## 1. Ancient Antecedents



It is not uncommon for historians to say seventeenth century advocates
of the distinction of P/SQs take it from the ancient Greek atomists,
Leucippus and Democritus. Evidence of this comes from fragments of
Democritus' works, such as:



> 
> 
> By convention sweet and by convention bitter, by convention hot, by
> convention cold, by convention colours; but in reality atoms and void.
> (Taylor 1999: 9)
> 
> 
> 



C. C. W. Taylor (1999: 175-9) is among scholars who hold that
this corresponds to the modern distinction of P/SQs, but the
attribution is problematic. It is true, that the properties of atoms
are sharply divided from things that are by convention, including some
SQs, but the problem is that the range of things by convention is
unspecified in the fragments. Other scholars cite ancient testimony to
the effect that it is meant to include everything apprehended by sense
perception (Lee 2011: 21-8; Pasnau 2007). If so, the ancient
distinction does not map onto a division between SQs and PQs that are
sensible. Because the textual context of the fragments is now unknown
and ancient testimony about it is conflicting, a definitive
interpretation is unlikely (Pasnau 2007).



The Atomists' theory is, however, in the spirit of an important
strain of the seventeenth century division of qualities. The ancients
hold that reality is grasped solely by intellect. They accept two of
Parmenides' intelligibility constraints on change--that one
thing cannot come from many things and many cannot come from one; they
aim to construct a metaphysical system in which change under these
constraints is possible and the experience of non-conforming change is
explained (Taylor 1999: 160-4). Anaxagoras and Empedocles share
a strategy for explaining a sort of change under these constraints:
basic things, conceived either as the four elements or as stuffs and
qualities, do not begin or cease to exist strictly speaking; but
states or relations of substances, conceived as either mixtures or
separations, have a qualified, or relative, being. Leucippus and
Democritus take a somewhat similar line: atoms moving in the void
always exist; they are indivisible, and never fuse along a common
boundary, but bitter, sweet and the like are aggregates of atoms that
come to be by convention, or relatively (Taylor 1999: 160-175).
Atoms and the void are an *a priori* solution to a problem of
intelligibility. Some later advocates of the P/SQ distinction, e.g.,
Galileo and Descartes, hold that intellect provides *a priori*
knowledge of the essences of things. But although variants of the
later distinction offer to explain the potentiality inherent in matter
and the means by which change is regulated, the issues are not
presented as a Parmenidian challenge to intelligibility. The target is
rather the Aristotelian view that matter is potentiality and the
bifurcated theory of accidental and substantial change. The influence
of Parmenides on metaphysics in the modern era is complex, but
advocates of the doctrine of P/SQs as a group show little of it beyond
adherence to *ex nihilo nihil fit*. Although atomism is
prevalent in the early seventeenth century, it is often coupled with
the Aristotelian theory of elements (Kargon 1964). LoLordo (2011)
argues that Gassendi's adaptation of Epicurean atomism makes no
distinction between PQs and SQs.


## 2. Galileo Galilei


### 2.1 The first known argument for a version of the P/SQ distinction



To the best of our knowledge, the doctrine of qualities is first
stated by Galileo Galilei. A paradigm-destroying natural philosopher
for whom "the book of nature is written in the language of
mathematics" (Galileo 1623 [2008: 179-83]; Crombie 1972).
Galileo argues for the distinction in a section of *The
Assayer* (1623). Having been asked to explain why he thinks it may
be possible that motion is the cause of heat, he produces a
non-technical sample of his distinctive method of answering questions
that arise in natural philosophy. The general method relies on
intellect, thought experiments, hypotheses about things that are
quantifiable, and sense perception of measurements that may testify to
their reality (Crombie 1972: 72-4, 81-7; Piccolino and
Wade 2008, *passim*)). The argument goes as follows. We cannot
conceive a corporeal substance without a determinate figure, size,
position, motion/rest, and number; nor can we imagine bodies separated
from any of these attributes. Galileo calls them "primary
affections" of matter. By contrast, it is not necessary to
conceive that a body have a color, taste, aroma, or make a sound; if
we lacked senses, intellect and imagination might never think of
them.



> 
> 
> Thus, from the point of view of the subject in which they seem to
> inhere [these things] are nothing but empty names, rather they inhere
> only in the sensitive body ... [I]f one removes the animal, then
> all these qualities are ... annihilated. (Galileo 1623 [2008:
> 185])
> 
> 
> 



Primary affections are essential to bodies; taste, smells, etc. are
inessential to bodies and sensory faculties are essential to tastes,
and the rest; so in the absence of sensory systems, there are no
tastes, and the like.



The argument is elaborated by means of a thought experiment. Suppose
you run a hand over a marble statue and over the body of a human
being. The hand does nothing but touch and move over the two bodies,
but when the animate body is touched, it feels sensations which vary
depending on where it is touched; e.g., a tickling sensation occurs
when the hand makes contact with the sole of a foot or pit of an arm.
The argument continues: "The sensation is entirely ours and not
at all in the hand ... [I]t would be a great error" to say
the hand has a property in addition to motion and touching, namely,
"the power to tickle". Thr arumgent continues: a tickle
sensation is entirely within the being who feels it; tickles plainly
have no reality unless they are felt; so if the sentient being is
removed, the tickle is nothing but an empty name; so it is possible
that tastes, etc. "have a similar and no greater reality"
(Galileo 1623 [2008: 185-6]).


### 2.2 A hypothesis about the corporeal causes of tastes, etc.



The basic hypothesis is that bodies need nothing other than
determinate sizes, figures, etc. in order to arouse tastes, etc. in
beings that have sensitive organs. Without ears, tongues, and skin
around armpits, there are no sounds, tastes, or tickles but only
various shapes, motions,
 etc.[1]
 In particular, some bodies are constantly divided into tiny
particles; some of them rise, enter the nasal passages and start a
process that ends with odors; others fall onto the surface of the
tongue, mix with saliva and eventually produce tastes; tiny vibrations
that pervade the air make contact with the ear drums which connect
with other organs that produce sounds; finally, there is a particular
sort of igneous particle found in fire which moves through pores of
the body and makes us warm. Many observations are brought in support
of these hypotheses (Galileo 1623 [2003: 188-9]). Thought
experiments establish the possibility of this account. Two features of
the hypothesis are particularly important. First, it sketches a
program of explanation for natural philosophy in which the distinction
of PQs and SQs has an essential part (Crombie 1972: 80-7).
Second, it entails that SQs are not in bodies that cause them but
internal to sentient bodies--effects of the mechanical affections
of insensient bodies on the sensory organs of living bodies. Piccolino
and Wade (2008: 1218-19, 1319-21) portray Galileo as
advocating the sort of explanatory program urged later by Hermann
Helmholtz and presently pursued in neuro-psychology. (On
Helmholtz' version of the P/SQ distinction, see Hatfield
2011.)


### 2.3 A non-Aristotelian account of the cognitive function of the senses



The mistake of supposing tastes, etc. are qualities that inhere in
inanimate bodies is charged not to our senses, but to the use of
language. It seems we give names to the primary affections which we
know to exist in bodies and we give names to the sensations of our SQs
as well. This makes us believe the names of the latter apply to bodies
just as the names of the former do. According to Galileo, the names of
SQs signify nothing when used in this way. This poses a challenge to
the traditional Aristotelian scholastic theory of sense perception.
According to Aristotle, the nature of sentient beings assures that
they acquire perceptual knowledge in an environment such as ours. Some
care is called for, but no special effort is generally needed. On
Galileo's theory, many sense perceptions are caused by
insensible particles whose properties are by no means easy to know.
When a fire makes us warm, it is easy to feel the warmth but the rapid
motion of particles that causes it has to be discovered by rigorous-s
use of an effective method of natural philosophical inquiry.



Galileo objects specifically to two principles of a contemporary
version of the Aristotelian stance: that mathematics is only
accidentally applicable to material things and the doctrine of proper
and common sensibles. A proper sensible is an object that is directly
perceived by only one sense--color, by sight alone; sound, by
hearing, etc. The faculty and its proper object are said to be fitted
to each other so that use of that faculty seldom results in
misperception of the quality special to it. By contrast, the common
sensibles--size. figure, motion/rest--are directly perceived
by more than one sense; as a result, we are supposedly more liable to
mistakes about them. Galileo's objection reconceives the
condition that makes perception of a body veridical or deceptive.
(Crombie 1972: 74-9; Piccolino & Wade 2008: 1312-15)
He argues that the appearance of a body corresponds to the properties
of the body that are its cause;. It is not up to . philosophy to say
how various apearances are related to the affections of the objects we
perceive; rather it requires the technical methods of natural
philosophy. For example, to avoid being deceived by the broken visual
appearance of an oar half in water, we need to find the physical cause
of the appearance. This will show that the visual appearance is
correct. According to Galileo's epistemology, we should assume
appearances do not exhibit the properties of their causes but rather
something from which we may be able to figure them out.


