulate a theory about
learning from experience. It is the theory of learning from experience
that will be the focus of the present study.


## 2. Varieties of Empiricism



"Names of philosophical positions," wrote W. V. O. Quine,
"are a necessary evil" (1995: 251). This is certainly the
case with the term "empiricism": while it is difficult
simply to abandon it, it can be misleading. Among historians of early
modern philosophy, for instance, the traditional distinction between
empiricists and rationalists has fallen out of favour, on the grounds
that there are few thinkers who can be clearly assigned to either
category (Norton 1981; Loeb 1981: 25-75). The problem here,
however, is not the category of empiricism, but its use as a
"pigeon-hole" into which one tries to place individual
thinkers. Empiricism need not be a global doctrine: a thinker may be
an empiricist in one domain but not in another. It is possible, for
instance, to be an empiricist about knowledge of the natural world,
but a rationalist with regard to a particular area of mathematics
(Markie 2015: sect. 1.2). So it is not surprising that individual
thinkers are hard to categorize. But empiricism is also an umbrella
term: it covers a variety of views regarding knowledge. Even with
regard to the one domain, a thinker may be an empiricist in one sense,
but not in another. So if we are to use the term, we need to
distinguish the different forms empiricism can take.


### 2.1 Explanatory Empiricism



The first is what I shall call *explanatory empiricism*, which
denies the validity of inferences from observable phenomena to causes
that cannot themselves be observed. Perhaps the best known modern
example is the empiricism of the physicist and philosopher Ernst Mach
(1838-1916). At least for most of his life, Mach rejected the
theory of atoms as an example of "hypothetico-fictive
physics" (Mach 1992: 136) and would respond to believers in
atoms with the words, *Habn S'eins gsehn?* (Have you seen
one?) (Meyer 1992: 151). (For a slightly different view of
Mach's atomism, see Banks 2003: 12-14.) A more recent
example is the "constructive empiricism" of Bas van
Fraassen, which holds that the aim of science is to produce theories
that are empirically adequate, a theory being empirically adequate if
"what it says about the observable things and events in this
world, is true" (van Fraassen 1980: 12). This view also rules
out inferences to unobservable entities (van Fraassen 1980: 72). While
this may sound like a modern position, it was held by the first people
to call themselves "empiricists," namely a group of
ancient medical writers, who rejected inferences to what is "not
evident" in favour of the recording of correlations between
types of observable events
 (sect. 3.3.1).


### 2.2 Genetic Empiricism



A second form of empiricism is summed up by the Latin slogan *nihil
est in intellectu quod non prius fuerit in sensu* (nothing is in
the intellect which has not first been in the senses). We can think of
this as a *genetic empiricism* (Lowe 1995: 189), which makes
claims about the origins of our ideas. These claims take two forms.
The first of these is what we may call *tabula rasa* (blank
slate) empiricism, which denies the existence of innate ideas or
principles of reasoning, holding that both our factual knowledge and
the concepts we employ in describing the world are drawn from
experience. The modern *locus classicus* for an empiricism of
this kind is the work of John Locke, who famously suggested that the
mind is originally "white paper, void of all characters, without
any ideas" (Locke *Essay* 2.1.2). (For the history of
this idea, see
 sect. 3.3.3
 and
 sect. 4.1.)
 But it also takes the form of what we may describe as a *cognitive
empiricism*. This denies that we have intellectual powers or
capacities that can give access to factual knowledge independently of
experience.



Cognitive empiricism may seem implicit in the *tabula rasa*
principle that "nothing is in the intellect which has not first
been in the senses." But the two positions are not identical,
for one can deny that we have innate ideas or principles while also
holding that the mind shapes what the senses provide. As G. W. Leibniz
(1646-1716) would later remark, "nothing is in the
intellect which was not first in the senses, except for: the intellect
itself (*nisi intellectus ipse*)" (Leibniz *NE*
1.2). There are two contributions the intellect could make. It could
predispose us to understand what we perceive in certain ways. Or it
may have the capacity to receive a belief-forming influence from an
external source, one not mediated by the senses. (For a medieval
version of the latter idea, see
 sect. 4.2.)
 A thorough-going cognitive empiricism would deny the existence of
either innate dispositions or non-sensory influences in the formation
of beliefs. It would hold that the mind forms its beliefs by
"selecting, combining, or otherwise manipulating simple ideas of
sensation" (Kahn 1981: 407).


### 2.3 Justificatory Empiricism



A final form of empiricism is what we may call *justificatory
empiricism*. In its strongest form, an empiricism of this kind is
indifferent as to the origins of our ideas and can tolerate inferences
to unobservable entities. It merely insists that wherever our ideas
come from and whatever explanatory claims we make, these cannot be
regarded as justified until they have been successfully tested against
experience. Within twentieth-century philosophy, this position is
associated with a sharp distinction between the "context of
discovery" and the "context of justification"
(Reichenbach 1938: 6-7), with justificatory empiricists focusing
on the latter. For Karl Popper (1902-1994), for instance, the
question of how a theory is discovered is irrelevant to its epistemic
status; what matters is whether that theory has explanatory power and
has withstood severe testing (Popper 1935 [2002: 7-8]). On the
face of it, this does seem a recent position. Perhaps no philosopher
before Popper would have regarded the origins of an idea as entirely
irrelevant to its standing. But there are hints of a modest kind of
justificatory empiricism in the work of some ancient and medieval
thinkers, who held that factual claims arrived at by deductive
reasoning lack credibility if they have not also been tested against
experience.


## 3. Ancient Empiricism



The discussion that follows will employ these categories in order to
analyse the thought of ancient and medieval philosophers. Once again,
this may be thought to run the risk of anachronism, insofar as such
categories are not themselves ancient. Although one school of ancient
thinkers did identify themselves as empiricists, no ancient or
medieval philosophers distinguished the three forms of empiricism I
have described. But this is not a fatal objection. We must be careful
not to distort the thinking of ancient or medieval philosophers by
forcing it into modern categories, as into a Procrustean bed. Nor
should we write intellectual history in a "Whiggish"
fashion, interested only in those features of ancient thought that
anticipate our own. But as Allen Wood once remarked (a little too
paradoxically), "people can mean things they can't think,
and therefore ... must be able to express thoughts they
can't have" (A. Wood 2002: 227). When it comes to ancient
and medieval philosophers, our modern categories can help highlight
aspects of their thought they themselves may never have made
explicit.


### 3.1 The Emergence of an Empiricist Attitude



If we think of empiricism as an attitude, rather than a clearly
formulated doctrine, then it dates from the very beginnings of Western
philosophy, in the work of some Presocratic philosophers. Some care is
needed here, since we know of the Presocratics only through quotations
and reports, found in authors who in some cases lived centuries later
and who themselves had philosophical axes to grind. One of our major
sources, for instance, is Aristotle. But Aristotle was not trying to
write a carefully balanced history of philosophy; he was attempting to
construct his own system and he sometimes reshaped his
predecessors' views with this goal in mind (Cherniss 1935: 347).
Judging from the quotations and reports we do have, it would be
difficult to describe the Presocratics as empiricists in practice.
There is little sign they engaged in what we would call
"empirical research": acts of observation intended to
discover new facts or test hypotheses (Lloyd 1979: 139). Even if they
had, many of their claims about the natural world are sweeping and
vague: it is far from clear how they could be tested empirically
(Barnes 1982: 48). Despite this, some of the Presocratics did express
sentiments that provide a foundation on which later empiricist
thinkers could build.



Of particular note are some sayings attributed to the Ionian
philosopher Heraclitus (fl. ca. 500 BCE). One fragment reads,
"The things of which there is sight, hearing, experience,
[these] I prefer" (DK 22B55; Graham 2010: 149). Although the
contrast here may be between knowledge by experience and knowledge by
hearsay (Kahn 1979: 106), it does suggest a confidence in knowledge
that comes from sense perception. It is, however, balanced by another
saying, which notes that "poor witnesses for men are the eyes
and ears of those who have barbarian souls" (DK 22B107; Graham
2010: 149). This suggests that while the senses do provide knowledge,
what they offer needs to be correctly understood. We find a similar
attitude in Empedocles (ca. 495-435 BCE), who, while emphasizing
the limits of human understanding, encourages his hearers to
"understand each thing in the way in which it is clear"
(*noei d'hei delon hekaston*, gleaning what
knowledge they can from the senses (DK 31B3; Graham 2010: 343).



Empedocles's saying can be understood as a response to the work
of the Eleatic philosophers, such as Parmenides (ca. 515-445
BCE) and Melissus of Samos (*fl*. ca. 440 BCE). Parmenides
relied on *a priori* arguments to reach conclusions that flew
in the face of experience, while his follower Melissus argued for the
unreliability of sense perception on the grounds that its deliverances
were contradicted by reason (Barnes 1982: 298-302). Such
teachings offered a challenge to the early scientific tradition
established by Thales of Miletus (*fl.* ca. 585 BCE), for they
suggested that "the phenomena which science attempts to
understand ... are figments of our deceptive senses"
(Barnes 1982: 305). If there was to be a scientific tradition, it
required a defence of sense perception over against pure reason.



Other Presocratic thinkers take empiricism beyond a simple defence of
sense perception; they also express suspicion regarding claims that go
beyond what can be observed. A key figure here is Xenophanes (ca.
570-475 BCE). Perhaps the best known of his sayings is fragment
34: "No man knows, or ever will know, the truth about the gods
and about everything I speak of; for even if one chanced to say the
whole truth, yet oneself knows it not; but seeming (*dokos*) is
wrought over all things" (DK 21B34; Kirk, Raven, & Schofield
1983: 179). This is a cryptic saying, made more so by the fact that it
lacks a context. But a plausible interpretation holds that in matters
inaccessible to perception, we cannot be confident about the claims
that are made, since we have no way of independently verifying them
(Hussey 1999: 22). Xenophanes's saying certainly expresses
caution about claimed knowledge of the gods. But if the phrase
"everything I speak of" refers to Xenophanes's
account of the physical world, it may also refer to knowledge of its
underlying principles (Barnes 1982: 139). Yet Xenophanes is no
sceptic: he believes that growth in knowledge (broadly understood) is
possible. As he is reported to have said, "the gods have not
revealed all things to men from the beginning; but by seeking men find
out better in time" (DK 21B18; Kirk, Raven, & Schofield
1983: 179). Indeed Xenophanes himself seems to have made bold claims
about both the physical world and divinity. His empiricism is limited
to the idea that in matters that go beyond what can be immediately
experienced we can expect to attain nothing more than "belief
[*dokos*] and verisimilitude" (Barnes 1982: 140).



A striking feature of ancient empiricism is its association with
medical writers, and Xenophanes's caution about knowledge of the
unseen is picked up by an early medical writer, Alcmaeon of Croton
(fl. ca. 500 BCE), to whom the following fragment is attributed:
"Concerning things unseen *(peri ton aphaneon*)
the gods see clearly, but so far as men may conjecture
(*tekmairesthai*)... " (DK 24B1; Kirk, Raven, &
Schofield 1983: 339). Once again, it would be foolish to read a body
of empiricist doctrine into such a cryptic (and apparently incomplete)
remark. Yet it, too, indicates a contrast, not only between what the
gods know and men may know (a commonplace of ancient Greek thought),
but between knowledge that can be obtained by observation and
knowledge of "the unseen". Yet Alcmaeon himself appears to
have made claims regarding "unseen matters" (*ta
aphane*), not merely (as a medical writer) about the interior
workings of the body, but also about the soul and its immortality. So
it would be mistaken to regard him as an explanatory empiricist,
rejecting inferences to what is unseen. Indeed in its original
context, the remark may have suggested that despite the difficulties
of knowing what is unseen, careful investigation allows us to make
such inferences (Huffman 2021: sect. 2.1). Indeed while
*tekmairesthai* can mean "conjecture", it can also
mean to infer from "signs" (*tekmeria*), a
phrase that would later be used for inferences from what can be
observed
 (sect. 3.3.3)



Staying within the medical tradition, a similar caution about
knowledge of "things unseen" can be found in some of the
works attributed to Hippocrates (ca. 460-370 BCE). The author of
one Hippocratic treatise argues against those who practice medicine on
the basis of some general principle or postulate
(*hupothesis*). Medicine, he suggests, does not require a
general theory "about human nature" (*ho ti estin
anthropos*), of the kind offered by Empedocles and other
philosophers (*AM* 20.1). Such general principles may be
required when dealing with with matters that are "obscure and
dubious", such as "the things in the sky or under the
earth" (*AM* 1.3). But medicine differs from such
speculative sciences, since its conclusions can be confirmed by
experience. Of particular note here is the phrase "the things in
the sky or under the earth," which picks out the main areas of
early Ionian science (Barnes 1982: 139; cf. Plato *Ap.*
18b8-9), regarding knowledge of which the author seems to be
expressing caution.



