## 1. Causality and Motion




The term 'motion', in Aristotelian philosophy, can stand
for a wide range of changes of state, and not simply changes of place
(the latter is usually known as *local* motion). Aristotle's
*Physics* is basically an exhaustive study of motion in this
very wide sense. In this section, we will first deal with motion in general,
and then with local motion. 


### 1.1 Motion in General



There is a large body of medieval literature which deals with general
change of state: it is important because it led to a great deal of
careful analysis of continuous change (under the heading of "intension
and remission of forms"), and this body of analysis was one of the
contributing currents which led to the invention of the calculus in
the sixteenth century. So, we can look at this literature from two
viewpoints: either from the tradition which it grew out of -
that of interpretations on Aristotle's *Physics* and the
picture of change found there - or from the viewpoint of what it
led to, that is, the analysis of continuous change. Either viewpoint
can be regarded as doing violence to the historical material, either
by disregarding what was new in it, or by isolating the innovation
from the tradition.



First we can look at the starting point. There
was a great deal of argument, in the thirteenth
and fourteenth century, about the precise interpretation of 
Aristotle's theory of change: this theory talks about the bringing
about of change, by an agent, in a patient, and, furthermore, 
it talks about the change happening because of the transmission of a form 
from the agent to the patient. The key text for this is 
Aristotle, *Physics* III, cc2-3 (202a9-20), and it is very obscure. 
Because of this obscurity, there is room for a good deal of disagreement
about the details: see (Lowe 2017) for a particularly baroque example. 



Next we can look at study of intension and remission of forms: (Goubier and
Roques 2017) is a good recent collection of work on this, Sylla (1982) shows
how this work was related to, and to some extent originated in, disputations 
about logic, and Dumont (2009) 
shows how these debates were related to their theological context. We must not 
forget, though, that modern scholarship on this topic
goes back to at least Anneliese Maier (1964), whose work goes back to the
1930s, and that she was working in the tradition of Pierre Duhem. So there is
a vast body of work on this topic, written by people with a variety
of agendas. Despite this, it is a very interesting body of literature, 
and, if it might seem impossibly complex, it is thus because it reflects
the variety of medieval and early modern viewpoints on this material. 



Motions are, in Aristotle's physics, classified into
*natural* and *violent*. A paradigmatic example of
natural (local) motion is the motion of a freely falling body, whereas
an example of violent (local) motion would be the motion of a thrown
body. The motion of a falling body is natural because
a heavy body has a natural tendency to move
downwards, or, as the thirteenth-century philosopher Peter of
Auvergne puts it, "being below is the principal end
of a heavy body" (Trifolgi 2015, p. 93).


### 1.2 Local Motion



Now if we throw a body, then it is, in this framework, relatively
unproblematic to account for the motion when it is in contact with our
hand: what is difficult is to account for its continued motion
thereafter. Aristotle's theory accounts for it by saying that,
when it is moving, a temporary vacuum is caused behind it, and, in
order to fill in this vacuum, air rushes around from the front, thus
leaving a void in front of the projectile which is filled by the
continued motion of the projectile. This explanation was vulnerable to
a large number of objections - for example, it is clearly easier
to throw a moderately heavy object, such as a stone, than a light
object, such as a bean, whereas light objects ought to be more
susceptible than others to motions of the air. And Aristotle's
theory, when confronted with the example of two stones thrown in
opposite directions so as to pass near to one another, cannot
consistently say how the air is supposed to move in the neighbourhood
of their close encounter. These objections were made by numerous
medieval authors, most significantly by John Buridan (*De Caelo et
Mundo* III, qu. 22, pp. 227ff.) and Nicole Oresme (*Du ciel et
du monde* II, ch. 25ff., pp. 525ff.).



This critique of Aristotle's theory of projectile motion did not
come out of nowhere. Aristotle relied on a concept of natural motion,
and *that*, in turn, relied on a concept of natural place:
natural motion was motion towards the natural place of a body (i.e.
motion downwards in the case of earth, and motion upwards in the case
of fire). (Aristotle, *Physics* IV.5, 212b30-213a5) Ockham is
already quite equivocal about the concept of natural place: and this is
for several reasons.