## 3. Rene Descartes


### 3.1 Roots of the distinction



Descartes formulates no distinction of P/SQs but familiar doctrines to
which he is committed point to one. He maintains that his mechanist
theory of matter and motion can explain everything in the corporeal
world (CSM 1.285; AT
 8.323).[2]
 He holds that bodies and minds are substances each of which can exist
independently of the other (CSM 2.54; AT 7.78). According to him,
knowledge of continuous extension, and the sizes, figures, positions,
and local motions of its parts, is innate in the human intellect, or
mind (CSM 2.44; AT 7.63). By contrast, we know SQs only with aid of
the body (CSM 1.217; AT 8.33-4). The Cartesian P/SQ distinction
implicit in these doctrines has three roots:


* the theory of ideas which is central to Descartes'
epistemology and psychology,
* the unity of a finite substance in general,
* the composite unity of a human being. Brief explanations
follow.


#### 3.1.1 Ideas



In *Meditations* an idea is said to be a mental act that is
"like an image" (CSM 2.25; AT 7.37). Most scholars take
this to mean that ideas are presentational and intentional: we are
acquainted with them and they direct thought to something else (M.
Wilson 1978: 102-103; 1992: 69-83;). To apprehend anything
at all (actual or possible) is to have an idea of it (CSM
2.2.25-26; AT 7.37). *Meditations* explicates
intentionality in terms of objective being: the idea of the sun is the
sun "existing in the intellect in the way its objects are
normally there" (CSM 2.75 and 27-8; AT 7.103 and 40; CSM
1.198, note 1). As Descartes also puts it, ideas represent things
(e.g., CSM 2.28; AT 7.40): an idea represents an object that is
*F* just in case it represents the object as *F* (CSM
2.145-6, 163; AT 7.207, 234). The representation may be sensory
or intellectual (CSM 2.126, 127-8; AT 7.179, 181): its cognitive
value may range from clear and distinct to confused or obscure (e.g.,
CSM 2.29-30; AT 7.43; CSM 1.207, 317; AT 8.22-3,
33-34).


#### 3.1.2 The substance-quality (mode) relation



Descartes argues, in general: if a particular quality or affection
exists, it must subsist or reside in a substance that makes it exist
(CSM 2.114; AT 7.61; CSM 1.210; AT 8.25). The substance must have the
capacity to enable this particular quality to exist and any other
quality in a range in which this one is included (CSM 2.124; AT 7.76)
The range is determined by its principal attribute, or essence: all
qualities (or modes) of a substance are referred to its essence and
unintelligible without it (CSM 1.210; AT 7.25; CSM 2.54; AT 7.78). So
the essence of a substance and all the qualities that do or can exist
in it have reciprocal relations of conceptual dependence (Bolton 2013:
65-6). Scholars characterize the relation in several
ways--as that between an occult quality and its known effects
(Malebranche 1674-75 [1980: 622]), the atomic structure of gold
and its sensible and laboratory properties (Rozemond 1998: 92; Beck
1965: 118-22), an absolute and its limits (Downing 2011:
122-25). But Secada (2000: 190-94) cites a letter in which
Descartes speaks of the relation as an instance of the more general
relation between two concrete things one of which is determined by the
other (To Mersenne for Hobbes, 21 April 1641, CSM 3.178-9; AT
3.355) Secada (also Bolton 2013) concludes the relation should be
understood as that between a highest order determinable and its
determinates.


#### 3.1.3 The union of mind and body



In addition to unities of nature (substances), Descartes recognizes
"unities of composition", for instance, the flesh and
bones of the same animal-machine (CSM 2.285, 157; AT 7.423, 222).
Other examples are a mouse, a human body and a human being. Such
things are *ens per se*: although they have parts that can
exist outside them, the parts are then regarded as incomplete. So the
mind/body unity is robust enough to tag a mind separated from its body
as incomplete (To Regius, December 1641; CSM 3.200; AT 3.461). Brown
and Normore (2019: 52-60) contend that things with this status
have a nature of the type Descartes calls "true and
immutable" (hereafter "TIN"). Anything with a TIN
has ground in reality indicated by its being the subject of
demonstrable truths; corporeal nature, geometrical entities and God
are mentioned in *Meditation* 5 (CSM 2.44-5; AT
7.63-5) and it is said to be true of Cartesian animals
(Conversation with Burman, 16 April 1648; CSM 3.143-4; AT
5.160). The bi-conditional test for things with a TIN is having a
property that arises from the conjunction of all its ingredients (CSM
2.84; AT 7.118; Brown & Normore 2019: 55-60). That is, the
composite possesses some property that does not already belong to one
of its parts, as in the case of Pegasus. For instance, a triangle
inscribed in a square has a property the triangle does not have on its
own, namely, having an area half that of the square that superscribes
it (CSM 2.83-4; AT 7.117-18). Descartes maintains that the
union of mind and body has a TIN which would, if known, account for a
human being's having appetites, emotions, and sensations of
colors, etc. (CSM 1.208-9; AT 8.23).


### 3.2 Issues



Four issues about Descartes' P/SQ distinction are prominent in
recent secondary literature:


* How he purports to show that SQs as we sense them do not inhere in
bodies
* The metaphysical status of SQs
* Whether ideas of SQs are mere (non-intentional) presentations or
also intentional and if the latter, whether their content is internal
or external
* How the intellect affects sense perception of PQs by comparison
with SQs.


#### 3.2.1 An argument that bodies do not resemble SQs as we perceive them



Scholars have difficulty finding an argument purporting to show that
heat, etc. as we sense it does not inhere in bodies. Some suggest that
his best argument is the intelligibility, simplicity, and explanatory
adequacy of his mechanist theory (CSM 1.284-86, 288; AT
7.121-123, 326). But it is conclusive only in conjunction with
the norm expressed by Ockham's razor. (Downing 2011:
113-14). Another problem is that Descartes writes that his
mechanist physics explains the nature of light, heat, etc. because he
presupposes that they are "only in our senses... and not in
the objects we perceive by sense" (Letter to Chanut,26 Feb 1649;
CSM 3.369; AT 5.291-2). It is doubtful Descartes would advance
the explanatory adequacy argument unless he had an independent
argument for the presupposition (for a different analysis, see Downing
2011: 114).



It might seem *Meditations* offers such an argument. It states
that our most common mistake about ideas in general consists in
judging that they resemble things outside us (CSM 2.26; AT 7.37). Some
scholars take this to be an attack on Aristotelian realism, but the
argument is elusive. The problem is that the context indicates that
assuming resemblance is an epistemic mistake but not necessarily a
factual one (e.g., CSM 2.57; AT 7.82-3). That is, making the
assumption is assenting to something we have no reason to think is
true; but it may nevertheless be true. Perceptual realism about SQs
tacitly goes by the board, but the opponent in view is the epistemic
stance of those who formed the habit of implicitly relying on their
senses as children and never reflected on its value for acquiring true
beliefs. (For this interpretation of Descartes' method of doubt,
see Patterson 2007.)



Lisa Downing (2011: 115-17) argues that the first part of
*Principles of Philosophy* (hereafter *Principles*),
articles 66-71 provides basis for an *a priori* argument
which Descartes could have had in mind but never stated. The articles
explain what we clearly and distinctly perceive about color, what we
might mistakenly judge about it, and what we can judge about its
relation to bodies without error. According to *Principles*
1.71, sensations of heat, color, etc. "do not represent anything
located outside our thought" (CSM 1.219; AT 8.35) Based on this,
Downing (2011: 118-28) constructs a valid argument to show ideas
of SQs as we sense them cannot exit in a substance with the principle
attribute extension. But she argues, it requires the principle
attribute of a body to be absolute geometrical extension and modes to
be its limits. She finds this too restrictive to be tenable because it
excludes modes such as force and inertia and it is a model of the
attribute-mode relation that cannot be applied to substances with the
principal attribute thought.


#### 3.2.2 The metaphysical status of SQs



A few scholars maintain that heat, e.g., is nothing other than a
sensation on the strength of *Principles* 1.70-71 (Vinci
1998: 199-200; MacKenzie 1990: 114-16). However, the
scientific writings offer mechanist accounts of light and color as
specific sorts of motions of insensible particles and suggest other
SQs, too, are micro physical configurations (e.g., *Optics*,
CSM 1.153, 155-6; AT 8.84-5, 88). Cottingham (1990:
237-38) among others cites a text that may seem to identify
sensible qualities with causal dispositions (CSM 1.285; AT
 8.323),[3]
 but Wolf-Devine (1993: 46-8) convincingly argues that the
passage refers to mechanist microstructures. Other commentators
maintain that Descartes either vacillates between mental and physical
views of color or has an inconsistent theory (M. Wilson 1992: 228,
234; Wolf-Devine 1993: 46-8). However several more charitable
interpretations accommodate both. According to Atherton (2004),
Descartes' opposition to certain scholastic doctrines motivates
a bifurcated theory of color consisting of a mechanist physiological
account paired with a mentalist psychological account of sense
perception. She finds nothing that unites the parts of an essentially
dualist theory of perception. According to Nolan (2011a), Descartes is
a nominalist about SQs: they are abstractions from experience that
have no *de re* counterparts. They are names we use to refer to
both certain ideas and certain bodily modes. However, it seems the
status of SQs should be understood as that of entities that emerge in
the mind-body union in an environment such as ours (see Alanen 2005:
147-164; also Brown & Normore [2019: 185-6] who argue
that the union is a subject of predicates without mentioning SQs).
This view of SQs is explicit in *Principles*, Part I (CSM
1.208-09; AT 8.22-23).