At first sight, this looks like an early expression of explanatory
empiricism: a refusal to make inferences to things that cannot be
observed. But as in the case of Xenophanes and Alcaemon, it is better
regarded as the expression of an empiricist attitude than an
empiricist doctrine. Even leaving aside the fact that "the
things in the sky" are not strictly unobservable, the same
Hippocratic work also speaks of discovering facts about what is inside
the body "from evident things outside [the body]"
(*AM* 22.3). In particular, it argues, we can draw conclusions
about the body's internal organs by noting the behaviour of
different kinds of observable objects: those that are solid,
stretched, spongy, porous, and so on. (The reasoning here is by way of
analogy, a common practice among the earliest Greek thinkers [Lloyd
1992: 254-55; Allen 2001: 10-11].) This appears
inconsistent with an explanatory empiricism, since it justifies
inferences from what can be observed to what is (at least in practice)
unobservable.



Talk of discovering what is unseen from what is evident calls to mind
the maxim attributed to Anaxagoras (ca. 500-428 BCE):
"Appearances are a vision of the invisible" (*hopsis
... ton adelon ta phainomena*) (DK 59B21a;
Graham 2010: 309). Not only was this principle employed in the
Hippocratic writings, but it was said to have been endorsed by
Democritus (ca. 460-371 BCE) (Sextus *M* VII 1.140). For
all his scepticism regarding sense perception (DK 68B6-10;
Graham 2010: 592-95), Democritus seemed to think it had an
evidential role (Asmis 1984: 345). In particular, he apparently
believed it could confirm the atomist theory which Leucippus
(*fl.* ca. 450-420 BCE) had originated and he himself had
embraced. There is, however, a difficulty here, as Democritus himself
may have realized, for in one report he has the senses utter the
following: "Wretched mind, after taking from us your evidence,
do you overthrow us? Our fall will be your defeat!" (DK 68B125;
Graham 2010: 597). The problem here is that while one can cite
observable facts (such as the fact of motion) in support of atomism,
that doctrine in turn suggests that appearances are deceptive (Barnes
1982: 559-64), since the world does not appear to be made up of
atoms. As another fragment reads, "by convention color, by
convention sweet, by convention bitter, but in reality atoms and
void" (DK 68B9, 125; Graham 2010: 597). This contrast between
what appears to be the case (which we accept "by
convention") and what is actually the case threatens the
undermine the atomist's appeal to the evidence of the
senses.



What we find, then, in the work of some of the earliest Greek
philosophers is not only the first outline of empiricist doctrines; we
also find hints of the difficulties such doctrines would encounter.
But to find a clearer statement of an empiricist view of knowledge, we
must turn to the work of Aristotle.


### 3.2 Aristotle's Empiricism



Aristotle discussed a variety of ways in which human beings can
achieve a grasp of reality. In particular, he mentions five ways in
which "the soul possesses the truth by way of affirmation or
denial" (*EN* 1139b15-17): art or craft
(*techne*), scientific knowledge
(*episteme*), practical wisdom
(*phronesis*), philosophical wisdom (*sophia*), and
comprehension (*nous*). These are, however, varieties of
knowledge; this remark does not tell us about the origins of what we
know. So was Aristotle an empiricist? Answering this question is not a
straightforward task. While Aristotle does offer us a description of
"systematic theoretical knowledge" (Pasnau 2013:
992)--namely, the *Posterior Analytics*--he offers us
no analysis of knowledge in general (Taylor 1990: 117).



We do know, however, that Aristotle believed the senses to be
reliable, and that we must rely on their deliverances for our
knowledge of factual matters. Although he insisted that "not
everything that appears is true (*ou pan to phainomenon
alethes*)" (*Met.* 1010b2-3), he regarded
things as they appear (*ta phainomena*) as the starting point
of any inquiry and conformity with them the indication of its success.
Although he uses the phrase *ta phainomena* broadly--it
embraces *endoxa*, "reputable opinions" as well as
what is immediately evident to the senses (Owen 1986:
240)--Aristotle regarded sensation (*aithesis*) as
particularly trustworthy. Each of the senses has its own proper
objects (such as color, in the case of sight) and our grasp of these
is "is true or [at least] admits the lowest degree of
falsehood" (*DA* 428b18-19; cf. 418a11). Not only
is sensation, at this lowest level, trustworthy, but it is our only
source of knowledge when it comes to contingent matters of fact. As
Aristotle writes, "no one can learn anything at all in the
absence of sense" and that even when we are not actively
perceiving, our thinking remains dependent on sensory images
(*DA* 432a7-9). Insofar as Aristotle's view of
knowledge insists on both the reliability of the senses and their
essential role in the gaining of knowledge, it can be thought of as
broadly empiricist.



To gain a better understand of the nature and limits of
Aristotle's empiricism, we must once again make some
distinctions. Aristotle can be classed as a *genetic*
empiricist, for he rejects the claim that we have innate ideas or
principles of reasoning. He is also, arguably, an *explanatory*
empiricist, although in a different sense from that found among
Hellenistic medical writers and Sceptics
 (sect. 3.3).
 We can regard him as a modest kind of *justificatory*
empiricist, who believes that scientific theorizing is answerable to
what can be observed. But what is contested is whether he can be said
to be a *cognitive* empiricist, given the role of intellectual
insight in his account of scientific explanation.



With regard to genetic empiricism, Aristotle rejects the doctrine of
innate ideas found in the work of Plato (427-347 BCE). He
strongly denies, for instance, that we have innate knowledge of the
principles of scientific demonstration (*APo.* 100a;
*Met.* 993a). This aspect of Aristotle's empiricism can
be illustrated by his attitude to mathematics. For Plato, mathematical
objects resemble the Forms--some may in fact be Forms (Shapiro
2000: 54-55)--which occupy a timeless realm distinct from
that of the objects of sensation (*Rep.* 510c1-511b2).
They are known, therefore, either by recollection
(*anamnesis*) or by a kind of direct intellectual
intuition. Along with his rejection of innate ideas, Aristotle also
rejects this conception of mathematical objects. What the
mathematician grasps are real features of perceptible objects, but
regarded in a particular way (Lear 1982: 168, 184). They are regarded
merely "as" (*hei*) having quantity and being
continuous or as objects of enumeration (*Phys.*
193b22-194a10; *Met.* 1061a28-1061b3,
1077b19-1078a4, 1078a22-28).



Is Aristotle an explanatory empiricist? Aristotle does accept the need
for inferences from what can be observed to facts that are not
immediately evident (Asmis 1984: 212-17). As he writes,
"to gain light on things imperceptible we must use the evidence
of perceptible things" (*EN* 1104a13-14). It
follows that something perceived can serve as a sign of something that
we would not otherwise know. Roughness of the tongue, for example, is
a sign (*semeion*) of fever (*DS* 462b26-32);
the large size of hailstones is a sign (*semeion*) they
have been formed close to the earth (*Meteor.*
348a33-36). But while knowledge of signs constitutes evidence,
it does not, in itself, represent scientific knowledge. Scientific
knowledge is knowledge not just of facts, but of their causes. Even if
the association between sign and signified is without exception, so
that one can infer the latter from the former, such an inference is
not the same as offering an explanation. It may be, for instance, that
fever is a reliable sign of illness (*Rhet.* 1357b18-19):
all those with fever have an underlying medical condition. But this
does not yet constitute an explanation, for it does not yet show that
the fever is a necessary consequence of the illness. A sign, in other
words, can establish only what Aristotle calls "the fact"
(*to hoti*); showing a correlation between sign and signified
does not, in itself, establish the "reasoned fact" (*to
dioti*) (Madden 1952: 372; cf. *APo.* 78a23).



So when it comes to establishing the "reasoned
fact"--the process of scientific explanation--is
Aristotle an explanatory empiricist? Would he deny the legitimacy of
inferences from observed facts to unobserved (or, perhaps,
unobservable) causes? Aristotle is cautious regarding our ability to
know about matters that cannot be directly observed. As he writes,
when it comes to "matters inaccessible to the senses," it
is sufficient if our investigation arrives at a possible cause
(*Meteor.* 344a5-7). Insofar as he allows inferences of
this kind, Aristotle is not an explanatory empiricist. But the phrase
"explanatory empiricism" does not quite fit the shape of
Aristotle's natural philosophy. Firstly, explanatory empiricism
stands in an epistemological tradition whose central question is
"What are we justified in believing?" That tradition has
its origins in later, Hellenistic philosophy
 (sect. 3.3);
 its concerns are not Aristotle's (Burnyeat 1981: 136). Second,
what Aristotelian explanations appeal to are the "causes and
principles (*aitia kai archai*)"--terms Aristotle
uses interchangeably (Witt 1989: 18)--by which phenomena are made
intelligible. Some of these "generative factors"
(Moravcsik 1975: 622) are inherent in (*enuparchousai*) the
things to be explained, while others are outside (*ektos*)
(*Met*. 1013a19-20; *APo*. 93a6-7). Only the
generative factors that lie outside the fact to be explained (in the
sense of being distinct from it) could be thought of as unobservable
entities. The generative factors that are inherent in things are not
unobservable in the same sense as atoms (for instance) would be. They
may still be "not evident" but in a different sense: it is
not always evident that a class of entities share a common nature.
Establishing this fact requires "induction"
(*epagoge*), to which I shall return in a moment.



What about justificatory empiricism? It seems that no ancient or
medieval thinker would have endorsed a strong form of justificatory
empiricism, which holds that the origins of a theory are irrelevant to
its epistemic status. But many did insist that all scientific
theorizing was answerable to experience. Aristotle certainly did so.
As he remarks, "we should accept what is evident to the senses
rather than [mere] reasoning," and reasoning only if it agrees
with the observed facts (*GA* 760b29-33). He had little
sympathy with the idea that the natural world could be explained in an
entirely *a priori* fashion, as Plato sometimes suggests (e.g.
*Rep.* 530b7-8, 531c1-5). He criticizes, for
instance, the Pythagoreans for forcing their observations of the
heavens to accord with theory, rather than seeking theories that
account for what can be seen to occur (*DC* 293a25-27).
Aristotle is particularly dismissive of the Eleatic philosophers, who
in their reasoning "transcended sense perception and disregarded
it on the grounds of following the argument." He notes that
while their opinions are supported by reason, it would be
"almost madness to believe them, when one considers the
facts" (*GC* 325a18-20). The facts in question are
those revealed by the senses.



If there is a feature of Aristotle's thought that may appear to
cast doubt on his empiricism, it is the account of scientific
explanation found in his *Posterior Analytics*. (I am assuming
this is an account of scientific explanation, rather than an account
of how we form everyday concepts [Scott 1995: 109-16].)
Aristotle's conception of scientific explanation is deductive:
it involves a derivation of the fact to be explained from first
principles, the latter being "true, primary, immediate, better
known than and prior to the conclusion, the latter being related to
them as effect to cause" (*APo.* 71b20-24). There
has been much debate regarding the role of such demonstrations. A
widely-held view is that they are not, for Aristotle, a means of
making scientific discoveries. Rather, they are the way in which such
discoveries should be presented (Barnes 1994, xii). Indeed Aristotle
expressly denies we can discover explanatory principles by way of
demonstration, holding that such demonstrations serve merely to make
them clear (*APo.* 93b16-18). This view seems consistent
with Aristotle's practice in his own scientific writings, as
well as the methodological remarks found in his biological works. He
argues, for instance, that when studying animals "we ought first
to take the phenomena that are observed in each group, and then go on
to state their causes" (*PA* 640a15). While we should not
confuse Aristotle's empiricism with that of Francis Bacon
(Harari 2004: 35), even his theorizing can be said to begin with what
is observed.