1. One is that - as we shall see later - he is generally quite
suspicious about teleology, and the concept of natural place is
basically a teleological one. Correspondingly, Ockham attempts - not
very successfully - to explain the kinematics associated with natural
place in terms of *efficient* causality. (Ockham, *Expositio
Physicorum* IV, c6: *Opera Philosophica* V, p. 78; Goddu
1984, pp. 122ff.).
2. Another reason is because of several examples which tend to
undermine the difference between rest and motion. Ockham, and many
other medievals, have reductionist accounts of place in terms of
contact between bodies; the place of a body is just the surfaces of the
bodies surrounding it (Ockham, *Expositio Physicorum* IV, c6:
*Opera Philosophica* V, pp. 55ff.). So, if we have a ship in a
river, which is flowing, is the place of the ship the surface of the
surrounding water? Is this, then, a moving place? And what, then, would
be the relation between that moving place and fixed places? Ockham
eventually decides that there are only fixed places, but his arguments
are not very strong, and one is left with the impression that the very
ideas of rest and motion have become somewhat problematic. (Ockham,
*Expositio Physicorum* IV, c7: *Opera Philosophica* V,
pp. 79ff.; cf. Goddu 1999)
3. The final reason is motivated by a theological example: we can
suppose that God could create another world than this, but, in that
case, what would the earth of that world do? Would it move towards the
centre of *this* world (which seems to us to be the natural
place of earth)? Or towards the centre of the other world? (Ockham, I
*Sent.*, d. 44: *Opera Theologica* IV, pp. 655-56; Goddu
1984, p. 124. See also Marsilius of Inghen, *Si essent plures
mundi* (1992).)




Correspondingly, both Buridan and Oresme are sceptical, not only
about Aristotle's theory of projectile motion, but also about the
related notions of natural place, motion, and rest. They both state -
Oresme far more emphatically - that it would be consistent with all we
observe if the earth were to rotate while the heavens remained at rest;
Oresme and Buridan have, on these grounds, been described as
"precursors of Galileo".




However, what is more interesting for us are the alternative causal
accounts that Buridan and Oresme adopted: they both said that
projectiles move violently because of a form inherent in them, which
led them to move in a non-natural direction, and which naturally
decayed. This form was known as 'impetus', and was a common
theme in the philosophy of the thirteenth and fourteenth centuries;
some version of an impetus theory goes back to the early thirteenth
century (Wood 1992). There was, especially in the fourteenth century, a
considerable amount of quantitative work on impetus which attempted to
establish such things as the law according to which impetus decayed
(Weisheipl 1982, pp. 535ff.).




What is significant here is that - despite the radical changes
in cosmology - this is is still an extremely *medieval*
theory: causation is due to forms inhering in substances, and there is
a division of the substances involved into agents and
patients. Instead of there being a single form involved in projectile
motion - the form of heaviness, responsible for natural motion
downwards - there are two, weight and impetus, and the two
conflict. The basic ontology is still the same, and the division into
agents and patients, though its details may have changed, still
persists. Furthermore, despite persistent doubts, there is still
something of a distinction between motion and rest, and motion can
only be the result of agency. Contrast this with Galileo's or
- still more - Newton's account: here uniform motion
and rest are treated on an equal footing, and, consequently, there can
be no unequivocal distinction between motion and rest. So, although
Buridan and Oresme are - in some sense - precursors of
Galileo, their causal ontology is still, in important respects,
thoroughly medieval (Maier 1964).


## 2. Causality, Self-Motion, and the Will




An example of motion in the wider sense is an act of the will: it is
a change of state of some entity (namely the mind or soul), but would
not have been thought of as *local* motion by most medieval
thinkers - thought and will were generally regarded as immaterial
processes (see Cross 1999, p. 75).




Aristotle has a picture of willed action in which actions are caused
by combinations of beliefs and desires: these belief-desire states are
not, of course, themselves actions (Normore 1998). This picture of the
will fits with one of Aristotle's major causal doctrines: that nothing
causes a change in itself.




However, Aristotle's picture of the will was not undisputed in the
Middle Ages: as early as the twelfth century, Anselm had outlined a
theory in which the will was a self-mover, and in which moral conflict
was explained by the presence of two wills in the same person (Normore
1998, p. 28). This position was later taken up, in conscious opposition
to Aristotle, by thinkers in the Franciscan school - Peter Olivi, and
then Scotus and Ockham.




Scotus follows a modified Anselmian line, speaking of a single will,
with two inclinations: one towards self-fulfillment, the other towards
justice). It is the presence of these two inclinations which
distinguishes willed causes from natural causes: natural causes are
determined to perform their acts (unless impeded), whereas the will is
not thus determined (Scotus, *Metaphysics* IV, 9: in Scotus,
*On the Will and Morality*, pp. 136ff.; Lee 1998; Cross 1999,
pp. 84ff.). The will is thus self-determining, rather than determined
by its end, and so Scotus affirms self-motion in psychology. In fact,
he goes further, and admits self-motion in physical cases as well: for
example, a falling object is actively moving towards its goal, and its
motion is caused by itself (because it is heavy); so this, too, is an
instance of self-motion (Effler 1962).