#### 3.2.3 The intentionality of ideas of SQs



Descartes repeatedly says we mistakenly suppose the things represented
by ideas of SQs resemble the ideas. Is it possible for ideas to
represent things and yet be misleading about what they are? The issue
comes to a head in remarks about "materially false" ideas
in *Mediation* 3. This is a non-judgmental sort of falsity
"that occurs in ideas when they represent non-things as
things"; for instance, ideas of heat and cold do not enable us
to know whether one is the privation of the other, both are privative,
or both are positive. Because there cannot be an idea that is not
intentional, or of something, the idea that represents it as something
positive should be called false (CSM 2.30; AT.7, 43-44). Arnauld
objects that on Descartes' theory, *X* cannot exist
objectively in the mind as something other than it is; were the idea
of *X* materially false, it would not be an idea of *X*
(CSM 2.145-46; AT 7.206-07). Three points in
Descartes' reply are reasonably clear. First, material falsity
does not have to do with the relation between an idea and its object
but with the mental operation that makes the object appear in a manner
that does not enable us to tell whether its object is privative or
positive (CSM 2.162-3; AT 7.232). Second, ideas of SQs are
confused and obscure; they involve sensations which are often wrongly
taken to be the objects the ideas represent. Third, Descartes agrees
with Arnauld: the idea of cold, e.g., cannot represent cold as
something it is not (CSM 2.164; AT 7.234; for a different
interpretation, see M. Wilson 1978: 114-16). The upshot is that
ideas of color etc. are confused. They provide opportunity for error
but are not erroneous representations (Hatfield 2011: 143-148).
Scholars disagree about how to interpret these texts in an internally
consistent way. There are two basic positions:


* Colors, etc. are non-intentional sensations;
* Ideas of color, etc. are intentional but confused or obscure.


##### SQs are non-intentional sensations



Several scholars urge that in Descartes' considered opinion, SQs
are sensations without representative content (M. Wilson 1978:
114-16; Vinci 1998: 196-98, 199-200; MacKenzie
1990). This is based on *Principles* 1.68 and 70, which say
that colors, etc, are clearly known when they are regarded merely as
sensations and we make no error if we judge there is something in
bodies whose nature we do not know which causes the sensations in us.
But these texts would mark a change of mind if they implied these
ideas represent nothing but what we clearly know. The scholars
mentioned in the next section are among those who are not persuaded
that Descartes abandons the view of ideas of SQs in
*Meditation* 3 and the exchange with Arnauld.


##### Ideas of SQs are intentional and confused



There are two things advocates of this view have to explain: what the
intentional objects of these ideas are and what is the source of the
confusion that stops short of misrepresentation. The literature offers
several carefully developed accounts but brief descriptions will have
to suffice here.


* SQ ideas do not intrinsically or presentationally represent
modifications of bodies but some texts imply they extrinsically
represent things in our environment such as their causes (M. Wilson
1990 [1999]; Schmaltz 1992). See De Rosa (2010: 69-93) for
critical discussion.
* Simmons (2001) maintains ideas of SQs have teleo-semantic content.
They represent things in the external world that are beneficial or
harmful to the perceiver. This is critically discussed by Detlefsen
(2013a) and De Rosa (2010).
* De Rosa (2010: 117-61) argues that the idea of color has two
components: a manifest phenomenal presentation caused by interaction
between parts of the body and the mind, and a latent clear and
distinct idea of the external corporeal cause of the idea. The idea
misrepresents the entity it represents because the representative
aspect is latent. See Paul and Morrison (2014) for critical
discussion.
* Lilli Alanen (2003: 147-164) offers a comprehensive, complex
and subtle account of materially false ideas. Two features of this
interpretation broadly considered can be mentioned here. Sensory ideas
in general represent elements and events comprised in the mind/body
union and do so by virtue of the complex relation that unites the mind
and body. To this extent, they are intrinsically representative and
ineliminably confused because the union is unintelligible to us. In
addition, sensory ideas are signs of the external objects that cause
them, informing us of their existence, changes, and effects on us. In
this capacity, they have extrinsic content which is, again,
incomprehensible to us. A second point is that sensory ideas comprise
sensations and unnoticed judgments which accrue to them in childhood;
some of these judgments ascribe qualities to objects they do not have.
In this respect, ideas of SQs, and perhaps some PQs, are materially
false.
* Gary Hatfield (2013: 138-148) offers another comprehensive
and subtle account. Here we can sketch three points. Ideas of colors,
etc. are intrinsically representational; they represent by virtue of a
similitude that is not phenomenally recognizable but understood with
the aid of metaphysics and natural philosophy. They manifestly
represent external objects as having colors, being red, e.g., and they
obscurely represent micro-structural modes of bodies. So they accord
with the account of material falsity in the Fourth Replies, i.e., they
give opportunity for false belief and still represent their objects as
they are. This comports well with relevant texts but the supposed
similitude between SQs as we perceive them and configurations of
insensible particles in motion may strain credulity.


### 3.3 Intellect's involvement in sense perception of PQs by comparison with SQs



A related issue is whether intellect is involved in perceptual
knowledge of PQs but not in perception of SQs, There are two main
positions:


1. Sense perception of secondary qualities involves only the sensory
faculties whereas perception of PQs deploys knowledge provided by
intellect (M. Wilson 1993 [1999: 26-40]; Maull 1980: 31 and
33);
2. Both intellect and the perceptual faculty have important parts in
producing perception of PQs and SQs (Simmons 2003; Ortin Nadal
2019).


#### 3.3.1 Perceptual knowledge is bifurcated



Advocates of the first position point to several texts:


* the proof that in general, sensory ideas are caused by a substance
that has all the qualities we clearly and distinctly perceive to exist
in corporeal substance; clear and distinct perception is intellectual
(CSM 2.55; AT 7.80; CSM 1.223; AT 8.40-41);
* the pragmatic non-theoretical purpose of sense perception (CSM
2.57; AT 7.82-3; CSM 1.224;AT 8.42);
* the theory that we see distance by a natural geometry
(*Optics*, CSM 1.170; AT 6.137-80);
* the doctrine that we know PQs and SQs of objects in quite
different ways: "our knowledge of what it is for an object to
have a shape is more clear than our knowledge of what it is for it be
coloured" (CSM 1.18-9; AT 8.34).



In opposition, critics note that most of these passages concern
features of sense perception in general and the one exception is a
passage that does not apply to sensory knowledge at all.


* Perception of the particular figures of particular things is less
certain than our knowledge that figures, etc. exist in the causes of
our sensory ideas in general (CSM 2.27, 55, 57; AT 7.39, 80, 83); we
make mistakes about the size and distance of the sun.
* Sense perception in general has a pragmatic purpose (CSM 2.57; AT
7.82-3).
* Our initial visual ideas of distance are, like our initial ideas
of all sensible qualities, direct effects of motions in and around the
pineal gland (*Optics*, CSM 1.167; AT 6.130; Hatfield
2015).
* The greater clarity of our knowledge of PQs is not manifested by
sense perception but by reflection on what we know *a priori*
about the nature of corporeal substance and its modes (Ortin
Nadal 2019: 1129-31).


#### 3.3.2 Perceptual knowledge is unified



In support of the second position, Simmons (2003) constructs an
argument based on the three grades of sensory response defined in
*Sixth Replies* (CSM 2.294-96; AT 7.437-39) and
physiological and psychological accounts,in *Optics* and
*Treatise on Man*, of the operations that form experience in
early childhood. In brief: the first grade of sensory involvement
starts with impingement of particles on the organs of sense and
includes all their physiological effects; the second grade comprises
the immediate effects produced in the mind as a result of its union
with a body provided with sensory organs; the third grade, which is
the first to involve intellect, comprises all the "judgments
about things outside us we are accustomed to making in
childhood" (CSM 2.295; AT 7.437). Simmons argues that the early
scientific works ascribe two sorts of judgments to
intellect--those that falsely project resemblance and those that
construct the appearances of sensible things. She finds that the
latter are associative cognitive acts that connect sensations of
qualities with images drawn from memory (Simmons 2003: 554; on images
and the intellect, see Letter to Mersenne, July 1641, CSM 3.186; AT
3.395). The model of clear and distinct perception promoted in
Descartes' later philosophical works hardly fits the
construction of images and ill-considered judgments that engage the
intellect of a young child (on the diversity of intellectual
operations, see Simmons (2003: 565-75,
 77-8).[4]
 The upshot is that sense perception of the world features PQs and SQs
and its purpose is practical rather than scientific (CSM 2.57-8;
AT 7.83; CSM 1.223-4; AT 8.41-42.) Once we attain clear
and distinct *a priori* perception of the essence and
properties of bodies, we are unable to conceive how a body could
resemble a sensory presentation of color, etc.



According to Ortin Nadal (2019: 1129-1132), the
fundamental difference between qualities of the two types is not that
ideas of SQs provide more material for error; in perception, specific
ideas of both sorts can misrepresent particular physical causes. They
differ with regard to the mistakes we can make about their causes. To
judge that a body resembles the sensation of a color is to error about
the nature of matter, whereas to make an error about the determinate
PQ of a particular object is still to know that it has the
determinable quality it comes under.