It remains true, however, that Aristotle's ideal presentation of
a scientific explanation is that of a demonstration from first
principles. Only such a demonstration, he believes, would make clear
the necessary connection that constitutes a causal relation. While
this ideal is not always attainable (Pasnau 2013: 993), it remains a
*desideratum*. So how does one arrive at those principles? It
is at this point that one might question whether Aristotle truly is an
empiricist. The principles to which scientific demonstrations appeal
are universals: they are properties common to all beings of a
particular kind which constitute them as beings of that kind.
Aristotle rejects the Platonic idea that such universal properties
exist *extra rem*, independently of the individuals that embody
them. So in order to arrive at the principles of scientific
explanation we need to recognize the identity of kind among the forms
of particular objects and thus the universal that is embodied in each
of them (Owens 1966: 168). The process by which we come to grasp the
universals required for scientific explanations is known as
*epagoge* (*APo.* 99b15-100b15), a term
commonly translated as "induction."



What Aristotle means by induction (*epagoge*) is
notoriously unclear (Ross 1949: 50; Harari 2004: 33-34), but
Aristotle customarily distinguishes induction from syllogistic
reasoning, in the sense of demonstration. As he writes in one place,
"all our beliefs come either through demonstration [*dia
sullogismou*] or from induction [*ex
epagoges*]" (*APr.* 68b13). The general
contrast here is that while "demonstration proceeds from
universals to particulars," induction takes one "from
particulars to universals" (Ross 1949: 47; cf. *Top.*
195a13-14). It is sensation (sense-perception:
*aisthesis*) that gives us knowledge of particulars
(*APo.* 81b6-7), while knowledge of universals is
required for scientific knowledge (*episteme*). So
what is *epagoge*, process by which we move from
perceived particulars to known universals?



Aristotle denies (as we have seen) that the universals that form the
principles of a scientific demonstration are themselves to be reached
by way of demonstration (*APo.* 72b19-20;
93b16-18). But a demonstration (*sullogismos*) is a
strong form of argument, in which conclusions follow necessarily from
the premises (*APr.* 24b18-20). So one might think that
some weaker form of reasoning could be involved, such as
"induction" in our modern sense: a generalization over a
collection of particulars. The latter gives rise to the notorious
"problem of induction"
 (sect. 4.2),
 of which Aristotle shows some awareness (*Top.*
131b19-36): how can any limited range of observations justify a
universal claim? But even leaving this problem aside, there is a
serious objection to understanding Aristotle's
*epagoge* in this way. The most such a generalization
could show that all individuals of a certain class have certain
characteristics (*APo.* 92a37-92b2): that all swans are
white, for instance. It could not tell us that this characteristic is
essential, one of the features that constitutes a swan as a swan. In
any case, Aristotle also suggests that each of the principles of
scientific explanation is in some sense self-evident: it
"commands belief of itself" (*kath'heauten
einai pisten*) (*Top.* 100b19-22). This seems to
exclude the idea that these principles are reached by a process of
reasoning.



If we reject the idea that Aristotelian *epagoge* is
a form of enumerative induction, we might be tempted to think of it as
a kind of "immediate apprehension of essences or forms"
(Harari 2004: 34), one that occurs in the very process of perceiving
particular individuals of a given kind. (This interpretation often
goes along with a conception of *nous* as a faculty of
intuition [Kahn 1981: 369], rather than a state of understanding
[Barnes 1994: 267-69].) But if *epagoge*
involves a process of intellectual intuition that goes beyond what
sense-perception delivers, it would undercut the claim that Aristotle
is a genetic (and, perhaps, justificatory) empiricist. Experience
would provide nothing more than the occasion on which the necessary
intuitive insight is gained.



So is there a way of understanding *epagoge* that
allows Aristotle to be thought of as an empiricist? One possible
solution begins with Aristotle's puzzling claim that while
"it is the particular that we sense (*aisthanetai men to
kath'hekaston*), sensation is of the universal (*he
d'aisthesis tou katholou estin*)" (*APo.*
100a17-b1). Why is this puzzling? It is puzzling since Aristotle
elsewhere states that sensation gives us knowledge of particulars, not
universals (*APo.* 81b6-7). So in what sense can
sensation also be "of the universal"? Various solutions to
this question have been offered (Hasker & Yurdin 2014:
127-130). One solution starts from a remark found at the
beginning of the *Physics*, where Aristotle writes that the
things that are immediately apparent to us (and thus the starting
point of inquiry) are "rather confused" (*mallon
sunkechumena*) (*Phys*. 184a22). Those rather confused
things are apparently general facts, for he goes on to remark that
"it is the whole that is better known to sensation" (Phys.
184a24-25). This suggests that "we begin by responding
perceptually to undifferentiated 'wholes'"
(Gasser-Wingate 2021: 149). Only later, by way of analysis
(*Phys*. 184a22-23), do we come "to recognize the
specific nature of that whose generic nature alone is at first
recognized" (Ross 1936: 458). On this view, experience
(*empeiria*) already gives us a knowledge of universals, but
one that is not yet precise. The "merely experienced"
person is responding reliably to the features that unite entities of a
kind, but has not yet understood what those features are.



Does this interpretation solve the problems created by an empiricist
understanding of Aristotle's idea of science? One objection is
that on an Aristotelian view the principles of scientific
understanding must be not merely properties that objects happen to
have; they must be properties that objects of this kind necessarily
possess (Kahn 1981: 389). Can we explain our ability to grasp such
necessary properties without invoking some purely intellectual power
of intuition? Some commentators deny that we can. (See, for instance,
Gaukroger 1978: 124.) One interesting suggestion is that our grasp of
necessary properties could arise from our interaction with things,
through our experience of the constraints that the world imposes on
that interaction (De Groot 2014: 69). But it is unclear if even this
kind of experience can yield the strong sense of necessity that an
Aristotelian conception of science demands.


### 3.3 Hellenistic Philosophy



A philosopher of whom Aristotle was very critical, but who is worthy
of mention in this context, is Plato's nephew and successor
Speusippus (ca. 408-339 BCE). Plato's school, the Academy,
eventually became the home of Scepticism
 (sect. 3.3.4),
 but before it did so Speusippus had come up with a modified view of
the capabilities of sense-perception. While not himself an
empiricist--Speusippus speaks of two criteria of truth, of which
sense perception is just one (Sextus *M* VII
1.145-48)--he does suggest that the capacity of
sense-perception (*aisthesis*) can be enhanced when it is
combined with reason (*logos*). In a way that bridges the
Platonic gap between the "intelligible" and the
"sensible", he speak of the resulting synthesis as
"cognitive sense-perception" (*epistemike
aisthesis*), which is the faculty that allows us to discern
objects and states of affairs (Dillon 2003: 79). It is, however,
consistent with Speusippus's Platonism that he would have
regarded knowledge of physical objects as merely "apparent
knowledge"; true knowledge was of the eternal mathematical forms
that underlay them (Dillon 2003: 80, 82).



For clearer examples of empiricist doctrine we can turn to the
thinkers of the Hellenistic age (a roughly 300-year period following
the death of Alexander the Great in 323 BCE, although my survey will
extend into the second century CE). These include medical writers and
Sceptics, Epicurus (ca. 341-270 BCE) and his followers, and the
school founded by founded by Zeno of Citium (ca. 332-265 BCE),
whose members are known as Stoics. The medical writers and Sceptics
offer a sophisticated form of explanatory empiricism. The followers of
Epicureans offer a strong view of the reliability of sense perception,
which is their "criterion" of truth, making them
justificatory empiricists. But they were not explanatory empiricists,
since they accepted inferences to what is "not evident"
(*adelon*), such as the existence and activity of atoms.
The Stoics hold to a form of genetic empiricism, in ways that resemble
modern uses of the *tabula rasa* metaphor, and share (broadly
speaking) the Epicurean view of the criterion of truth. But at least
when it comes to moral notions, they modify their empiricism by
insisting that humans are predisposed to form certain beliefs, beliefs
about what is just and good. No more than the Epicureans are the
Stoics explanatory empiricists: they accept the legitimacy (under
certain conditions) of inferences to what is not evident to the
senses. The strongest critics of the Stoic view were the Sceptics, who
argued that while we can "go along with" what is evident
for practical purposes, we should practise suspension of belief
(*epoche*), so as to attain tranquillity of mind
(*ataraxia*).


#### 3.3.1 Medical Writers



The first people to describe themselves as empiricists
(*empeirikoi*) were a group of medical writers of the
Hellenistic period. We know of these thinkers only indirectly, through
the work of other ancient writers, in particular Galen of Pergamon
(129-ca. 200 CE). As medical practitioners, their primary
concerns were practical: finding the correct method of diagnosis and
treatment. But in discussing these matters they put forward some
general epistemological claims, developing the empiricist tendencies
already found in the Hippocratic writings
 (sect. 3.1).
 In particular, they set themselves in opposition to those whom they
called "rationalists" (*logikoi*) or
"dogmatists" (*dogmatikoi*). What set them apart
was a refusal to posit causes beyond those that can be observed.



In spelling out their position the medical empiricists distinguished
between two styles of inference. What they rejected was the variety of
reasoning they called "analogism" (*analogismos*),
which "starts from what is apparent but ends up with what is
entirely unclear" (Galen *SB* ch. 5). In its place they
advocated what they called "epilogism"
(*epilogimos*), which deals "solely with what is
apparent," but can be used "to discover what is currently
unseen" (Galen *SB* ch. 5). The idea here is that while
the cause that is inferred may be currently hidden, the inference is
based on an association that is, at other times, directly observable.
*Epilogismos*, the empiricists argued, is a form of reasoning
used by all human beings, which leads to a consensus, while
*analogismos* leads to disagreement and conclusions that can be
neither confirmed nor denied (Galen *ME* ch. 24).



The tradition of medical empiricism influenced the work of Galen
himself, although his preferred method involved the use of both
*logos* (reason) and *empeiria* (experience). As Galen
wrote, "*logos* alone discovers some remedies,
*peira* ['experiment' or 'trial']
without employing *logos* [discovers] others, while some
require both of them working together" (Galen *Op.* 13,
pp. 886-887; Hankinson 2022: 104). Galen is what I have called a
justificatory empiricist in that he believes all factual knowledge to
rely on the evidence of the senses, which are assumed to be broadly
reliable (Hankinson 2009: 214-15). Galen's justificatory
empiricism is revealed in his conviction that there are philosophical
questions that simply cannot be resolved, because perceptual
experience gives us no way of adjudicating them. Such questions
include whether the world is created or uncreated, whether its extent
is finite or infinite, and whether God is corporeal or incorporeal
(Hankinson 2009: 228-29). Galen is not, however, an explanatory
empiricist, since he thinks there are non-evident causal factors that
we can discover. Indeed the proper practice of medicine requires us to
discover them (Hankinson 2022: 91). It is for this reason that he
opposes the belief of the medical empiricists that experience alone
suffices for the practice of medicine. Rejecting, as they do, the
search for underlying causes, such practitioners have "no
systematic way of determining therapy in cases which are unlike those
which they have come across before" (Hankinson 2022: 87).