Ockham expands on Scotus's theory of the will to deny that actions
are properly explained by their ends: we are *influenced* by
ends, but our actions are not *necessitated* by them and thus
not caused by them (Ockham, *Quodlibet* I, qu. 16: *Opera
Theologica* IX, pp. 87ff.). A free agent is one which, under
exactly the same circumstances, could have chosen otherwise; and so a
free agent can reject the Beatific Vision (and, in fact, actively turn
to any other object whatever). (Ockham, *Quodlibet* IV, q. 1:
*Opera Theologica* IX, pp. 292ff.).


## 3. Causal Accounts of Perception



Perception was, throughout the Middle Ages, a contentious topic, and
it was also a topic in which the answers to strictly *causal*
questions could influence philosophical positions in other areas (for
example, on whether certain knowledge of external entities was
attainable). The "traditional" view, dating back to Roger
Bacon in the mid-thirteenth century, was that physical objects were
known because they caused a succession of likenesses, or
*species*, first in the medium between the object and the perceiver,
then in the senses, and finally in the intellect, of the perceiver (Tachau
1988, pp. 3ff., Normore 2010, pp. 255f.). This position was attacked by
thinkers such as Henry of Ghent, Peter Olivi, and Duns Scotus. Interestingly,
many of these criticisms tend towards a relational account of perception, in
which - although *species* still play a role - the role that they play
is to be a means by which we know things, and in which the *species*
themselves are not known directly but only by reflection. (Tachau 1988, p. 66}.
(Panaccio 2010, pp. 242f., Normore 2010, pp. 255f.)



Ockham then radicalised these critiques by denying that there are any
such *species* at all: perception and other phenomena which
were usually explained by *species* - the sun heating or
illuminating physical objects, for example - were now explained by
action at a distance (Tachau 1988, pp. 130ff., Stump 1999). There was
a similar debate about the causal mechanisms behind memory, where,
again, Ockham denied a *species*-based account; however, in the
case of memory, he replaced species not with action at a distance but
with habits (Wolter and Adams 1993).



There was a similar late medieval debate on intentionality,
that is, the relation
between mental states and their objects. It is no surprise, given Ockham's
arguments about *species*, that he took an externalist line on these,
too: he poses the question by asking whether, if an angel saw the contents of
my mind when I was thinking about some singular object, could the angel
(supposing that it did not know about anything external to my mind) tell which
object I was thinking about? Ockham answered no: and this thought experiment
seems to show that my mental state (i.e.\ my thinking of this particular
object) is not determined by the contents of my mind alone (Panaccio 2010, pp.
242f., William of Ockham 1981, pp. 378f.). This is clearly *some sort
 of* externalist position, but describing this position exactly has proved
to be surprisingly difficult (Panaccio 2010, Normore 2010, Brower-Toland 2007).
Correspondingly although many medieval thinkers were sympathetic to arguments
of this sort, the details of the positions they adopted were by no means
uniform. For example, followers of Scotus, such as Francis of Meyronnes, held
positions on intentionality which were intricate, were relational in some
sense, but were not straightforwardly externalist (Cesalli 2010). Debate on 
these issues continues into our era, and has invited comparison with, for
example, Sellars (1960): Sellars' position, although interesting, does
not seem (as he claims) to reproduce Aquinas' position on 
intentionality (King 2010). This only shows that there are a very large
number of different accounts of intentionality, and that the
medievals, even though they were surprisingly inventive in discovering them,
did not manage to discover all of them. 




Ockham denies species not on the basis of empirical evidence, or on
the basis of epistemological arguments, but purely and simply on the
basis of his razor: if we deny *species*, then we can give an
account of the phenomena which uses fewer entities, because
*species* are entities. Although this position of Ockham's did
not have much influence on his contemporaries or followers - it is,
after all, extremely implausible - it is a good example of how causal
reasoning is affected by tacit ontological assumptions: the fact that
*species* were seen as entities and the fact that Ockham had a
programme of reducing the number of entities, led to an account of
perception which tried to do away with *species*. On the other
hand, action at a distance was, despite its implausibility, entirely
unaffected by Ockham's critique. And, similarly, Ockham's account was
not noticeably *simpler* than the accounts it criticised, which
shows how far Ockham's own razor was from the principles of simplicity
and the like, which are usually considered to be its modern
equivalents.