## 4. Robert Boyle



Robert Boyle's variant belongs more to philosophy of science
than theory of perception. It is integral to the corpuscular theory
(hereafter "CT"), the mechanist natural philosophy he
promotes. He is an avid experimentalist and prolific author of works
on ethics, theology, natural religion, and both alchemical and
mechanist natural philosophy (Stewart 1991: xi-xii,
xv-xvi; Anstey 2000: 9). Boyle strongly opposes any intrusion of
metaphysical and logical entities in natural philosophy (Boyle 1666
[1991: 3, 8, 22, 28]; Stewart 1991: xiii). The targets are
Suarez's real qualities and scholastic accidental and
substantial forms. The former, as Boyle understands them, are entities
that naturally inhere in material substance but have independent
causal powers (see Boyle 1666 [1991: 21-2]). Boyle avoids
controversial metaphysical doctrines that divide mechanists, such as
Epicurean atomism, innate motion, Cartesian essences and the endless
actual separation of parts of bodies (1666 [1991: 7-8]).
According to CT, observable bodies are composed of very small
relatively stable corpuscles, composed of *minima naturales*
with tightly cohering parts rarely separated in nature (Anstey 2000:
10-11, 43-4).



Boyle's best known work in natural philosophy is *The Origin
of Forms and Qualities According to the Corpuscular Philosophy*
(1666) (hereafter "OFQ"). A large part of OFQ is devoted
to reports of experiments intended to show the superiority of CT over
its scholastic and chemical competitors (OFQ [1991: 18]; also Boyle
1674). According to the theoretical part, matter is one extended
impenetrable substance to which motion in accord with contingent laws
is added by God. Local motion, the "grand agent" of
natural change (OFQ [1991: 19]), divides matter into parts, each
modified by a determinate size, shape, and local motion. The
corresponding determinable modifications are inseparable from every
part of matter, and designated "primary" because they are
simple, primitive, and sufficient to derive all qualities (OFQ [1991:
97]). Texture, a fourth primary modification, is the configuration,
i.e., mutual orientation and order, of the corpuscular parts of a body
(OFQ [1991: 30]). Texture is composed of the primary affections of
many insensible particles which jointly form a novel modification that
none of the particles has by itself, much as TINs do for Descartes.
All four primary affections are intrinsic properties. Boyle calls them
"mechanical affections" (hereafter "MAs")
because, he says, the operations of machines are commonly explained by
them (OFQ [1991: 18-21]). The stated aim of OFQ is



> 
> 
> to make it probable by experiment ... that almost all sorts of
> qualities ... may be produced ... by corporeal agents
> ... that work [only] by virtue of motion, size, figure, and
> contrivance of their own parts. (OFQ [1991: 17])
> 
> 
> 



Qualities are entities of a different type: causal capacities and, for
the most part, relations between two bodies (OFQ [1991: 13,
23-30]). The terms "quality" and "power"
are treated as synonyms. The quality-making relation is not fully
explicit. A relation of congruence, or fit, between the MAs of the two
bodies is often mentioned; spatial temporal co-incidence is implicit;
connections with other things in the environs are suggested
(O'Toole 1974; Kaufman 2006: 166-173). The relational
theory is motivated by arguments of two sorts (OFQ [1991:
23-30]). One is illustrated by the legend that the first lock
was forged without a key; Boyle argues that when a piece of metal of a
certain size and shape was subsequently made, the lock acquired the
power to be worked; so the power to be worked and the power to work
are relations between the key and the lock; but in the bodies
themselves there is nothing distinct from their sizes, figures,
motions and those of their parts. The second is an argument from
parsimony. If every new quality a body acquired added something to the
body itself, it would contain a vast number distinct entities that
serve no physical purpose.



Anstey (2000: 52) maintains that qualities have a reductive structure:
MAs are not powers and qualities, which are powers, are reduced to
their respective MAs. The quality is, then, explained by its
constituents: the congruent mechanical affections of a pair of bodies
explain what happens when one is duly applied to the other in terms
that are intelligible and familiar from our understanding of machines.
Boyle repeatedly claims that in the bodies that constitute qualities,
there is nothing distinct from their MAs, but often remarks that
qualities derive from or depend upon the MAs of bodies. Anstey is one
of several scholars who find a tension between these two claims. The
worry is that "nothing distinct from" expresses
ontological reduction; that the dependence in view is asymmetric; and
a thing cannot have an asymmetric relation to itself.



The tension is at the center of one of the two main scholarly
disagreements about Boyle's theory of qualities. The other
questions whether he consistently adheres to the relational theory or
retreats to a monadic notion of dispositive qualities in some
passages.


### 4.1 Are qualities reduced to mechanical affections of bodies or dependent on them?



This is usually discussed in connection with qualities of the sort
Boyle calls "sensible". These are our SQs--in fact,
on two occasions, Boyle refers to them as "secondary
qualities". Sensible qualities are present in bodies that
operate on human beings so as to cause the sort of ideas on account of
which we ascribe colors, etc. to bodies. Names given to the various
ideas of SQs are also used to designate the corresponding qualities,
so "color", "green", etc. can be used to stand
for qualities in the scope of corpuscularian explanation (OFQ [1991:
24, 30-4, 51-2]). Boyle is a Cartesian mind-body dualist.
Although he discusses perception in several texts, he has no well
developed account (see Anstey 2000: 68-85). OFQ sticks to the
physical project announced at the start.



Scholars have taken five basic positions on this question.


* Peter Alexander (1974: 229-238; 1985: 73-85) argues
that qualities that are sensible are textures, but colors, green,
blue, etc. are nothing but mind-dependent sensations. But textual
evidence of this is compromised by the ambiguity of names of SQs
(Anstey 2000).
* Mackie (1976: 14-15) suggests SQs are dispositions based on
the intrinsic MAs of the bodies to which the qualities belong, The
main problem is that the texts do not clearly articulate the
distinction between a disposition and its basis in things that are so
disposed. Boyle's terminology is unstable across different
works, and less than precise (Stewart 1991: xiv), and the texts focus
on the horizontal two-place relation between bodies (or bodies and
perceivers) that form a quality without saying much about the vertical
presumably three-place relation between a quality and two sets of
MAs.
* According to Anstey (2000: 105-07), Boyle's natural
philosophical work shows a deep interest in framing hypotheses about
the corpuscular constitutions of specific qualities, i.e., physical
reductions of qualities deemed possible, but his work also exhibits a
deep investment in the CT which states that MAs are basic and
qualities are dependent entities which are explained by them. Anstey
concludes these commitments are inconsistent and irreconcilable.
* Keating (1993) argues that Boyle's strong inclination toward
the reduction of qualities to MAs pushes him to say that colors, etc.
are mind-dependent sensations.
* O'Toole (1974: 307-13; followed by Kaufman (2006:
190-1)) argues that for Boyle, qualities are relational
properties of bodies rather than intrinsic to any one body. He notes
that the texts taken to say the SQs of a pair of bodies are identical
to their MAs actually say that in each body there is nothing but its
MAs while saying the quality is a relation over and above that We
mentioned congruance and spatial and temporal co-existence; means of
applying one body to the other are also suggested.


### 4.2 The "dispositive sense" of having a sensible quality



Sensible qualities pose a particular problem, one that is said to be
the "chiefest difficulty" for CT: "it seems evident
that [colors, odors, and the like] have an *absolute* being
irrelative to us" (OFQ [1991: 32]). Two reasons are given:


1. snow would be white and a glowing coal would be hot if there were
no sentient beings and
2. the heat of the coal would still melt ice and warm wax if sentient
beings were taken away.



Boyle does not deny that the objection has some basis in fact but
contends it is consistent with CT (OFQ [1991: 32-37]). That is,
the absolute, non-relational, nature of colors, etc. seems evident for
misguided reasons that Boyle traces to Suarezian metaphysical
assumptions. With regard to (b), his long response makes the point
that it is not heat but the insensible corpuscular constitutions of
bodies that are hot that have the power to melt ice according to CT.
The response to (a) is a matter of controversy. It specifies a
"favorable sense" in which it can be said that bodies
would have sensible qualities if there were no sentient beings; but,
Boyle contends, it does not suffice to establish that SQs are
absolute.



As Boyle explains it, the dispositive sense is conveyed by this: If
there were no perceivers, snow, which is actually cold, would in
virtue of its corpuscular constitution, be such that if it were
applied to the tactual sensory organs of an animal, it would produce
"such a quality (sic)", whereas a body, which does not
actually have a SQ, would not produce such a quality. This is to be
"dispositively" cold, or "fit" to be cold, but
actually to be endowed only with certain MAs (OFQ [1991: 31]). Why
would the lack of competent perceivers prevent snow's being cold
given that snow need not be perceived in order to be cold?



Anstey (2000: 103) takes this to be a retreat from the relational
theory. Curley (1972: 448-50) finds Boyle ambiguous and
mistaken. Stewart (1991: xiv) holds that instability in Boyle's
use of terms precludes a definitive interpretation. However, Kaufman
(2006) argues that the dispositive sense is consistent with the
relational theory and as Boyle says, part of his defense of it. As
Kaufman sees it, the theory of qualities is a consequence of
Boyle's method in natural philosophy, especially his commitment
to reasoning from nothing but observational data and antipathy to
*a priori* argumentation. It is possible here to give only a
rough sketch of two important points of this interesting, detailed,
and complicated interpretation.