Galen shares what appears to be Aristotle's view: that sense
perception already gives us a knowledge of the first principles of
scientific knowledge, although such knowledge can be made explicit
only by reasoned analysis. The reasoning in question is akin to that
of a geometer, "working back from a problem ... to the
axioms from which the problem can be solved, or the facts
explained" (Hankinson 2009: 223). The doctor's craft
(*techne*) therefore involves not just experience but what
we may call "reasoned experience" (Hankinson 2022: 104).
Such reasoning allows one to discern the conditions under which a
therapeutic substance will manifest its powers (van der Eijk 2005:
289-90). Galen refers to experience of this kind as
"differentiated experience" (*diorismene*
*peira*) (Galen *Op.* 11, p. 685), which can allow one
to discover causal powers even from a single trial (Hankinson 2022:
106). This idea of differentiated experience brings us very close to
the theory (if not the practice) of what we call "controlled
experiments" (Singer 2022: 185).



Experience in this context is not passive, a mere reception of
sense-data. As in the case of Speusippus's "cognitive
sense perception", which like the performance of a musician can
be improved by practice (Sextus *M VII* 1.146), the perceptual
ability of the skilled practitioner can be improved by reason and
effort. Galen's example of this is his ultimately successful
struggle to perceive the systole of the pulse, which reason had taught
him must exist (Singer 2022: 171-72). In the case of experience
trained by reason and practice, the gap between observable effect and
unobservable cause narrows to the point of disappearing. If we use the
term "perceiving" broadly (Galen *Op.* 8, p. 793),
trained experience can be said to make manifest the causal factors
that produce our experience (Singer 2022: 174).



Such claims seem overly optimistic. Galen fails to take into account
the degree to which theory is underdetermined by data. Skilled Chinese
physicians, when taking a pulse, "perceived" quite
different causal factors, since their perception was informed by a
differing kind of medical practice and body of theory (Kuriyama 1999:
25, 37). In this respect, Galen's philosophy of science faces
the same difficulties as Aristotle's (to which it is closely
related). Can it explain how observation could lead to an explanation
that is necessarily true, given the evidence (Hankinson 2022:
110)?


#### 3.3.2 Epicureans



As will already be evident, the early medical writers were also
philosophers. But among the specifically philosophical Hellenistic
schools, the first I shall consider is that of the Epicureans.
Epicurus and his followers had little interest in a study of the
natural world for its own sake. The Epicurean movement can be regarded
as a "secular religion" (Milton 2002: 182), which aimed to
achieve a knowledge of nature only insofar as this promoted peace of
mind. (For a more vigorous defence of the scientific credentials of
Epicureanism, see Asmis 1984.) But in working out their philosophy the
Epicureans develop a distinctive form of empiricism. It is, first of
all, a genetic empiricism, holding that the evidence of the senses is
"the base and foundation" of all our knowledge (Sextus
*M* VII 1.216). But it differs from some other forms of
empiricism both in regarding as infallible the immediate deliverances
of perception and allowing for the mind to be affected by external
objects in ways independent of the five senses. Epicurean empiricism
is also more tolerant than that of the medical writers and Sceptics,
since it accepts inferences to unobservable causes. It is not, in
other words, an explanatory empiricism. Indeed this seems to be the
reason why the sceptical Sextus counts Epicureans among the
"dogmatists" (Sextus *PH* 1.3).



In a work entitled *On the Criterion or The Canon* (*Peri
kriteriou e kanon*), Epicurus is said to have
argued that all sensations (*aistheseis*) are reliable,
there being nothing that can refute them (Diogenes Laertius [DL]
10.31). Sextus sums up this Epicurean doctrine as the view that
"all sensed things ... are true and existent" (Sextus
*M* VII 2.9). On the face of it, this seems an untenable
position. Do our senses not deceive us? Do we not see sticks as bent
where they enter water or a distant large object as small? There are
two responses an Epicurean can make. The first is to make a clear
distinction (comparable to that made by Aristotle
 [sect. 3.2])
 between our sensations and the opinions we form on the basis of what
we sense. When it comes to our opinions, Epicurus held that
"some them are true and others are false, since they are
judgements of ours concerning what appears, and we judge sometimes
rightly and sometimes wrongly" (Sextus *M VII* 1.210).
When we make mistakes, in other words, it is not the senses that are
to blame. Secondly, the senses are verdical because sensations are not
"purely private mental phenomena" (O'Keefe 2010:
100). They are brought about by the objects of which they testify,
which emit a continuous flow of images or *simulacra*
(*eidola*) from their surfaces (Epicurus *Ep. Hdt.*
46-51). On this view, the senses are reliably informative,
faithfully reflecting the objects that produced them, even if we later
attribute to them a false propositional content.



Epicurean empiricism is, however, a relatively tolerant form of
empiricism. It is tolerant, first of all, insofar as it does not
restrict perception to what is received by way of the five senses.
Epicurus and his followers seem to have regarded the mind itself as a
kind of "supersense" (DeWitt 1954: 207-9), capable
of receiving *eidola* on its own account. (This doctrine
bears some resemblance to one of the Buddhist beliefs mentioned
earlier
 [sect. 1].)
 The Epicurean writer Lucretius (ca. 95-54 BCE), for instance,
claims that some of the *simulacra* thrown off by objects are
exceedingly fine, such that they can penetrate the body and affect the
mind independently of the five senses. This allows him to account for
the images we see in dreams, which accurately reflect aspects of the
world, although in a manner that deceives us (Lucretius *RN*
4.722-31, 5.1168-71).



Epicurean empiricism is also tolerant insofar as it rejects what I
have called "explanatory empiricism": it does allow for
explanatory inferences to unobservable entities. More precisely,
Epicurus speaks of two kinds of inferences from what is perceived to
what is currently unobserved. The first is an inference to "that
which awaits confirmation" (*to prosmenon*); the second
is an inference to "that which is non-evident" (*to
adelon*) (Epicurus *Ep. Hdt.* 38.10). Discussing the
views of Epicureans and other dogmatists, Sextus makes a similar
distinction (*PH* 2.97-98). He speaks, firstly, of an
inference to something that is "for the moment" (*pros
kairon*) non-evident, but which will become evident in due course.
His example is the city of Athens, which (assuming I am not at Athens)
will become evident to me if I travel there. This corresponds to
Epicurus's "that which awaits confirmation" (DL
10.34). But Sextus also speaks of entities that are "non-evident
by nature (*ta ... phusei adela*)", a category
that corresponds to Epicurus's "non-evident." Some
dogmatists, Sextus writes, regards the existence and activity of such
as knowable by inference, an example being the Epicurean (and
Democritean) belief in atoms moving in the void (Sextus *M* VII
2.319). Note that Sextus adds to these two a third category: that of
matters which are "altogether non-evident" (*ta
kathapax adela)* (*PH* 2.97), which even dogmatists
agreed could never be known by any kind of inference, his example
being whether the number of stars is odd or even.



On the Epicurean view, what kinds of inference could be used to
supported belief in unobservable entities? In the case of entities
unobservable only "for the moment," the answer is clear.
One can make a conjecture and then test it when the entity in question
becomes observable (Sextus *M* VII 1.212). But in the case of
things that are "non-evident by nature," only indirect
confirmation is possible. Epicurus's view appears to have been
that a hypothesis consistent with experience--one that is not
"witnessed against"--can and should be accepted
(Sextus *M* VII 1.216). But of course two or more hypotheses
may survive this test. Remarkably, Epicurus and his followers seem to
have held that in these circumstances they can all be accepted, not
just as possible, but as true, being realized at some time or in some
place (Allen 2004: 95; cf. Lucretius *RN* 5.526-33).
While this may have seemed plausible in a universe infinite in extent
and unlimited in time, it leaves open the question of which of these
explanations applies to the particular case we are interested in.
Epicurus seems to have held that we should not expect to be able to
answer this question, in every case (*Ep. Pyth.* 86). Nor do we
need to do so; the knowledge that an event has some natural
explanation is sufficient to free us from a superstitious fear of the
gods (*Ep. Pyth.* 87). (For the more restrictive view that this
tolerance of multiple explanations applied only to "explanations
of celestial phenomena", see Sedley 1982: 269-71.)


#### 3.3.3 Stoics



A second school of Hellenistic philosophy that can be classed as
empiricist is that of the Stoics, whose view of knowledge owes much to
the thought of the Epicureans (Scott 1995: 162, 185). Picking up (it
seems) an image from Plato (*Theaetetus* 191c9-10),
Aristotle had spoken of the mind using a metaphor akin to that of the
*tabula rasa* (*DA* 430a1). But Aristotle's use of
the metaphor is not identical with that of later empiricists, for what
he was speaking of was not so much the initial state of the mind as
its capacity to receive knowledge (Polansky 2007: 454). The
Stoics' use of a similar metaphor is more clearly an expression
of a genetic empiricism: the idea that we have no innate ideas or
principles of reasoning. "When a man is born", they are
said to have claimed, "he has the regent part of his soul like a
sheet of papyrus (*chartion*) well-prepared for making a
transcript" (Aetius, *Placita* 4.11; Mansfeld &
Runia 2020: 1591). What complicates the attribution of a genetic
empiricism to the Stoics is that they held that certain moral
notions--ideas about what is just and good--are acquired
"naturally" (*phusikos*; DL 7.53). In itself,
"naturally" could mean nothing more than arising
spontaneously from experience, "without attention or
teaching" (Scott 1995: 180-81). But in this context ideas
that are acquired "naturally" are distinguished from those
acquired "by experience" (*kata periptosin*),
that is to say, by way of sensation. This suggests something akin to
innate knowledge (Scott 1995: 203-204).



There is, however, more than one way of thinking about innate
knowledge. A claim to innate knowledge could mean that we are born
with certain ideas already implanted in our minds, or it could mean
simply that we have a disposition to form certain beliefs when
presented with the right experience
 (sect. 2.2).
 Stoic innatism with regard to moral notions is better understood as
the second of these: a dispositional innatism. One way in which they
expressed this idea was to say that "nature gives us uncorrupted
starting points" (*he phusis aphormas didosin
adiastrophous*); it is external influences that lead human beings
astray (DL 7:89). On this view, "we are naturally disposed to
form certain patterns of behaviour" (Scott 1995: 206). But in
order to form the morally correct pattern of behaviour, we must be
predisposed to form correct moral beliefs. One can argue that this
does not violate the tenets of a genetic empiricism, since on the
Stoic view the mind is not entirely passive in forming ideas. Under
the influence of experience it "inscribes for itself"
(*enapographetai*) certain notions on the blank sheet of
papyrus (Aetius, *Placita* 4.11; Mansfeld & Runia
2020: 1591). It is consistent with such a view to say that the mind is
predisposed to write one thing rather than another (Scott 1995:
207-8).



In addition to this modified form of genetic empiricism, the Stoics
also embraced a justificatory empiricism, in which the presentation
(*phantasia*) of a thing is the criterion by which the truth
about it is known (DL 7.49). But they held that not just any
presentation can play the role of a criterion, only one that is
"apprehensive" (*kataleptike*; Annas
1990: 189 n.18). An "apprehensive" presentation is
"one caused by an existing object and imaged and stamped ... in
accordance with that existing object, of such a kind that it could not
be caused by a non-existent object" (Sextus *M* VII
1.248; see Reinhardt 2023: li-lii). The question this raises is:
how do we know which presentations are "apprehensive" (and
thus a reliable criterion)? The Stoics had already recognized that an
apprehensive presentation required certain conditions to be in place,
particularly regarding the state of mind of the person who receives
the presentation (Annas 1990: 191). But it seems that under pressure
from the Sceptics (Reinhardt 2023: xcix) the later Stoics suggested
that an apprehensive presentation was a necessary but not sufficient
criterion, since it was subject to a further condition. "An
apprehensive presentation", they argued, "is the criterion
of truth provided that it has no obstacle" (Sextus *M*
VII 1.253). An "obstacle" (*enstema*) is some
fact about the circumstances that makes a person reluctant to believe
what the presentation suggests (Sextus *M* VII 1.254). If, for
instance, we were to see a person who was actually present, but we had
good reason to believe she was elsewhere, the proper move would be to
withhold assent (Reinhardt 2023: c n.288).