One should note that, although Ockham was very innovative, and altered the
landscape of medieval thought quite decisively, there were very few, if any,
people who adopted his position without modification: he produced a large
number of interesting arguments, but, on the basis of these arguments, he
adopted positions which many of his contemporaries were not convinced by (Rode
2016, Courtenay 2008). So, although his arguments forced a reexamination of
many philosophical and theological doctrines, the thinkers influenced by him
took up a broad spectrum of positions, depending on which of his doctrines they
assented to and which of them they rejected. Walter Chatton, for example,
denied Ockham's position on intuitive cognition - namely, that when
we know singulars we do so by a separate intellectual faculty distinct from
sense perception - in favour of a position that saw intuitive and sensory 
cognition as very much interrelated, and this, in turn, followed from a rejection
of Ockham's position that the sensitive and intellectual souls were distinct 
(Schierbaum 2016).


## 4. Causality, Knowledge and Necessity




There is a persistent supposition - see, for example, (Gilson
1937) - that Ockham, and many of his fourteenth-century
followers, had a basically Humean position on causality; this
supposition has deep historical roots (Nadler 1996), but is inaccurate
(Adams 1987, pp. 741ff.).




The supposedly Humean position has three basic assertions: that
there is nothing more to causality than the regular sequence of
phenomena, that such a regular sequence cannot give a necessary
connection, and that, consequently, we can have no certain knowledge of
causal relations.




One item in this chain of argument has some textual support in
Ockham: he did not believe that the relation of efficient causality was
a *thing* distinct from its *relata* (Ockham,
*Quodlibet* VI, qu. 12: *Opera Theologica* IX, pp.
629ff.) However, one can still believe this and hold that causality is
a real *relation*, and Ockham *did* so believe (Adams 1987, p.
744; White 1990b). So this link in the chain is not found in
Ockham.




The "Humean" argument, in addition, makes a detour through
psychology: as Adams analyses it, it relies on a premise like "There
can be nothing more in concepts than there actually is in intuitions"
(Adams 1987, p. 744). But such a detour through psychology, though
widely practiced in the eighteenth century, was somewhat foreign to
medieval thought (White 1990a). And one should generally be very
circumspect in interpreting medieval texts on issues like
these: key terms tend to be used in subtly different ways 
than in the modern literature, and controversies tend to be
about issues which are rather different from our controversies
(Zupko 2001).




Even though pseudo-Humean arguments of this sort cannot reasonably
be ascribed to Ockham or to most other medieval thinkers - with the
possible exception of Nicholas of Autrecourt - there still remains the
question of what their views on these questions actually were. Since
the medievals generally did not conflate ontological and
epistemological issues, there are two questions: first about the
necessity of causality, and second about whether we can know causal
propositions with certainty.


### 4.1 Causality and Necessity




Medieval thinkers believed that the world was created by God, and so
a question like "Is proposition P contingent?" were seen as equivalent
to the question "Could God have created a world in which P does not
hold?". So our question can be reduced to one about divine power.




A very common theme in medieval thought is the distinction between
God's absolute and ordered, or ordained, power (*potentia
absoluta* and *potentia ordinata*). This distinction goes
back to early medieval thought (Moonan 1994), and was extensively used
in later medieval philosophy (Courtenay 1971; Adams 1987, pp.
1186ff.).




God's absolute power is unrestricted power. According to this power,
God can create a huge variety of possible worlds. One frequently used
principle is this: given two distinct entities, God can create a world
in which one of them, but not the other, exists, or, in *this*
world, God can destroy one of them, leaving the other intact. We should
note that this is not exactly innocuous; ontologically, it amounts to
some sort of logical atomism. See (White 1990b).




But God will, in practice, not exercise absolute power: as Aquinas
puts it, "what is attributed to the divine power insofar as the command
of a just will executes it, God is said to be able to do with respect
to His ordered power". (Aquinas, *Summa theologiae* I, qu. 25,
a. 5, ad 1) So there are limits to God's ordained power (which come
from the concept of a just agent): inside the space of worlds which God
could create by absolute power, there is a space of worlds which could
be created by ordered power. It is this smaller space of worlds which
is relevant for our question of the necessity of causal connections.
And, with respect to God's ordered power, there was a wide range of
causal assertions which were regarded as necessary by medieval
thinkers.


One of the significant aspects of this distinction was that -
entirely apart from its theological motivation - it gave
medieval authors very powerful and flexible analytical tools. 
Buridan, for example, deploys this distinction in a very subtle
analysis of some extremely obscure arguments in Aristotle (Knuuttila 2001).
Scotus uses different, but related, arguments to investigate
modal questions such as that of the contingency of the
present. To some extent (although there is
considerable argument about this area) these methods allowed
a far-reaching reformulation of the metaphysics of modality 
(Normore 2003; cf. Knuuttila 1993). 