First, Kaufman (2006: 175-85) notes that if a body that has MAs
in virtue of which it is dispositively cold is not actually cold, then
it might seem that the relational account of qualities is expendable.
One reason is that from the fact that a body is dispositively cold and
the tenets of any adequate mechanist theory, we can predict and
explain what the body would do if it were applied to the tactual organ
of a perceiver; so it might seem that CT can do without relational
qualities. According to Kaufman, this is not right. This is said to be
because by Boyle's empiricist lights, we have no way of knowing
what would happen if snow and the right sort of perceiver were to come
together except by observing the event. The point is that we cannot
attribute dispositive coldness to a body unless such bodies have, or
once had, the relational quality cold.



Second, as Kaufman (2006: 186-90) describes it, Boyle maintains
that the MAs of snow in the actual world are *de re* identical
to the MAs of snow in a world without perceivers, but Boyle wants to
say snow is cold in the actual world and only fit to be cold in the
perceiverless world. This is problematic because it seems there is no
respect in which they could be different. Kaufman suggests a solution
along the following line. Because CT states that a body gains or
looses a quality if its relatum begins or ceases to exist and Boyle
recognizes a difference between a body's having a quality and
someone's knowing that it does, he maintains that a body has a
dispositive quality in virtue of how someone thinks of its MAs. That
is, under the condition that animals are, or are assumed to be, absent
from the world, a corpuscularian may reasonably think of the MAs of
snow as being fit to manifest cold were they to operate on a competent
perceiver. That marks the difference between the MAs of snow in a
perceiverless world and in the actual world, yet in the latter snow is
not cold. On this interpretation, dispositive qualities are beings of
reason with a foundation in things (Kaufman 2006: 190).


## 5. John Locke


### 5.1 Locke's argument for the distinction of P/SQs



Although Locke's *Essay Concerning Human Understanding*
(1689-90) (hereafter *Essay* or E, cited by Book,
Chapter, and Section number) contains the canonical distinction, it is
an inquiry into the origin, limits and sufficiency of human cognition.
It is not meant to contribute to theoretical natural philosophy, but
to expose abuses of language that hamper it (E [1975: 9-10]).
Locke's attitude toward CT is mixed: he points to its failure to
account for the cohesion of the parts of matter and communication of
motion (E 2.23.26, 28) but holds it up as the hypothesis that
"goes farthest in an intelligible Explanation of the Qualities
of Bodies", (E 2.23; 4.3.12-13).



That PQs and SQs are placed in the same category, that of things that
subsist in substance, reflects Locke's interest in what we know
by means of the senses. For him, substance and quality are inseparble.
So if we know that qualities exist, we can rightly infer there is a
substance in which they inhere, and we know qualities exist because we
receive ideas of sensation caused by substances. Ideas of sensation
are one of two sorts of ideas which are the simple elements of all our
thoughts and can only be acquired in experience according to Locke.
*Essay*, book 2, chapter 8, which exposits and defends the
distinction of PQs and SQs, comes immediately after five chapters that
define and classify simple ideas. The sixth breaks down the relation
between simple ideas of sensation and qualities in order to show that
most of the former do not resemble the later.



The argument of *Essay* 2.8 can be divided into seven
stages.



First, sections 1-6 urge that privations, e.g., rest and
darkness act as causes just as their positive opposites do.



Second, sections 7-8 distinguish qualities and ideas:



> 
> 
> Whatsoever the Mind perceives in itself, or is the immediate object of
> Perception, Thought, or Understanding, that I call *Idea*; and
> the Power to produce any Idea in our mind, I call *Quality* of
> the Subject wherein that power is. (E 2.8.8)
> 
> 
> 



A snowball has powers to produce ideas of white, cold, and round; the
mind possesses the ideas. The point is that ideas are intentional and
ontologically distinct from the qualities. Yet Locke says he will
sometimes speak of ideas as in the subjects but should be understood
to mean qualities of the subjects that cause the ideas in us. This is
said elsewhere to comply with ordinary ways of speaking (E 2.32.2).
Locke apparently sees little harm in indulging this once it is
understood that it cannot be correct. This is one of several ordinary
but inexact uses of language that play a role in Locke's theory
of qualities; on civil and philosophical uses of language, see
*Essay* 3.9.3.



Third, sections 9-10 introduce the distinction of primary
qualities, secondary qualities, and a sub-species of the latter. There
is a criterion by which PQs are selected: inseparability from bodies
in whatever state they may be--in perception, conception, all
alterations bodies undergo, and in an imaginary act of repeatedly
dividing a sensible particle into insensible parts. Secondary
qualities are characterized as



> 
> 
> nothing in the Objects themselves, but Powers to produce various
> Sensations in us by their primary Qualities, i.e., by the Bulk,
> Figure, Texture and Motion of their insensible parts. (E 2.8.10)
> 
> 
> 



The sub-species includes attributes such as the sun's power to
melt wax and thereby give it the power to cause ideas of warmth.
According to Locke, these attributes are commonly regarded as powers
but are "as much real Qualities in the Subject, as those which I
to comply with the common way of speaking, call *Qualities*,
but for distinction, *secondary Qualities*," (E 2.8.10; E
2.31.2) Recall that Boyle uses the traditional term "sensible
qualties" to signify warmth, color, etc. So SQs are designated
by the term "quality" but are wrongly supposed to be
qualities. More on this in stage seven.



Fourth, sections 12-14 declare it is "manifestly by
*impulse*" that ideas are caused in minds (E 2.8.11). A
little reflection is taken to show that when we perceive a body,
motions of insensible particles convey motions to parts of our bodies
that ultimately result in sensory ideas. If it is true that bodies
acton our sense only by impulse, this is a reason to accept the
characterization of SQs in *Essay* 2.8.10.



Fifth, sections 15-21 state and defend what is commonly called
the "Resemblance Thesis" (hereafter "RT"). It
has a positive and a negative part: ideas of primary qualities
resemble features of the bodies that cause them--"their
Patterns do really exist in the Bodies themselves"--but
"there is nothing like the ideas [of SQs] in the bodies
themselves" (E 2.8.15). This is presented as an evident
consequence of the claims in sections 8 through 14. The sections that
follow argue directly for the positive and negative assertions.



The negative claim is supported by at least two lines of thought. The
first ascribes the common belief in the resemblance of SQs and their
ideas to a misguided metaphysical theory implicit in a common way of
speaking: "Flame is denominated *Hot* and *Light*
... from the ideas [it produces] in us" (E 2.8.16). But,
Locke argues, fire causes ideas of both heat and pain and we have no
more reason to think heat resembles something in the fire than that
pain does (E 2.8.16). Many scholars take the remark about use of
language to say we give the same names to sensations and the qualities
that cause them (e.g., Ayers 1991: 63-4), but although this
might account for the assumption that ideas are in substances, which
Locke indulges, it hardly explains belief in idea-quality resemblance.
Jacovides (2007b) convincingly argues that the usage to which Locke
refers is paronymous, i.e., the name of one thing is given to
something which has a certain relation to the one but with a change of
ending. Locke speaks of the names of general ideas of kinds of things
in this way; e.g., we are denominated "human" from the
abstract idea, humanity (E 3.8.1). In *Essay* 2.8, ideas
produced by SQs are almost always given names of abstract ideas:
"sweetness", "warmth", etc. Jacovides suggests
Locke's point is that the paronymous use of names of ideas of
SQs may convey that there is something in objects that gives them
their colors, etc. The second argument against resemblance questions
why warmth, etc. is supposed to resemble something in bodies but pain
is not (E 2.8.16).



In support of the positive resemblance between PQs and ideas of them,
Locke appeals to practical action: it is by being moved from
*A* to *B* that a body causes the idea of its motion
from *A* to *B*. so the idea represents motion "as
it really is" in the body (E 2.8.18); "A Circle or Square
are the same whether in *Idea* or Existence." (E 2.8.18)
Quick as this argument is, it is taken to show that motion and figure
"are really in bodies whether we take notice of them or
no." (E 2.8.18) The following sections 19 through 21 propose
thought experiments regarding the color of porphyry in the presence
and absence of light, the taste of an almond before and after a
pounding, and a bowl of water in which one person has placed both
hands one of which has been in ice water and the other in hot water.
Curley (1972) decisively argues that none of this is meant to show
that appearances of SQs--unlike PQs--are perceiver relative
unreliable bases on which to judge the properties of things we
perceive. But scholars have had difficulty seeing what the examples
are intended to show: whether to substantiate the resemblance thesis,
the corpuscular theory, or the reality of qualities and non reality of
powers. For different views, see, e.g., Alexander (1985:
124-29), Atherton (1992), Lowe (2005: 51-2), Rickless
(2014: 88-93), McCann (2011: 175-6). Robert Wilson (2004:
204-06); on the color of porphyry, Stuart (2003) and Jacovides
(2007b: 641-2).



Seventh, sections 22-6 stress that PQs are real qualities
whereas SQs are imputed qualities. Two things are said to mark PQs as
real:


* mind-independence: PQs are in bodies whether they are perceived or
not (E 2.8.23); bodies would have them if there were no sensible
beings (E 2.31.2) and
* the dependence of SQs on PQs of insensible particles (E
2.8.23).



The vast secondary literature on this doctrine raises a large number
of debatable questions. The following issues are considered here
because they are particularly important for understanding what is
distinctive about Locke's version of the P/SQ distinction:


* Whether SQs are a sort of quality or a sort of power
* The epistemic basis of the criteria of PQs
* The metaphysical status of SQs
* What it is for the idea of a PQ to resemble a PQ of a body
according to Locke.