Were the Stoics what I have called "explanatory
empiricists", denying the legitimacy of inferences to unobserved
(or, perhaps, unobservable) entities? A Stoic argument regarding
divination defends the practice on the grounds that we can observe the
reliability of certain signs even if we cannot explain why they are
reliable (Cicero *Div.* 1.12-16), a defence that reminds
one of the views of the medical empiricists (Allen 2001: 163). There
is also evidence of Stoic caution about inferences to what is
"not evident" (*adelon*). The Stoic thinker
Strabo (ca. 64 BCE-24 CE remarks of Poseidonius (ca.
135-51 BCE) that the latter's philosophy is too influenced
by Aristotle in its "aetiologizing"
(*aitiologikon*), a practice that "our school avoids
because of the concealment of causes" (*dia ten
epikrupsin ton aition*) (Strabo *Geog*. 2.3.8).
This is consistent with a remark attributed to the leading Stoic
thinker Chrysippus (ca. 280-208 BCE). When discussing the
location of the ruling part (*hegemonikon*) of the
soul--a matter the Stoics regarded as "non-evident by
nature" (Tieleman 1996: 271)--Chrysippus is reported to
have said that there are no "sure signs"
(*tekmeria*) from which one could infer a conclusion
(Arnim *SVF* 2.887; Galen *Op.* 5, p. 268). This remark
reflects Aristotle's usage of *tekmerion* to refer
to a sign from which one can arrive at a necessary conclusion
(*Rhet*. 1357b4-5). As we shall see in a moment,
Chrysippus did arrive at a conclusion, but his remark suggests a
greater caution about such inferences that we find in the more
rationalist thinker, Galen (Allen 2001: 165).



Despite this caution, the Stoics were not explanatory empiricists:
they went beyond what the medical empiricists would permit. Like the
Epicureans, they allowed for inferences to what is "non-evident
by nature", an idea that laid them open to attack from the
Sceptics. An example commonly attributed to the Stoics (although see
Sedley 1982: 241) is the inference from the observable fact of sweat
passing through the skin to the existence of "invisible pores
(*noetoi poroi*)" (Sextus *PH* 2.140). Some
such inferences--demonstrations (*apodeixeis*) in the
strict sense--were based on rational insight into the nature of
things, a notion that resembles the Aristotelian ideal, to which the
Stoics seem to have become increasingly sympathetic (Allen 2001:
178-79, 182). But on other occasions the Stoics appear to have
been satisfied with evidence that was merely "plausible"
or "persuasive" (*to pithanon*), that is to say,
evidence that "leads us to assent", even though the
conclusion may be false (DL 7.75). At least in some contexts, *to
pithanon* (the plausible) is synonymous with *to eulogon*
(the reasonable) (Allen 2001: 166). Using the latter term, Philodemus
(ca. 110-30 BCE) attributes to a Stoic
philosopher--probably (*pace* Hankinson 2003: 79)
Dionysius of Cyrene (*fl.* ca. 150 BCE) (Allen 2001:
207)-- the idea that "it is sufficient, concerning ...
things ... which derive from experience, for us to be convinced
in accordance with what is reasonable (*kata ten
eulogian*)" (LS 42J). The example offered in this context is
that when we set sail in summer it is reasonable to believe we will
arrive safely.



There is, however, a debate regarding the role of merely plausible
considerations in Stoic epistemology. A first view is that merely
plausible considerations are sufficient to guide our action, although
they would not justify a claim to knowledge (Burnyeat 1982: 237). A
second is that where certainty is unobtainable, merely plausible (in
the sense of probable) considerations can justify a claim to knowledge
(Gould 1971: 135). A third is that we can start with what is merely
plausible and through the clarification and mutual corroboration of
such insights arrive at the apprehension (*katalepsis*) of
a truth (Tieleman 1996: 284). (For a discussion of the role played by
mutual corroboration in the Stoic conception of knowledge, see Annas
1980: 101-103.) Either the second of the third of these
interpretations would explain Chrysippus's willingness to argue
that the location of the ruling part of the soul is the heart, despite
the absence of "sure signs".


#### 3.3.4 Sceptics



As I suggested a moment ago, it was the Sceptics of the Pyrrhonian
school, as represented by the writings of Sextus Empiricus (ca.
150-225 CE), who most vigorously contested the Stoic view. The
aim of the Sceptics was not to achieve knowledge of the natural world,
although Sextus's title *empiricus* suggests an
association with the Hellenistic medical school of the same name.
Their aim was to attain "tranquillity in matters of
opinion" (*kata doxan ataraxia*) (Sextus *PH*
1.15). Nor could Pyrrhonian Sceptics have a theory of knowledge, for
to have a theory of knowledge would entail holding beliefs, and their
aim (more radical than that of modern sceptics) was the suspension of
belief (Annas & Barnes 1985: 7). One might argue that Pyrrhonian
Scepticism was compatible with belief in a minimal sense of the word
(Smith 2022: 289-92). But this seems to have meant nothing more
than "going along with" the impressions that are forced
upon us (Sextus 2000: 61 n. 253) - even the sceptic can admit to
feeling cold (Sextus *PH* 1.13) - without any
"inclination towards" (*aneu prospatheias*) what is
being affirmed (Sextus *PH* 1.230).



Sextus does, at times, appear to endorse empiricist views. He remarks,
for instance, that "if the senses cannot apprehend external
objects, neither can the mind" (Sextus *PH* 1.99), since
the senses are the mind's "guides"
(*hodegoi*) (Sextus *PH* 1.128). This looks like a
variety of genetic empiricism, denying that the mind is
"innately equipped with a set of concepts" (Annas &
Barnes 1985: 116). But not only would an empiricism of this kind
entail holding a philosophical doctrine (which a Pyrrhonian Sceptic
ought to avoid), but Sextus's "modes"--his ways
of inducing suspension of judgement--have the general pattern of
contrasting appearances, showing that we have no reason to accept one
rather than another (Annas & Barnes 1985: 25). So (*pace*
Smith 2022: 260) it would be surprising if Sextus were here endorsing
empiricism. He is more likely to be mentioning a view taken for
granted by his "dogmatist" opponents in order to undercut
their beliefs, a practice that is common in his writings.



Among his criticisms of dogmatist views, Sextus includes a critique of
induction (*epagoge*), which assumes that
*epagoge* is simply a process of enumerative
induction. As we have already seen
 (sect. 3.2),
 this is unlikely to be what Aristotle understood by the term. But
given this understanding of induction, Sextus is quick to point out
its inadequacy. Those who follow this procedure must survey "all
the particulars or some of them." If they survey only some, the
induction will be insecure, since it may be that some of the omitted
particulars lack the feature in question. But if they try to survey
all, "they will labour at an impossible task, since the
particulars are infinite and indefinite" (Sextus *PH*
2.204). The argument is not strictly sound, since there are cases in
which we know (or believe we know) all the instances. An example that
would later be offered by John Buridan (1295-1358) is that of a
proposition regarding all the (six) planets (*SD* 6.1.5). But
if we set aside such instances, Sextus's challenge anticipates
what would later be called "the problem of induction". The
Stoics had had a response to this challenge, which is that the world
is governed by a divine reason, which guarantees its orderliness and
predictability (Allen 2001: 165). But it was not a response likely to
satisfy a sceptic, ancient or modern.



On the face of it, the Sceptics' project of living without
belief seems impossible. One might argue, with David Hume, that
"all human life must perish", if the principles of ancient
scepticism were "universally and steadily to prevail"
(Hume 1902: 160 [SS128]). But Sextus's response to this
problem was remarkably close to that offered by Hume (Smith 2022:
197), namely that we can (in practice) rely the observed association
of particular kinds of events. The question here is whether this
broadly empiricist attitude is consistent with Pyrrhonian
Scepticism.



A key distinction here is that which Sextus makes between two types of
sign: the "commemorative" or "recollective
(*hypomynestikon*)" and the "indicative
(*endeiktikon*)" . An indicative sign is thought to
signify some reality that has not directly been observed. It does so
on account of what Sextus calls "its proper nature and
constitution" (Sextus *PH* 2.101). The idea here seems to
be that of a necessary connection between the sign and what it
indicates, known by reason rather than by experience. The inference
from movement to the possession of a soul, for example, makes use of
an indicative sign, for the soul can never be directly observed. In
Sextus's view, inferences of this kind cannot be justified. (If
Sceptics were to allow themselves to hold a philosophical doctrine,
this would be an instance of explanatory empiricism.) Sextus is,
however, happy to endorse the use of commemorative signs, which are
based on nothing more than the past experience of the association of
two kinds of phenomena. Smoke, for instance, is a commemorative sign
of fire; a scar is a commemorative sign that the bearer has received a
wound (Sextus *PH* 2.102).



On the face of it, this looks inconsistent with Pyrrhonian Scepticism,
since it appears to involve holding a belief (such as "this is
smoke") and the making of an inference (such as "smoke,
therefore fire"). But Sextus suggests that neither belief (in
the sense of adherence to the truth of a proposition) nor inference
(in the sense of a logical operation) is required. He argues that if
reliance on commemorative signs did involve inferences, you would
expect those who had no knowledge of logic to be less skilled in the
relevant activities. But, as he writes, "illiterate helmsmen and
farmers with no experience of dialectical principles are expert judges
of signs" (*akros semeiountai*), being able to
predict (in one case) a coming storm or calm weather and (in the
other) a good or a poor harvest (Sextus *M* VII 2.270). Indeed,
even non-human animals gain knowledge in this way. As Sextus writes,
"the dog, when it tracks an animal by its footprints, is
actually using signs" (*semeioutai*), although it is
not making inferences. In Sextus's words, "it does not
formulate the judgement 'if this is a footprint, a beast is
here'" (Sextus *M* VII 2.271). One way of
understanding this is to say that just as we cannot avoid "going
along with" certain sense impressions, so we cannot avoid being
influenced by such observed correlations. If this deflationary
interpretation is correct, a reliance on commemorative signs would be
consistent with Sextus's scepticism.


## 4. Medieval Empiricism



It is hardly surprising, given the influence of Christianity, that
neither ancient Scepticism nor Epicureanism found much of a following
among medieval thinkers. Stoicism was influential, but more for its
moral teachings than for its view of knowledge. However, the attitudes
associated with the medical empiricism of the Hellenistic age did not
entirely disappear: we find at least hints of these among late
medieval writers. A striking example is the polemical work
*Antipocras* by Nicholas of Poland (*fl.* 1278). While
himself an educated man, trained within the scholastic tradition and
assocated with the medical school at Montpellier, Nicholas advocated a
medical practice based on experience, praising the knowledge of common
people over against that of the scholars. This may have been a
reaction to the more theoretical style of medicine developing at
Montpellier as a result of the discovery of works by Galen and some
Arab writers (Eamon & Keil 1987: 195). But for evidence of more
general empiricist doctrines, we must turn to the followers of
Aristotle.


### 4.1 Varieties of Medieval Empiricism



The medieval followers of Aristotle shared his confidence in the
reliability of the senses. Thomas Aquinas (1225-1274), for
example, follows Aristotle in holding that sensation is not deceived
with regard to its proper objects, such as colours and sounds
(*ST* 1a 17.3), provided that the sense organs are functioning
properly and in the right circumstances (Pasnau 2002:
188-89).



Did medieval philosophers embrace a form of genetic empiricism? The
Latin phrase characterizing an empiricism of this kind--*nihil
est in intellectu quod non prius fuerit in sensu* (nothing is in
the intellect that was not first in sensation)--was something of
a commonplace. It is found (in variant forms) in the work of
Bonaventure (1221-1274), where it is attributed to Aristotle,
Aquinas, Matthew of Aquasparta (ca. 1240-1302), and Duns Scotus
(1266-1308) (Cranefield 1970: 78-79). Not all accepted the
idea, as we shall see, but Aristotelians generally took it for
granted. Aquinas follows Aristotle (*DA* 432a7-9) in
holding that all the objects of our understanding "have their
being in the objects of perception," rather than in some
separated, Platonic realm. It follows that "without sense
perception no one can either learn anything new, nor understand
matters already learned." Even when we are thinking about
something we already know, our thought is always accompanied by an
image (a "phantasm"), which is a likeness of something
that can be perceived (Aquinas *In DA* 3.13 [para. 791]).