### 4.2 Knowing Causal Propositions: Demonstration




As far as our knowledge of causal propositions is concerned, we can
again draw a distinction. One question is this: do medieval thinkers,
in practice, establish causal propositions on the basis of argument?
And the other is this: what sort of metatheory of causal argument do
the medievals have?




The answer to the first question is quite straightforward. Ockham,
like other fourteenth-century theologians 
- see, for example, (Biard 2000) on Buridan - frequently gives instances
where we can make reliable causal inferences and come to know causal
propositions on the basis of experience (Ockham, *Ordinatio*
Prologue, qu. 2: *Opera Theologica* I, p. 87) These arguments
frequently rely on a theory of natural kinds: for example, Ockham
writes



> Because someone sees that, after eating such an herb,
> health follows for someone with a fever, and because he can eliminate
> all other causes of health for that person, he knows evidently that
> that herb was the cause of health; and thus he has knowledge
> (*experimentum*) in the singular case. It is, however, obvious
> to him that all individuals of the same kind have an effect of the same
> kind in a patient of the same kind; and thus he assents evidently, as
> to a principle, that every herb of such a kind cures fever. (Ockham,
> *Ordinatio* prologue, qu. 2: *Opera Theologica* I, p.
> 87)




The second question is that of a metatheory. Here the story becomes
somewhat more complicated. There was a generally accepted metatheory,
namely that of Aristotle's *Posterior Analytics*, according to
which scientific demonstrations were syllogistic proofs, based on
necessary and self-evident premises. There were two sorts of these:
proofs of the simple fact (*demonstrationes quia*) and proofs of
the reasoned fact (*demonstrationes propter quid*). In the
latter, the syllogisms involved must have middle terms that are
*causes* of the state of affairs which is to be demonstrated.
This gives a theory of scientific reasoning in which the structure of
the arguments is intimately tied up with the structure of the causal
chains that they demonstrate.




There is, indeed, an extensive literature of medieval commentaries
on the *Posterior Analytics*, and much of this literature is
very important; we find in it a great deal of material on the authors'
attitudes to necessity, the structure of science, the relation between
various sciences, the autonomy of philosophy *vis-a-vis*
theology, and the like. However, it cannot be taken to be automatically
relevant to the *practice* of reasoning in the Middle Ages: the
logical metatheory (that of the syllogism) is far too restrictive, and
the conditions placed on scientific demonstrations are far too
stringent, for it to be a plausible description of very many actual
processes of reasoning, in the Middle Ages or at any other time.


## 5. Final Causes




We often find in Aristotle and in the literature influenced by him
an enumeration of four types of cause: formal, material, efficient and
final. The first two are uses of 'cause' in a somewhat
wider sense than is current nowadays: the term here simply means
'explanation in general' (Ockham, *Expositio
Physicorum* II, c11: *Opera Philosophica* IV, p. 348), and
explanations by means of matter and form were common both in Aristotle
and in the literature. Efficient causes are what we would now simply
call 'causes'. Final causes, however, are problematic: a
final cause is an end or a purpose, and, whereas it is clear that
*rational* agents act for the sake of ends, it is not clear that
much else does. Furthermore, it also seems clear to us that the
causality of a rationally pursued goal can be reduced to efficient
causality.




Aristotle, however, has a much stronger position on final causality:
he believes that there are processes in nature (the growing of a tree,
for example) which are completed and regulated by a final state, or
end, towards which they tend. As Adams puts it,



> According to Aristotelian metaphysics, natures are
> complexes of powers. When appropriately coordinated, the collective
> exercise of such powers converges on an end. In the sublunary world,
> elemental powers are simple and deterministic. Even where more complex
> living things are concerned, the "coordination" of their
> powers is "built-in" in such a fashion that - given
> relevant circumstances - they function to achieve their
> end. (Adams 1996, p. 499)




Aristotle's natural science tends to be governed by the biological
paradigm, and it is clear that, for him, final causes in this strong
sense are extremely pervasive. He also argues in the *Physics*
that natural processes cannot all be explained by final causality
alone, which implies that final causality cannot, in general, be
reduced to efficient causality.




The medieval literature is far from unanimous on these questions.
William of Ockham, for example, who wrote several commentaries on
Aristotle's *Physics*, and who discusses these questions at
numerous places in his commentary on Aristotle's *Physics*,
hardly has a uniform position. He is quite happy with explanations of
natural phenomena by means of efficient causes in general, but he will
also often speak of final causes: what is unclear is whether the final
causes he speaks of (with varying degrees of strength in different
works) have any explanatory role to play that cannot be reduced to
efficient causality (Adams 1998; cf. Goddu 1999).