### 5.2 Issues


#### 5.2.1 Are SQs qualities or powers?



*Essay* 2.8 speaks of qualities, real qualities, imputed
qualities, powers, and mere powers. Which difference coincides with
the distinction of P/SQs? Locke's report of what he calls
"quality" in the subject, unlike the report of what he
calls "idea in the mind", is often taken to be a
definition (Curley 1972: 443; Rickless 2014: 84; Jacovides 2007a:
110-11; Stuart 2013: 34-9). Accordingly, Curley (1972:
443-45) argues roughly as follows. For Locke, all qualities are
powers to cause ideas by definition, but SQs are such powers and only
imputed to be qualities. So SQs are not consistently said to be
qualities; but they are consistently said to be powers. He concludes
that there are, for Locke, two sorts of powers and the famous
distinction is based on RT. One problem is that RT is presented as a
consequence of the P/SQ distinction, not the source of it (R. Wilson
2015: 200; Rickless 2014: 88).



Jacovides (2007a: 111-12; also Mackie 1976: 14-5) holds
that qualities are defined as powers and argues that this is
consistent with saying that SQs are powers and not qualities because
Locke's notion of power is ambiguous as shown by this:


1. power is an ability to do something which neither causes nor
explains its manifestations, e.g., the ability to digest does not
digest (E 2.21.20)
2. power is "the source from which action proceeds", and
"when they [sc. powers of substances] are exerted into act, they
are called 'causes'" (E 2.22.11).



But these texts contain only one notion of power: (a) argues that
powers are not exerted by powers, but by substances--the will
doesn't will; (b) identifies substances that exert their powers
with causes.



We can suggest a different solution. Locke's remarks about what
he calls "idea" and "quality" in E 2.8.8 are
not definitions by his lights; see E 3.3.15-7 on real and
nominal essence. They are more like sufficient conditions for being
designated by the terms "idea" or "quality."
Being a power is sufficient for being designated by
"quality," as shown by color, etc., but it is not
necessary, as shown by the PQs of insensible particles which are not
powers to cause ideas, but constituents of such powers. McCann (2011:
59-67) argues that the PQs found in the microstructures of
bodies are the causes of both ideas of SQs and ideas of the PQs of
sensible bodies. He urges a divsion of types of PQs based on a
division of labor--insensible PQ are primary PQs whereas PQs of
gross bodies are secondary PQs. For other solutions to the present
issue, see Rickless (2014: 85), Stuart (2013: 34-9), R. Wilson
(2015: 207-8).


#### 5.2.2 Epistemic basis of the criteria for PQs



The view that Locke argues for the distinction of P/SQs, at least in
part, from the truth or adequacy of the CT (held by Alexander 1985:
111-30; Mackie 1976: 17-8) is met with two objections:
Locke's main interest in *Essay* Book 2 is in ideas
(Atherton 1992) and Locke endorses neither the truth nor adequacy of
CT (Ayers 2011: 136-7; Downing 1998; McCann 2011:
179-85).



Robert Wilson (2015) argues that there are four inseparability
criteria for primary qualities; two pertain only to sensible bodies
and the others form a transdictive inference, i.e., an inductive
inference from what is true of all sensible bodies to what is true of
insensible ones (on transdictive arguments, see R. Wilson 2002).
Stuart (2013: 46-52) objects that limiting two modes of
inseparability to sensible bodies lacks textual support. Stuart
maintains that the criteria for PQs are the same as those for ideas
included in the general abstract idea of body. But Jacovides (2007a:
115) points out that Locke's account of this idea includes only
extension, solidity, and capacity to communicate motion, not the full
roster of Locke's PQs (see E 2.23.22).



Michael Ayers (2011: 138-42) suggests Locke's distinction
of qualities can be seen not as a consequence of the virtues of CT but
as a source of its perspicuity. On this reading, the argument of
*Essay* 2.8 is a pre-theoretical philosophical reflection on
the variety of affections bodies are observed to have and the
intelligible mechanical interactions among observable solid rigid
bodies.



Downing (2007: 354-59) argues that the inseparability criteria
are part of a corpuscularian argument for the distinction of qualities
which accords with our natural understanding of physics. Locke does
not endorse the adequacy or truth of CT but, she contends, he regards
it as an imperfect example of an ideal demonstrative physics which
would deduce all operations of bodies from their real essences, or
internal constitutions (see E 3.3.15; 4.6.11). She suggests
*Essay* 2.8.8 through 14 makes an argument that relies on the
implicit ideal (see E 3.3.17). (See Ayers 1981, also Downing 1998)



McCann (2011) objects that Downing's account accords undue
epistemic weight to the natural line of thought in *Essay* 2.8.
He maintains Locke's reasoning is non-theoretical and unable to
justify assent. Its force is limited to showing how "ordinary
folks have to think" about the qualities ordinarily ascribed to
bodies (McCann 2011: 184-6). The discussion has weight enough to
overtop the common views about the resemblance of ideas of SQs to
their causes, which is all Locke intends it to do (McCann 2011: 175,
171-2).


#### 5.2.3 Metaphysical status of SQs and powers in general



According to Curley (1972), Alexander (1974: 168-171), and
others, SQs are textures. Alexander also maintains that colors, etc.
are not SQs but ideas in minds. But the explicit account of SQs in
*Essay* 2.8.10, repeated in E 2.8.23, 24; 2.21.73, stands
against both of these interpretations. Passages that make an analogy
between warmth, etc. and pain suggest secondary qualities are
subjective sensations (e.g., 2.8.17; also 2.8.21), but Robert Wilson
(2015: 199) rightly objects that the sub-species, which includes the
sun's power to melt wax and thereby give it the power to cause
ideas of liquidity, shows SQs to be powers to affect bodies as well as
minds.



George Berkeley's *Principles of Human Knowledge* (1710)
(hereafter "PHK") and *Three Dialogues between Hylas
and Philonous* (1713) (hereafter "TD") have encouraged
this interpretation of Locke's doctrine of SQs. According to
Berkeley, the philosophers' doctrine of P/SQs states that
extension, size, motion, etc. inhere in mind-independent material
substance and cause ideas that resemble them, whereas colors, tastes,
etc. are ideas that cannot resemble anything without a mind. In his
view, philosophers are right about SQs and wrong about the status of
PQs. He opposes the doctrine because it promotes a form of
materialism. The distinction figures in his idealist, anti-materialist
argumentation as a philosophically reputable means by which the
admitted mind dependence of SQs can be used to leverage that of PQs.
One tactic is to claim that arguments supposed to show that color,
etc. cannot exist without a mind can be used against PQs, too, where
the arguments in question are his own perceiver relativity
considerations (TD [2008: 167-70]; PHK [2008: 88]). This is
paired with the contention that we cannot abstract the idea of a PQ
from all ideas of SQs, e.g., the sensory idea of a figure cannot be
separated from all other visual or tactual qualityies. Either both
exist in the mind or neither does (PHK [2008: 70-1, 86]; TD
[2008: 115-6]) Many subsequent philosophers wrongly assume that
Berkeley's report of the philosophers' distinction applies
to Locke's doctrine of qualities (see Stroud 1980; M. Wilson
1982 [1999]])



Most recent scholars hold that *Essay* 2.8.10 states what
colors. etc. are according to Locke: causal capacities with intrinsic
or causal ground in the PQs of insensible particles. Mark Johnston
develops this in terms of a sophisticated account of a dispositional
property understood as a "constituted disposition":
"a higher order property of having the properties which,
oddities aside, would cause the manifestations of the disposition in
circumstances C." (Johnston 1992: 230-34) This comports
with the structure of a SQ according to *Essay* 2.9-10,
but the text says nothing about conditions or circumstances of
manifestation (see discussion of *Essay* 2.30.2 below in this
sub-section).



Scholars also disagree about the metaphysical status of powers that
derive from the micro-structures of observable bodies. Are they
entities that exist in, say, an almond even though they depend on its
basic constituents and presuppose co-existence of a competent
perceiver? Or is there nothing in the almond but the real qualities of
it and its parts?



Jacovides (2007b) argues for the leaner alternative. He maintains that
for Locke, the explanatory entities are PQs, bodies, ideas, and minds.
Because they constitute the facts that determine the truth or falsity
of our assertions about SQs, we have no need of ideas that track their
derivatives (Jacovides 2007b: 630). As he sees it, the doctrine that
SQs are powers is not about what is in the world but Locke's
solution to a semantic problem: a way we can speak that makes
assertions about SQs true and certain and forestalls the mistaken
suggestions of the paronymous use of SQ terms (Jacovides 2007b:
641).