But even when medieval writers speak of the mind as a *tabula
rasa*, we should not attribute to them the kind of genetic
empiricism later found in the work of John Locke. The blank tablet of
which medievals speak, following Aristotle (*DA* 430a1), is
what they called the "passive" or "potential"
intellect, which was only one element in the process of cognition
 (sect. 4.3).
 When it comes to the larger picture, medieval philosophers sometimes
combine the Aristotelian idea that all knowledge comes through the
senses with an Augustinian view, which speak of divine illumination
 (sect. 4.2),
 or with the idea of innate knowledge. Even Aquinas held that we have
a kind of innate capacity, ultimately given by God, to grasp the first
principles of scientific knowledge (Pasnau 2002: 307-8). This is
probably best thought of as a kind of a *dispositional*
innatism: we are disposed to form certain ideas when presented with
the right kind of experience.



At times Aquinas seems to go further. He writes, for instance, that
our knowledge arises "partly from within and partly from
without" and that there is some truth in the idea that "we
already know that which we learn" (Aquinas *DV* 10.6).
Once again, however, this looks like a dispositional innatism: the
principles we employ in forming our beliefs are implicit in the
structure of the mind and its operations (Allers 1942: 58) (see
 sect. 4.3).
 Other writers are clearer in combining the *tabula rasa* idea
with a commitment to the idea of innate knowledge. Roger Bacon
(1215-1292), for instance, suggests that the human soul is
created with the *species* (aspects or forms) of all things
within it, which reflects an Augustinian conception of knowledge. But
since the soul's union with the body hinders our access to this
knowledge, the human mind (as potential intellect) can be described as
a "blank slate" (*tabula nuda* or *rasa*)
(Bacon *In Phys.* pp. 7, 9; R. Wood 2007: 54). A similar view
is found in the work of Robert Grosseteste (ca. 1165-1253), who
also combines a modified Augustinian belief in divine illumination
 (sect. 4.2)
 with the Aristotelian idea that we gain knowledge of scientific
principles from experience (van Dyke 2009: 688-89).



While these look like puzzling combinations of empiricist and
non-empiricist views, their rationale will become clear shortly
 (sect. 4.3).
 But alongside this qualified genetic empiricism, we also find
expressions of a justificatory empiricism. In particular, the work of
Roger Bacon sets out two forms of justificatory empiricism, both of
which fall under the heading of what he calls *scientia
experimentalis*. The first involves confirming well-founded
deliverances of reason by "trying them out," as it were,
for oneself (Hackett 2015: sect. 5.4.3). Bacon's rationale for
urging this kind of testing is that arguments alone are insufficient
to convince us of the truth of any practical matter.
"Reasoning," he writes,



> 
> 
> draws a conclusion and forces us to grant it, but it does not confirm
> it nor remove doubt so that the mind may rest in the knowledge of the
> truth, unless this is found by way of experience. ... For if a
> man ... should prove by way of adequate arguments that fire burns
> and harms and destroys things, still the mind of the hearer would not
> rest on this account nor would such a person avoid fire before he had
> placed his hand or some combustible material in fire, so that he might
> test by experience [*ut per experientiam probaret*] what the
> argument had taught. (Bacon *OM* 6.1)
> 
> 
> 



It follows (in a phrase for which Bacon has become famous) that
"without experience nothing can be sufficiently known"
(*sine experientia nihil sufficienter sciri potest*)
(*OM* 6.1).



A second form of justificatory empiricism holds that some beliefs can
be defended *only* by reference to experience, there being no
arguments from prior principles that can be adduced in their support.
These include, first of all, matters within the domain of the other
sciences which mere reasoning could never have revealed, such as the
practical discoveries made by alchemists (Bacon *OM* 6, 2a
prae.). But there are also matters that fall outside the domain of any
speculative science, fields of inquiry in which *scientia
experimentalis* "by its own power investigates the secrets
of nature" (Bacon *OM* 6, 3a prae.). To appreciate what
this kind of investigation entailed, we need to understand the class
of phenomena to which it applied: that of the secrets of nature.



The domain of natural philosophy was "the common course of
nature" (Biard 2001: 79, 80): it dealt with what occurred (in
Aristotle's phrase) "either always or for the most
part" (*e tou aei e tou hos epi to polu*)
(*Met.* 1027a21-22). But there existed phenomena that
fell outside this domain, namely miracles and marvels. Miracles, in
the strict sense, were supernatural occurrences, which fell in the
domain of theology. But marvellous phenomena were often thought of as
"preternatural," that is to say, as brought about by
occult but natural properties and powers (Daston 1991: 95-100).
Such powers included the influence exercised by the heavenly bodies on
earthly affairs, as well as mysterious properties of sublunary
entities, such as the lodestone. These powers were occult in the sense
of being hidden. But they were also mysterious in that their workings
were not explicable in terms of what Aristotelians believed to be the
four simple substances (earth, air, fire, and water) and their
associated primary qualities (the hot and the cold, the moist and the
dry) (Pasnau 2011: 44). It is for this reason that they could not be
proved to exist by demonstrative arguments, but could be known only by
experience. This gave rise to a particular form of empiricism
associated with the study of "natural magic."



An example can be found in the work of an author already mentioned:
Nicholas of Poland, the radical medical empiricist of Montpellier. In
his *Antipocras*, Nicholas claims that although the presence of
a divinely given power in the most humble of objects cannot be proven
by reason, it "is made evident by its effect" (*hoc
patet effectu*) (Nicholas of Poland *Ant.* ll.
202-3). In this respect Nicholas seems to have been influenced
by another work more closely associated with magic, the *De
mirabilibus mundi*. This states that there are "certain
effects manifest to the senses to which we can assign no
reason," as well as effects known only by way of reason. The
marvellous powers of natural objects belong to the former category,
which are known by way of practical experience (*per
experientiam*) (Ps-Albertus *PA-DM* para. 49-50).
While this work was falsely attributed to Albert the Great (ca.
1200-1280), Albert himself adopts a similar line when defending
the mysterious powers of certain stones: these, too, are known
"most convincingly by experience," although they lack a
scientific explanation (*A-DM* 2.1.1).



When medieval writers speak of knowing matters *per
experientiam* (by experience), they are understanding the term
*experientia* very broadly. The same may be said of the term
*experimentalis*, so that the phrase *scientia
experimentalis* should be translated as "experiential
knowledge" rather than "experimental science."
Although Bacon's *scientia experimentalis* did include
practical procedures (*experimenta*), as were used in alchemy,
as well as the use of instruments (as in the study of optics and
astronomy), it also included simple observation as well as reliance on
second-hand reports of dubious value. (We are still a long way from
the motto of the seventeenth-century Royal Society: *nullius in
verba*, "take no man's word for it.") Nor is
there any consistent link with quantification. In only a few fields,
such as astronomy, are the procedures employed by natural philosophers
associated with careful measurement.



In the century following Bacon's work, we do find the
development of what we may call "quantified concepts,"
particularly in the study of local motion. We find this, in
particular, in the work of the Oxford *calculatores* associated
with Merton College, who are perhaps best known for their formulation
of the "mean speed theorem." This was the idea that



> 
> 
> a uniform acceleration produces the same ... distance travelled
> in a given time, as a uniform velocity equal to the instantaneous
> velocity at the middle instance of the time of acceleration. (Crombie
> 1961: 153)
> 
> 
> 



But while such developments mark a break with the Aristotelian
tradition of a purely qualitative physics, they remain largely
theoretical. While the discussions giving rise to the mean speed
theorem emerged from everyday observations of falling bodies, the
theorem itself was proven arithmetically and geometrically (Clagett
1959: 256, 259-60). The principles developed were not
"operationalized," that is to say, expressed in forms that
would allow for experimental testing (Clagett 1959: 206). Their
experimental testing had to await Galileo's inclined plane
experiments (Crombie 1961: 153-54).



This broad understanding of experience is evident, too, in medieval
conceptions of the origin and scope of perceptual knowledge. With
regard to the origins of such knowledge, later empiricists would focus
almost exclusively on the five "exterior senses." But
medieval authors, following Avicenna (Ibn Sina;
980-1037), commonly list up to five "interior
senses" (Bates 2010: 138-39) alongside the exterior ones.
Aquinas reduces these to four: the common sense, the imagination, the
*vis aestimativa* (or *cogitativa*), and the sense
memory (Aquinas *ST* 1a 78.4). The work of these interior
senses involves an active role in shaping our perceptions, even those
of our physical environment. The mind, in other words, is not entirely
passive, even at its basic levels of operation. The *vis
aestimativa*, for instance, refers to the power that even
non-rational creatures have of making judgements about what is
dangerous or useful: it shapes our perceptions by adding an evaluative
component.



With regard to the scope of experience, this was generally thought to
include knowledge of our own interior acts. That we have some kind of
immediate access to such knowledge was "one of the truisms of
Augustinian thought" (Leff 1975: 28). Its mechanism was
unclear--Scotus says simply that we know these things "by
way of a certain sense, that is, an interior perception"
(*quodam sensu, id est, perceptione interiore*) (Scotus
*Ord.* IV d. 43, q. 2)--but its existence was not in
doubt. As an anonymous scribe wrote in the margin of one of
Ockham's manuscripts, the idea that "there is nothing in
the intellect unless it was formerly in the [exterior] senses"
applies to "external objects, not to its own acts" (Leff
1975: 28 n.116). In Bacon's work, this focus on inner experience
is taken a step further, insofar as his conception of experience
embraces not only our everyday mental states, but also spiritual
states up to and including "raptures" in which people have
visions of things "of which it is not lawful for human beings to
speak" (Bacon *OM* 6.2). These are experiences brought
about by the grace of God, rather than any purely natural process of
cognition.



I have been arguing that medieval thought included a qualified form of
genetic empiricism (to which I shall return) and a distinctive form of
justificatory empiricism, coupled with a broad understanding of
"experience." What about explanatory empiricism? Insofar
as medieval natural philosophy was influenced by Aristotle it shared
his view of scientific explanations. For the most part these did not
invoke unobservable entities that are distinct from the
*explanandum*, but generative powers that stem from very nature
of the object being investigated
 (sect. 3.2).
 Medieval thinkers did not deny that we could know about unobservable
entities. God, after all, was an unobservable entity, and thinkers
such as Aquinas thought that "natural reason" could know
something about God, albeit by way of analogy. (William of Ockham [ca.
1285-1347] would later downplay the extent of such knowledge,
without dismissing it altogether [Leff 1975: 382-98].) But there
were limits to what we could know about the unobservable features of
the natural world (Hutchison 1982: 235-36). Aquinas, for
instance, appears to endorse Aristotle's view that when it comes
to matters that are "not manifest to the senses"
(*immanifesta sensui*) we should not demand "a certain
and necessary demonstration"; a merely possible explanation may
be the best we can achieve (Aquinas *In Meteor.* 1.7 [lect.
11]). One could regard this as a very modest kind of explanatory
empiricism.



One late medieval writer did embrace a stronger version of explanatory
empiricism. This was Nicholas of Autrecourt (1299-1369).
Nicholas's central idea is that (outside of the articles of
faith) we can have certain knowledge only of propositions whose denial
would result in a contradiction. But since we can, without
contradiction, affirm the existence of something while denying the
existence of that which is thought to be its cause, we can have no
certain knowledge when it comes to causation. In a similar way, we
cannot infer the existence of an underlying substance from appearances
of things (Thijssen 1987: 243-44). More generally, Nicholas
writes, "from the fact that something is known to be, it cannot
be inferred evidently ... that there is some other thing"
(Letter to Bernard 2, SS11 [p. 65]). These observations amount to
a vigorous attack on the Aristotelian ideal of scientific
understanding: the idea that we can (at least in principle) have
certain knowledge of the essences of things (Stiefel 1992: 299).