Without objecting to this account of the semantics of names of SQs, we
can add that the doctrine of SQs solves an epistemic problem arising
from Locke's thesis that the limited knowledge we have is
sufficient for our needs (E 1.5). The solution to this problem is in
part about the world. Jacovides overlooks Locke's semantic
theory of simple ideas of sensation in general and its function in his
epistemology. The key texts are *Essay* 2.30.1-2,
2.31.1-2. and 4.4.4. In outline, God sustains steady causal
connections among the different internal constitutions of bodies, the
constitutions of the bodies of perceivers, and all the simple ideas of
sensation that enter their minds. With this foundation in place, the
fact, as Locke has it, that we are aware that simple ideas of
sensation come from without us enables these ideas to be, and be taken
to be, reliable signs of things that are founded in reality (E 2.30.2,
31.2; 4.4.4); for a defense of the natural sign interpretation of
simple ideas of sensation, see Ayers 1991: 60-64. Ideas of SQs,
in particular, are not ideas of micro-constitutions; they do not
inform us of the nature of what they represent. They are ideas of
powers that stand in for the causal foundations that underwrite sense
perception; see Ayers 1997 on the place-holder function of this idea
of power. We cannot have perceptual knowledge of the nature of the
foundations. But because ideas of SQs track collections of PQ of
insensible particles on which the ideas depend, we have the cogntiive
ability to distinguish different objects we have to do with, discern
their qualities, and take them for our needs (E 2.30.3; 4.4.4).


#### 5.2.4 Resemblance of ideas of PQs and qualities of bodies



Locke's RT has spawned confusion on at least two points of
doctrine:


* How resemblance between an idea of sensation and a quality
existing in actual things is to be understood;
* Why RT is important, or what Locke intends to convey by asserting
it.


##### How is "resemblance" to be understood?



Ayers (2011: 151-57) observes that Locke has more than one
account of what ideas are. According to a Cartesian view, ideas are
intentional mental acts by which their objects are presented and
represented to the mind (e.g., *Essay* 1.1.8; 4.1.2)). On this
view, it is not difficult to understand what it is for an idea to
resemble, or not to resemble, its object: it presents the object as
having a feature that resembles (does not resemble) a feature the
object has. But *Essay* 2.8 introduces a different notion of
ideas: simple ideas of sensation are effects of the operations of
bodies on our sensory organs. They are non-intentional sensations akin
to raw pains. They acquire a representative function in virtue of
their contingent causal connections to the constitutions of bodies
around us. In Ayer's view, this makes the claim that ideas of
PQs resemble qualities existing in bodies "problematic, if not
absurd" (Ayers 2011: 154). Here it seems "resembles"
must mean "shares a property with", not "depicts a
thing as having a property it has". Problems with the former
include that essentially mind-dependent ideas cannot be like anything
that can exist without a mind, as Berkeley says. Also, how can we know
what the qualities of bodies are like in themselves given that we know
the qualities only by means of the ideas they cause in us? Also, it is
difficult to understand how an idea can share with a body the quality
being 10 feet long, or moving away from the perceiver at 20 miles per
hour, or being solid, and so on. Most scholars are resigned to saying
or tacitly allowing that if simple ideas are taken to be
non-intentional qualia, the positive resemblance thesis is deeply
problematic.



But Jacovides (1999), undertakes to defend the literal interpretation
of the positive RT. His defense is limited in two ways: it applies
only to visual ideas that are products of experience and metnions only
respects of resemblance that Locke and his contemporaries would have
found acceptable, such as the image of a tri image of a square and a
square. Two broad problems can be mentioned:


* Locke appears to assert that all ideas of PQs--tactual as
well as visual--resemble qualities of their causes and
* on this interpretation, the resemblance might be far from exact;
but the texts speak of "perfect resemblances" and mirror
images (E 2.8.16).


##### Why does Locke want to establish RT?



More central to Locke's distinction of P/SQs is the question why
idea-quality resemblance and non-resemblance are important in
Locke's view. RT is said to be a consequence of the argument
that establishes the P/SQ distinction in *Essay*
2.8-14--what does the further thesis do for Locke? In
sections 16-19, the resemblance/non resemblance dichotomy and
the real/imputed quality dichotomies coincide (Rickless 2014:
88-90). E. J. Lowe (2005: 51-2) suggests that in denying
resemblance in the case of ideas of SQs, Locke "expresses the
view that the ideas mislead us into taking a dispositional property to
be a real non-dispositional quality". But although we do
commonly make this mistake according to Locke, the fault is not with
what ideas represent but our tendency to conflate ideas and
qualities.



McCann (1994: 64), among others, suggests that Locke's purpose
in denying that ideas of SQs resemble bodies is to discredit the
scholastic theory that we perceive qualities of objects by receiving
resemblances of them which are emitted by the objects. One objection
is that Locke wholly repudiates this scholastic theory, so this fails
to account for the positive part of RT.



Jacovides (2007a: 108-110) suggests that epistemic concerns
motivate Locke's adherence to both parts of RT. As he sees it,
Locke is worried that because ideas are the immediate objects of our
knowledge, they may not represent external objects. So he thinks we
should have a presumption against assuming that ideas of sensation
resemble things without us. The pain analogy provides a reason in
support of this presumption. However, Jacovides maintains, Locke
thinks it is overcome in the case of PQs because they play a part in
CT which is our most intelligible account of how things happen in the
world. There is some truth to this general description, but it has no
clear connection with Locke's interest in RT. The presumption is
not explicitly stated, as far as I know, and the epistemic worry about
ideas is stated only in *Essay* 4.4 where RT is not
mentioned.



The arguments addressed to RT turn out to show that a quality is real
just in case it resembles the idea of it. *Essay* 2.8, sections
17 and 18 explicitly state that PQs are real qualities, i.e. qualities
that are in a body whether it is perceived or not, and SQs are not
real qualities; the point is reiterated in sections 22 and 23.
Rickless takes the emphasis on reality and non-reality to show that
for Locke, the most fundamental difference between PQs and SQs is that
the former are real qualities of bodies and the latter are not in the
strictest sense. We can suggest that although this may be the basic
difference from the perpective of natural philosophy, Locke's
interest in the P/SQ distinction derives from his more fundamentl
interest in the relation between perception and reality (see E 2.8.7,
.22), From Locke's perpective, the basic difference is at the
level of sorts of ideas of sensation and the difference has to do with
how they represent the real qualities present in bodies perceived by
the senses.


## 6. Thomas Reid



Thomas Reid, known for his theory of perception and an epistemic
theory he calls "common sense", urges a version of the
distinction of qualities in two of his main philosophical works:
*An Inquiry into the Human Mind on the Principles of Common
Sense* (1764) and *Essays on the Intellectual Powers of
Man* (1785). Both use an empirical method to investigate basic
faculties of the human mind such as perception, conception, and
judgment. Qualities of bodies are identified as the objects of
perception. Among them, Reid finds the foundation of a division
aligned with the standard lists of PQs and SQs presented as a
difference in how we know them. Some scholars maintain his doctrine of
P/SQs is entirely epistemic (Lehrer 1978; McKitrick 2002), but others
contend it depends on a metaphysical distinction (Van Cleve 2015:
110-12).



Reid is also known for his extended critique and unrelenting
repudiation of the way of ideas. He finds it in the whole of
philosophy from Democritus to Berkeley and Hume. Characterizing it as
the view that there is a single sort of mind-dependent entity that we
must immediately apprehend in order to apprehend anything else, he
contends it precludes all cognition of things outside the mind (Reid
1785 [2002: 27-32, 199-200]; Nichols 2003; Van Cleve 2015:
57-98). In his view, it is the source of most false and mistaken
philosophical accounts of perception (Reid 1785 [2002: 249,
261-5]).



Reid's concept of sensation is a main instrument of his
opposition to the theory of ideas. He holds sensations to be
non-intentional objects of consciousness which have potential to be
conceived; they are what they are felt to be and felt to be what they
are (Reid 1785 [2002: 36-8, 193-4]). Perceptions, by
contrast, are cognitive. An episode of perception breaks down to an
act of conceiving an external object and an act of believing in its
present existence. Sensations are, then, wholly extrinsic to
perceptions. But with one exception, perception of all sorts of
objects is attended by a simultaneous sensation which is the effect of
the object operating on sensory organs of the perceiver (Reid 1785
[2002:
 193-4])[5]



Two terms are used for the mind's shift from a non-intentional
sensation to a perception. The sensation is a "sign" to
the mind of the presence of the quality that causes it, in particular,
a natural sign grounded in the constitution of our nature (Reid 1785
[2002: 119]). The sensation "suggests" a certain
conception and firm belief; the one is "naturally and
necessarily annexed" to the others. The transition is
non-inferential, not due to habit, experience, or education. It
follows, according to Reid, that the conjunction of acts is a
consequence of the naturally inexplicable constitution of human nature
(Reid 1764 [1997: 7-8, 40-44]; 1785 [2002:
96-101]).



That sensations and perceptions of the qualities that cause them are
distinct mental acts is, like the theory of ideas, a point on which
philosophical theories of perception founder as Reid sees it (e.g.,
Reid 1785 [2002:199-200]). His empirical evidence shows that
sensations and the corresponding qualities have the same names in
virtually all human languages: "smell of a rose" is
ambiguous as between an aromatic feeling and a quality possessed by a
kind of flower. The quality and the sensation are generally confounded
by imagination as if they were the same operation (Reid 1764 [1997:
43]; 1785 [2002: 193-99]). This is benign for ordinary purposes,
according to Reid, but a thorn in the side of philosophers. It
obscures a difference among the various qualities perceived by sense
that Reid's decisive division of sensation and perception brings
to light.