Nicholas remains an empiricist rather than a thorough-going sceptic,
for he believes that we cannot, without contradiction, deny what
appears to be the case. I cannot, for example, affirm both that a
"white patch appears to me and that it does not appear to
me" (Copleston 1974: 253). The problem is that this certainty is
limited to appearances; it does not extend to the things themselves.
To escape from scepticism, Nicholas has to revert to merely probable
knowledge. As he writes "if we have any certainty about things,
it is probable that everything that appears to be is (*omne illud
quod apparet esse sit*), and that everything that appears to be
true is true (*omne illud quod apparet esse verum sit
verum*)" (*EO* p. 228). This merely probable
knowledge is itself conditional: it holds on the condition that
"we have any certainty about things." What makes Nicholas
confident this condition is fulfilled is his firm conviction regarding
"the goodness and purposiveness of things" (Weinberg 1948:
127), a view that reminds us of the Stoics
 (sect. 3.3.4
 Nicholas believes that "since falsehood is an evil of the
intellect, the complete goodness of the universe would have to be
denied if the human desire to know were eternally thwarted"
(Weinberg 1948: 193). But this principle of goodness is itself a
matter of merely probable knowledge (Weinberg 1948: 127-28).


### 4.2 The Eclipse of Divine Illumination



I have already noted a certain ambivalence among medievals writers
regarding what I have called genetic empiricism, insofar as they
combine the idea of a 'blank slate' with that of an innate
capacity of the mind to form ideas. Some medievals, as we saw, went
further, holding that the mind was incapable of attaining scientific
knowledge without a divine illumination. It is time to look more
closely at this idea of divine illumination: the reasons why it was
held and its gradual decline.



Followers of St Augustine shared Plato's doubts about the
capacity of sense perception, in particular about its ability to give
rise to scientific knowledge. Such knowledge, they held, did not arise
from sense perception alone; it required a further input, provided by
God himself: a divine illumination. This can be a difficult idea for a
modern reader to grasp, since it is easily confused with another
belief, which has to do with a supernatural source of knowledge
accessed by faith. The two are analogous, insofar as they both involve
a divine initiative. The act of faith requires a divine initiative
"on the volitional side," to enable us to accept
propositions as divinely revealed. The doctrine of divine illumination
also speaks of a divine initiative, but that initiative is directly
cognitive rather than volitional (Pasnau 2015: sect. 1). It holds that
divine illumination is required for a number of apparently ordinary
cognitive activities, which are engaged in by believers and
non-believers alike.



While this doctrine has theological roots, it was also motivated by
two features of human knowledge. The first has to do with those
principles of thought that appear to be necessarily, not contingently,
true. These include mathematical truths and principles such as that of
non-contradiction or sufficient reason. While such principles are
essential to thought, it is difficult to see how they could be derived
from sense perception. (Aristotle had provided an empiricist account
of the origin of mathematical ideas
 [sect. 3.2],
 but this was, and remains, contested.) A second motivation was
related to the Aristotelian ideal of science. Scientific understanding
(*episteme*, or *scientia*), as we have seen,
involved a grasp of the essential properties of perceived objects. But
this, too, involves a kind of insight that can seem to lie beyond the
capacity of the senses
 (sect. 3.2).
 Advocates of divine illumination argued that these kinds of knowledge
were possible only because the human mind has access to the mind of
God. More precisely, it was possible only because God bathed the human
mind in his own, uncreated light. This allowed it not only to grasp
necessary truths, but to understand the essences of created beings by
reference to their exemplars in the divine mind.



The development of medieval empiricism involved a rejection of this
doctrine. It is already opposed by Aquinas, who holds that that the
human mind requires a special divine illumination only for those
matters that entirely exceed its natural powers, which are known by
divine revelation (Aquinas *ST* 1a 2ae 109.1). But the most
vigorous rejection of the doctrine is to be found in the work of Duns
Scotus, who opposes the (modified) illuminationist teaching found in
the work of Henry of Ghent (ca. 1217-1293).



Henry's version of the illuminationist doctrine was already
influenced by Aristotelian thought (Marrone 1985: 145; 2011: 24),
which in this context took the form of a desire to attribute as much
as possible to the power of the human mind. But Henry was reluctant to
attribute all we know to our unaided powers. In particular, he made a
distinction between knowing what is true (*id quod verum est*)
about an object and knowing its truth (*eius veritatem*). The
first is a very basic level of knowledge, existing even in non-human
animals. It does grasp its object as a something distinct from the
knower, but fails to grasp its essential characteristics. Such
knowledge is not sufficient for science, which requires a grasp of
what the object truly is (*quid sit in rei veritate*) (Henry of
Ghent *Summa* 1.2). The latter kind of knowledge requires
access to the exemplar of the object in the divine mind, something
only God can give.



It is this idea that Duns Scotus opposes. He would not, of course,
have opposed the idea that the human mind was created by God and thus
owes all its powers to God. (Scotus was, after all, a theologian.) But
he did deny that scientific knowledge involved a special divine
illumination. His argument proceeds in two stages. He insists, first
of all, that Henry's restrictions on the powers of the human
mind, rather than leading to scientific certainty, lead inevitably to
scepticism. If the human mind is naturally incapable of grasping these
truths, then no amount of divine light will help it. He then argues
that there are four kinds of naturally attainable knowledge that give
rise to the certainty required for science. There is, first of all,
knowledge of "matters knowable in an unqualified sense,"
such as the fact that a triangle has three angles equal to two right
angles. The second is knowledge of "matters knowable through
experience," such as that the moon is eclipsed. The third is
knowledge of "our own actions," such as the fact that I am
awake. The fourth is knowledge of "matters we are currently
aware of through the senses," such as that the object in front
of me is white (Scotus *Ord.* I dist. 3, pars 1, qu. 4,
adnot.).



Perhaps the most critical of these is knowledge through experience,
for it relates to the way we grasp the principles of scientific
explanation. Scotus's suggestion is that we can know these
principles through repeated exposure to particular instances. As he
writes,



> 
> 
> even though a person does not experience every single individual, but
> only a great many, nor does he experience them at all times, but only
> frequently, still be knows infallibly that it is always this way and
> holds for all instances. He knows this in virtue of this proposition
> reposing in his soul: "Whatever occurs in a great many instances
> by a cause that is not free, is the natural effect of that
> cause." (Scotus *Ord.* I dist. 3, pars 1, qu. 4)
> 
> 
> 



Scotus seems to regard this proposition as self-evidently true. But as
Nicholas of Autrecourt would point out, it merely begs the question
(Thijssen 1987: 249). In any case, Nicholas writes, if Scotus means by
a cause "that which brought an effect in a great many instances
in the past and will do so in the future," then there is a
problem. For "even if something has brought about an effect in a
great many instances, it is still not certain it will continue to do
so" (*EO* p. 237; Thijssen 1987: 249 n. 44). So we
cannot, with certainty, form general statements on the basis of the
kind of observations to which Scotus appeals. (This is, of course, the
very problem of induction to which Sextus Empiricus had referred
 [sect. 3.3.4].)
 Even if we could formulate such general statements, we could not be
certain that they embody causal claims. It is true that at least one
contemporary, John Buridan, would weaken the traditional Aristotelian
idea that a scientific explanation should make its conclusion
necessary (Grant 2010: 237). But the issue Nicholas raised would
remain a challenge to even Buridan's more relaxed ideal of
scientific knowledge.



We shall come back to this question
 (sect. 4.3),
 which was made more pressing by the nominalism of Scotus's
successor, William of Ockham. But even if Scotus had no satisfactory
alternative to the doctrine of divine illumination, that doctrine was
soon eclipsed. By the mid-fourteenth century, it seems to have been no
longer considered a viable option. While the idea did not entirely
disappear--it re-emerges, for instance, in the work of Nicolas
Malebranche (1638-1715)--its rejection by late medieval
thinkers marked an important step on the road to a more thorough-going
empiricism.


### 4.3 A Cognitive Empiricism



I shall conclude this discussion by returning to the strong form of
genetic empiricism that I have called "cognitive
empiricism": the idea that the mind has no access to sources of
knowledge that are independent of the senses. It is clear that
advocates of the doctrine of divine illumination would not count as
cognitive empiricists. But what about those who rejected this
doctrine? One might question whether even they were cognitive
empiricists, for the same reason one might ask this question of
Aristotle. While recognizing that all factual knowledge dervives from
the senses, they attributed to the mind a remarkable ability to grasp
the universals embedded in the singular objects of sense perception.
What we see in the work of William of Ockham is a downgrading of this
power, which goes hand-in-hand with a rethinking of the idea of
universals. Ockham's work thus laid the foundations on which
early modern empiricists would build.



The power of the mind to grasp universals was traditionally attributed
to what was called the "agent intellect," the intellect as
active or productive. The idea of the agent (or active) intellect is
one of the most complex and contested aspects of medieval
epistemology. It has its origins in some cryptic remarks made by
Aristotle (*DA* 430a10-19), in which he distinguishes
between two aspects of the mind. There is the mind as "that
which becomes all things" (*nous toi panta
ginesthai*) and there is the mind as "that which makes all
things" (*ho ... toi panta poiein*). Behind
these descriptions is the idea that the mind "becomes" the
object of its knowledge, in the sense of taking on its intelligible
form without the matter in which it was originally embodied. The mind
as passive--as that which becomes all things--stands in
relation to the mind that makes all things in the same way that prime
matter, which is pure potential, stands in relation to its shaping
principle. Aristotle describes the mind as active or
productive--the *nous poietikos*, to use a phrase
coined by his commentators--in terms that suggest something
immaterial: it is "separable, unmixed, and impassible."
This may be because it is not so much a substance, as an operation or
function; indeed Aristotle describes the *nous poietikos*
as "by nature activity" (*tei ousiai ...
energeia*).



As that last interpretive remark suggests, this passage demands
commentary and commentators disagreed about how it was to be
understood. (For a classic survey, see Brentano 1992.) Broadly
speaking, there were two views. The first held that the agent
intellect was separate from each human mind, such that the
intelligible forms "flow" from this immaterial entity into
the mind to produce an act of understanding. This was the view held,
in two quite different forms, by Ibn Sina (Avicenna) and Ibn
Rushd (Averroes) (1126-1198). Such a view implies that
there is only one agent intellect, common to all human beings, which
may or may not be identified with God himself. (When it is identified
with God, this view closely resembles the Augustinian doctrine of
divine illumination.) The alternative idea, vigorously defended by
Aquinas, held that both the agent intellect and the potential
intellect are aspects of the human mind. Even among those who held
that latter view, there were different ways in which the agent
intellect was thought to produce knowledge. In some versions of the
doctrine (such as Aquinas's) the agent intellect was thought to
abstract the intelligible forms from the images produced by the
senses. In other versions, the input of the senses was little more
than the occasion on which the agent intellect drew on its own innate
resources, in the process of which the mind could be thought of as
enlightened by God. Thomas of York (d. ca. 1260), for instance spoke
openly in this context of innate ideas and of a knowledge that came
"from above and not from the senses" (Bowman 1973:
265).



The second of these views, in particular, seems incompatible with both
a *tabula rasa* and a cognitive empiricism. So it is not
surprising that the development of a more thoroughgoing cognitive
empiricism went hand-in-hand with with an effective abandonment of the
idea of the agent intellect.



The work of the agent intellect had seemed essential because of a
widely held view regarding the object of knowledge. This held that
while the senses deal with particular individuals, the intellect can
grasp only their universal properties. As Aquinas writes, citing
Aristotle (*APo.* 87b37-39), "sense has to do with
particulars, whereas intellect has to do with universals"
(*sensus particularium, intellectus ... universalium*)
(Aquinas *In DA* 2.5 [sect. 284]). (The idea that the intellect
has direct knowledge only of universals seems distinctively
Aristotelian; it was not shared by other ancient philosophical schools
[Normore 2007: 114].) Aquinas's reason for holding this view is
that the intellect, as an immaterial substance, cannot directly grasp
material singulars. Intellectual knowledge therefore requires the work
of the agent intellect, abstracting the immaterial intelligible forms
from the perceptible forms grasped by the senses. We do, of course,
know singular objects, for all the objects of sensation are,
ultimately, singulars. But in Aquinas's view, the intellect
knows these objects only indirectly, by directing its attention back
to the images yielded by the senses and linking these with the
associated general concept (Aquinas *ST* 1a 86.1).



William of Ockham, on the other hand, holds what Aquinas expressly
denies: that "the singular object is that which is first known,
in a manner simple and proper to itself" (Ockham *Quodl.*
I qu. 13). He holds, too, that the immateriality of the intellect is
no barrier to this, since the relation involved is a merely
intentional one (Ockham *Ord.* I dist. 3, qu. 6; *In
Phys.* 1.1 [sect. 2]). Ockham, however, goes further in denying
the very existence of universals. All Aristotelians, of course, denied
that universals exist *extra rem* (distinct from singular
objects). But Christian Aristotelians such as Aquinas continued to
regard the similarities between individuals of a given kind as the
product of a common idea, which existed *ante rem* (before the
existence of particular objects) in the mind of God (Aquinas
*ST* 1a 15.2). As aspects of the divine mind, these were
thought to share in the divine nature. Indeed they were nothing less
than "the divine essence as imitable outside itself"
(Klocker 1980: 357). This gave the natural kinds discoverable by
science a metaphysical grounding. While paying lip service to the
Augustinian notion of ideas in the divine mind, Ockham reduced these
to the ideas God has of individual objects in the act of creating
them. These ideas refer, in the first instance, to the creatures
themselves: the only reason for positing them is that when God creates
he knows what he is doing (*Deus est rationabiliter operans*)
(Ockham *Ord.* I dist. 35, qu. 5). It follows, as Ockham
writes, that



> 
> 
> no universal ... exists in any way outside the mind and anything
> that can be predicated of many is by its nature in the mind either
> psychologically [*subiective*] or logically
> [*obiective*], and ... no universal is of the essence or
> nature [*de essentia seu quidditate*] of any substance. (Ockham
> *Ord.* I dist. 2, qu. 8)
> 
> 
> 



This means that the features that unite entities of a kind, which are
expressed in the general propositions of science, no longer have a
metaphysical foundation. While this view is generally described as
"nominalist," it is more accurately characterized as
"conceptualist," since the "names" to which
universals were reduced were *nomina mentalia* (mental names),
or (as we would say) concepts (Maurer 1999: 63-64). But with
this qualification in mind, I shall continue to refer to it as
"nominalist."



What implications does this have for the idea of the agent intellect?
Despite his nominalism, Ockham continues to believe that there are
real similarities lying behind our classificatory schemes, rejecting
the idea that these are entirely matters of convention (Ockham
*Ord.* I dist. 2 qu. 8). But although real similarities exist,
they are not expressions of common essences or natures. Coupled with
the idea that the mind has an immediate grasp of singular objects, it
follows that a special faculty was not longer required in order to
abstract the universal from the particular. It was enough for the mind
to be able to identify similarities among particular individuals.
(Note that the status of these similarities then became a problem,
which Ockham may never satisfactorily have resolved [Goddu 1984:
92-93].) So it comes as no surprise to discover that Ockham
denies any real distinction between the agent intellect and the
potential intellect. "The same intellect," he writes,
simply "has different names" (Ockham *Ord.* I dist.
3, qu. 6). Since there was now no distinctive task for the agent
intellect to perform, Ockham could wield his famous razor to abandon
the idea that it represented a distinct power of the mind (Ockham
*Rep.* II qu. 20).



There is another dimension to the empiricism of this period, which is
a new emphasis on experience as a means of discovering causal
relations. Any Aristotelian, of course, would have agreed that we know
of causal relations by means of experience. But what we find in the
work of Scotus and Ockham is a particular focus on this idea, coupled
with descriptions of how one may isolate particular causes. Historians
have long associated this with a renewed emphasis on God's
absolute power and the idea that the order of the world depends
entirely on his will, which makes that order radically contingent and
discoverable only *a posteriori* (Foster 1934: 464-65;
Henry 2009: 88). It is, however, difficult to find a contemporary
expression of this idea and it has recently been contested (Harrison
2002: 63-64). Nor should one overstate the degree of novelty in
the thought of Scotus and Ockham. Grosseteste had already outlined the
idea of a controlled experiment (Lewis 2013: sect. 11), which itself
had been anticipated in the work of Galen
 (sect. 3.3.1).
 Interestingly, Grosseteste's example had also been a medical
one: he notes that we can come to know that scammony (*convulvulus
scammonia*) purges red bile by administering it "after all
other causes purging red bile have been isolated and excluded"
(Grosseteste *In APo.* 1.15; Crombie 1953: 74). It is true,
however, that more detailed accounts of how to isolate causal
relations are found only in the late thirteenth and fourteenth
centuries. Scotus outlined several such methods (Vos 2006:
312-19), all of which involved separating out possible causes
and testing them by seeing in which cases the effect follows. At one
point, for example, Scotus writes:



> 
> 
> How, from perceptible effects, does one arrive at knowledge of the
> cause? I respond, by dividing thus: in [a situation of type] A are
> present [factors] B, C, D, if you wish to know which of D, B, or C is
> a cause, consider the case where you find B without C. If then D
> follows B and not C, then in [a situation of type] A, B is the cause
> of D. (Scotus *In Met.* I qu. 4)
> 
> 
> 



Ockham picks up on this idea. He does not deny that we can know of
some causes by simple reasoning, on the basis of self-evident or
well-established metaphysical principles. We can know, for instance,
that the will causes its own acts (Ockham *Ord.* I dist. 1, qu.
3) or that there exists a first conserving cause of this world (Ockham
*Ord.* I dist. 2, qu. 10]). But when it comes to the discovery
of causes within the natural world, we must rely on observation. If a
causal agent is of a very specific kind (in Ockham's terms,
known by way of its *species specialissima*), just one
observation will suffice. A single observation of a particular kind of
herb curing a particular illness, for example, can safely be
generalized. It can be generalized on the basis of the principle that
"all agents of the most specific kind will bring about effects
of the same nature" (Ockham *Ord.* I prol. qu. 2). If the
suspected cause is of a more generic kind, then many instances will be
needed (Ockham *Ord.* I prol. qu. 2). But in either case, the
process is thoroughly empirical: it involves the isolation of causal
factors by way of experience. As Ockham writes,



> 
> 
> although I do not intend to make any general claim about the nature of
> immediate [secondary] causes, I shall say what suffices for something
> to be an immediate cause, namely that when it is present there occurs
> the effect, and when it is not present ... the effect does not
> occur. And everything that has this character with respect to another
> is the cause of that other ... That this is sufficient for one
> thing to be the cause of another, seems manifest. Because if not,
> there remains no way of knowing something to be the cause of something
> else. (Ockham *Ord.* I dist. 45, qu. 1)
> 
> 
> 



It is tempting to see this as a reduction of the idea of causation to
that of the regular association of kinds of events. One could even
argue that such a view would be a logical consequence of Ockham's
nominalism (Klocker 1980: 351). But Ockham's point here seems to
be epistemological, not metaphysical: this is how we discover a cause
(Adams 1979: 23). It follows that the only kind of cause we can
discover is that which produces a novel (and therefore observable)
effect. Without such an effect, there would (once again) be no way of
knowing something to be a cause (Ockham *Sum.* II 3). Coupled
with the effective abandonment of the idea of the agent intellect,
this brings us very close to a thoroughgoing cognitive empiricism.


## 5. Conclusion



Those sympathetic to empiricism may be inclined to portray this
history as a progress from darkness to light. But a moment's
reflection suggests it deserves a more cautious evaluation. These
ancient and medieval thinkers certainly laid the foundations on which
early modern and modern empiricists would build. But they also
bequeathed a series of problems, some of which have never been
satisfactorily resolved.



There are, first of all, the problems faced by the Epicureans, who
were genetic but not explanatory empiricists, favouring the atomist
theory of Leucippus and Democritus. As Democritus seems to have
realized, while the atomist doctrine can be supported by observable
facts (such as the fact of motion), once formulated it seems to
undercut a belief in the reliability of such observations. At the very
least, it suggests a distinction between what would later be called
"primary" and "secondary" qualities, narrowing
the range of the kind of experiences to which natural philosophers
would appeal.



The Epicureans also seem to have come up against another enduring
problem: that of the underdetermination of theory by data. If the
senses cannot directly "bear witness" to the existence of
what cannot be observed, being able only to bear witness against it,
there may be a number of apparently equally acceptable hypotheses
about the unobservable. One could argue, as Epicureans seem to have
done, that these should all be regarded as true, in the sense of being
realized somewhere and at some time. But this seems unsatisfactory
when it comes to explaining particular events.



More broadly, there is the problem of just what experience can show.
Can experience be a source of mathematical or logical knowledge? Can
it reveal the causal relations that are traditionally regarded as the
goal of science? Aristotle, as we saw, seems to have held that the
mind has the power of identifying the causal powers that belong to
particular kinds of entities. But it was never clear that experience
alone could achieve such an insight. Medieval discussions of the
"agent intellect" tended to regard it as an external
power, or, at least, a human faculty that participates in a divine
light. Thinkers such as Henry of Ghent went further, embracing the
Augustinian idea that an act of divine illumination is required on
each occasion we attain such insights. Under the impact of
Aristotelian naturalism this idea was abandoned, but without (it must
be said) any clear alternative.



Later empiricists could (and did) follow Ockham in denying that there
are universals to be discovered; they professed to be undisturbed by
the fact that we cannot grasp the "real essences" of the
objects we perceive (Locke *Essay* 3.6.9). But the problems of
empiricism did not go away. Even one of the empiricist's
favorite ideas--the "covering law" account of
scientific explanation--assumes we can arrive at true
generalizations, propositions that apply to all members of a
particular class. But for those who take Ockham's nominalism to
its logical conclusion, a law of nature can be nothing more than a
contingently true proposition. Since the similarities that fix class
membership have no metaphysical grounding, not being based on a shared
nature, we cannot know the truth of the law by any kind of rational
insight. The only way of testing such a proposition is by observation.
It is at this point that there arises the "problem of
induction," first discussed by Sextus Empiricus and picked up by
Nicholas of Autrecourt, but made famous in the eighteenth century by
David Hume.



In our own time, the wheel has turned. On the one hand, talk of
"essences" and necessary truths has become fashionable
again, at least in some circles (Haldane 2018). In particular, if we
want to distinguish laws of nature from accidental generalizations,
contingently true propositions do not seem sufficient for science.
Laws of nature on this view must embody modal knowledge, particularly
as this relates to contrary-to-fact conditionals (Armstrong 1978:
274). This brings us back to Aristotle, for his conception of
scientific knowledge (*episteme*) involved knowing
that given a posited cause, the fact to be explained cannot be
otherwise (*APo*. 71b11). The problem for the empiricist is:
How can sense perception yield modal knowledge of this kind? On the
other hand, William of Ockham's nominalism can be thought to
anticipate the work of contemporary thinkers who would "lower
the bar" (as it were), no longer demanding that we gain
knowledge of what is necessarily the case. Taken to its logical
conclusion, Ockham's view can be thought to hold that the
"universals" that were the goal of Aristotelian science
are no more than "sets of competencies in classifying things
together", which have no grounding outside the practices in
which we engage (King 2003: 225). (For a less sceptical reading of
Ockham, see Colvert 2000: 265.) Is a radical nominalism of this kind
tenable? The question has not yet been answered.



This article began with the observation that we should not approach
ancient thinkers in a "Whiggish" fashion; nor should we
force their thought into modern categories. But what this history
shows is that there is no need to do so. Once we understand what these
thinkers were saying, we can see that we are still grappling with the
issues they raised.