According to Reid, when experiencing the smell of a rose, you have a
pleasing sensation and directly perceive the quality of a body that
causes it. The sensation is felt; the external cause is conceived and
firmly believed to exist. By contrast, an experience of hardness or
softness involves faint and uninteresting sensations that serve just
as signs to which the mind immediately responds by conceiving and
believing in the bodily qualities they stand for. Pressing a hand on
the top of a table, you have a sensation which is typically unnoticed
and you notice a property of the table, a cohesion of its parts
sufficiently firm to resist any change of its shape. Tactual
perception of figure is similar: when you press your hand around the
surface of a ball, you feel the figure; although the feeling can be
noticed with attention and practice, it is usually disregarded (Reid
1785 [2002: 193-7]; 1764 [1997: 55-7]).



The contrast provides the foundation of Reid's distinction
between qualities listed as primary and secondary. The senses give us
"a direct and a distinct" notion of the primary qualities
and inform us "what they are in themselves", but
"only a relative and obscure" notion of the SQs, one that
tells us "only that they ... produce in us a certain
sensation" and "leave us in the dark" about
"what they are in themselves" (Reid 1785 [2002:
200-203]; 1764 [1997: 61-2]). By a "distinct
notion", Reid means one that provides clear and distinct
understanding of its object. Our notion of hardness is said to inform
us perfectly of what that quality is; "it is manifest to
sense" (Reid 1785 [2002: 203]). By a "direct
notion", he means one that is not relative. A relative notion of
a thing, "is, strictly speaking, no notion of the thing at all,
but only of some relation which it bears to something else"
(Reid 1785 [2002: 200-03]). For instance,



> 
> 
> That smell in the rose is an unknown quality ... which is the
> cause ... of a sensation which I know well...The same
> reasoning will apply to every secondary quality. (Reid 1785 [2002:
> 202])
> 
> 
> 



Two of Reid's reflections on this contrast are especially
important. First, there are no disputes about the natures of the
primary qualities because the are exhibited to sense. Primary
qualities are investigated by the *a priori* methods of the
mathematical sciences. Reid attributes this to the distinctness of our
concepts of them and the knowledge of their natures the concepts
provide. They are subjects of conceptual necessary truths. But because
secondary qualities are obscure to sense, they are an appropriate
subject of the sort of formal inquiry conducted in natural philosophy.
Their natures are not known directly by the senses but may be
discovered by observation, reasoning, and deft use of hypotheses;
e.g., we know vibrations cause sounds and reflected light causes
sensations of colors (on the proper method of natural philosophical
inquiry (see Reid 1785 [2002: 267-75]).



The second reflection concerns the "capitol part"
sensations bear in the "notion we form of the [object
perceived]" (Reid 1785 [2002: 204]). Unlike the easily forgotten
sensations that accompany perception of PQs, many sensations caused by
SQs have a salience that makes us attend to them and form concepts of
them. They are not just signs of the qualities when we perceive them:
"the thought of a secondary quality always carries us back to
the sensation it produces", This is because although each
perception is conjoined with a sensation proper to it (Reid 1785
[2002: 199]), relative notions specify every object as an unknown
cause. We have only their known effects by which to reflect on them
and mark them as different from each other. This has been called the
"capitol part" thesis. In effect, post-perceptual
apprehension of SQs is mediated by conception of the sensation that is
its sign--apparently, a mediation Reid considers innocuous (see
Reid 1785 [2002: 134]; for different views about what that might be,
see Nichols 2007: ch. 10; Copenhaver 2004; Buras 2011; Van Cleve 2015:
297-300). The implications of this for the meanings of SQ terms
is disputed by scholars.



Two main issues are debated in the literature on Reid's P/SQ
distinction:


* Whether SQs as known by our senses are dispositions or causal
bases of them;
* Whether or not the capitol part thesis commits Reid to the view
that sensations are semantic constituents of the meanings of SQ terms
(Lehrer 1989: 27).


### 6.1 Are SQs dispositions or causal bases?



Some texts suggest that secondary qualities as known by the senses are
dispositions, notably:



> 
> 
> [color] is a certain power ... in bodies that in fair daylight
> exhibits to the eye an appearance which is very familiar to us
> although it hath no name. (Reid 1765 [1997: 87])
> 
> 
> 



But there are more texts that identify SQs with the unknown causes of
familiar sensations, such as:



> 
> 
> The qualities in bodies we call *heat* and *cold* are
> unknown. They are only conceived by us, as unknown causes or occasions
> of the sensations which we give the same names. (Reid 1765 [1997: 54;
> especially 86]; 1785 [2002: 202-04], among others)
> 
> 
> 



Nowadays it is generally accepted that a disposition and the cause of
its manifestations are distinct; it is not possible that they be the
same thing.



Wolterstorff offers this argument:



> 
> 
> if green were a disposition in things to cause certain sensations
> under certain conditions and not the physical basis of that
> disposition, we would know what it was. (Wolterstorff 2000: 112)
> 
> 
> 



This might well be based on a paragraph in *Essays on the
IntellectuaI Powers of Man* (Reid 1785 [2002: 203-04]). It
states that feeling informs us that the fire is hot but not what heat
is in the fire; it notes that this may seem to be a contradiction but
says it ought to be accepted. It allows that if we had no notion of
heat we could not understand the assertion that fire is hot, and
concludes "we have a notion, but it is a relative notion. We
know they are the causes of certain known effects" (Reid 1785
[2002: 203-04]). This might naturally be understood to invoke
the obscurity of relational specifications of qualities to dispel the
seeming contradiction.



Van Cleve (2015: 105-110) dismisses Wolterstorff's
argument on the ground that there is an equally good counter argument
as follows. It is an epistemic principle that you know *X* is
*F* only if you know what it is to be F; Reid thinks we know of
many things that they are green; so, we must know what it is to be
green. Someone might object that Reid adheres to a different epistemic
principle in the passage we just discussed. In a note citing this
text, Van Cleve admits Reid's position is unclear but takes the
last part of the passage to say the relative notion enables us to know
what it is to be green, namely, to manifest certain sensations. In his
opinion, Reid favors this view of SQs. Reading the passage this way
renders the contradiction irrelevant but leaves the many texts that
would seem to identify SQs with the unknown causes of known effects
out of account. As Nichols (2007: 6.4) puts it, the dispositional
interpretation makes nonsense of Reid's frequent claims that he
does not know what SQs are. Nichols finds evidence of the causal basis
theory in Reid's use of SQ terms in talking about the causes of
SQ sensations and concludes that all relevant texts considered, we
should take him to accept a physical microstructural account of SQs
(Nichols 2006: 6.6).



Van Cleve elaborates on the dispositional interpretation in two ways.
First, noting that Reid is drawn to the view that SQs are unknown
causes, he suggests that although it was not available to Reid, a
second order dispositional account might accommodate the conflicting
texts: to have a SQ is to have a property *P* such that
*P* would cause certain sensations in circumstances *C*
(Van Cleve 2016: 109). Second, he notes that if SQs are dispositions
manifested by human responses, they are relational properties, whereas
PQs seem to be intrinsic. He argues that the latter is borne out under
a test of intrinsicality along lines proposed by G.E. Moore (Winkler
2016: 225-31 challenges this; Van Cleve 2016: 240-44 is
his response). From this, Van Cleve concludes that Reid's P/SQ
distinction rests on a metaphysical foundation (Van Cleve 2016:
110-12).


### 6.2 The capitol parts thesis and the semantics of SQ terms



This is an issue on which more recent scholars tend to disagree with
some earlier ones. The capitol part thesis states that we cannot
reflect on or distinguish among SQs without conceiving the sensations
that signify them. This suggests a close conceptual connection between
qualities and their proper sensations. Lehrer (1989: 27) maintains it
commits Reid to a view on which the meaning of, say, "*X*
is red" essentially contains concepts of sensations of red. If
Reid holds that SQs are dispositions manifested by sensations. he is
committed to this semantic claim. But the present question is whether
or not the capitol part thesis commits him to the semantic claim.



The thesis about post-perceptual conception of a SQ entails two claims
that are directly relevant: that we cannot form the concept of a SQ
without at the same time forming the concept of the sensation it
causes and that we cannot have the concept of a SQ unless we have
experienced the quality. The need for experience is often taken to
support the view that PQs and SQs differ in respect of how their
meanings are specified: in particular, it is said that
"*X* is red" means that *X* manifests
sensations of red under certain circumstances, whereas the meaning of
"X is solid" can be specified without reference to
sensations of solidity. But recent scholars argue that the semantic
claim about SQ terms is not entailed by the capitol part remark. They
point to reasons for a person's needing to have the concept of
the sensation caused by a quality in order to have the concept of that
quality--the capitol part thesis--that are independent of
the truth or falsity of the semantic claim. Van Cleve points to the
use Kripke might make of the perception of SQs to fix the reference of
"red" in its rigid use, i.e., use as a name for just those
qualities that cause a certain sort of sensation in the actual world.
Nichols (2006: 6.4) offers a different reason for a similar
conclusion. But although recent scholars agree about this, they
disagree over how Reid intended the capitol part thesis to be
understood.



Van Cleve reports his belief that Reid meant it to entail that SQs are
dispositions which cannot be conceived unless conceived as manifesting
certain sensations. SQs are, then, essentially phenomenal or human
response dependent (Van Cleve 2016: 107-9). No argument is
offered other than the one purporting to show that Reid can be read as
urging a dispositional theory of SQs (see above). There seems to be no
consensus on the interpretative issue.