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A Physicalist Theory of QualiaThe Monist, 68 (4), October 1985, 491-506.

Austen Clark

Department of Philosophy U-54University of ConnecticutStorrs, CT 06269-2054

AbstractAlthough the capacity to discriminate between different qualia is typically admitted to have

a definition in terms of functional role, the qualia thereby related are thought to eludefunctional definition. In this paper I argue that these views are inconsistent. Given a

functional model of discrimination, one can construct from it a definition of qualia. The

problem is similar in many ways to Goodman's definition of qualia in terms of 'matching',and I argue that many of his findings survive reinterpretation into a physicalistic basis

which employs 'indiscriminability' as its primitive term. I show how one can identify the

critical properties to which discrimination capacities are sensitive, and then identify their

order. A problem arises concerning the different ways in which qualitatively distinctexperiences can differ (hue, shape, and so on). Physicalist accounts have often been

accused of relying in a circular fashion on some antecedent understanding of phenomenal

properties in order to specify those differences. This account avoids such an accusation:ordering of critical properties is determined by the dimensionality of discriminations, and

the latter is given by the structure of the discrimination pair lists. Once a topology of

quality is constructed, qualia names can be defined by their relative location within theorder. In the conclusion I argue that psychophysics employs physicalist techniques to define

a topology of quality, and that it can provide what Thomas Nagel calls an "objective

phenomenology."

The qualitative content of experience is commonly thought to elude functional definition,

as qualia seem to have intrinsic non-relational properties. In contrast, the relationship of

qualitative similarity is typically ceded to functionalism, as there seem to be no intrinsicnon-relational properties involved in the capacities of discrimination. In this paper I will

argue that these intuitions are inconsistent. If one admits the possibility of a functional

definition of qualitative similarity, one can construct from it a definition of qualia.

The key moves in this construction are inspired by Goodman's Structure of Appearance.

Starting from a phenomenalistic base, Goodman showed how identity criteria forqualia could be framed given the following conditions:

(N) Ifx andy do not match,x andy present distinct qualia,and

(S) Ifx andy present distinct qualia, then there is somezwhich matchesx but noty.

It is difficult to see how either (N) or (S) could sensibly be denied. Denial of (N) would

amount to the claim thatx andy may sometime present identical qualia even though they

do not match. (S) is much weaker than the claim that matching provides a sufficientcondition for qualitative identity; to deny it one must find instead that x andy each match
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the same sets, yet fail to be qualitatively identical. Denying either condition seems

implausible. But Goodman showed that with them one can achieve the seeminglyimplausible project of defining "qualia."

This achievement has important consequences for the debate concerning functionalism and

qualia. For example, physicalist accounts of secondary qualities and sensations have

repeatedly been accused of circularity. The physicalist has traditionally attempted to definephenomenal properties in terms of phenomenal similarities. Smart analysed "I have ayellowy orange afterimage" as "Something is going on in me like what goes on when I see

an orange." This account was challenged as not specifying the respect in which the

experiences were alike. Having a yellowy orange after image is like seeing anorange in one respect, but it is also like seeing a basketball. To define phenomenal color,

one must somehow specify that it is the latter aspect that is relevant, not the former. But, the

argument goes, to do that, one must name the phenomenal property which the experiencesshare. So any physicalist reduction of phenomenal properties is circular, resting ultimately

on unanalyzed phenomenal properties.

A major task to this paper is to show how a definition of phenomenal properties can avoid

such circularity; how, that is, one can specify the way in which two experiences need belike one another without presupposing any notion of their having the same phenomenalproperties.

Goodman's system employed a phenomenalistic, nominalistic, and realistic base; the qualia

debate now typically assumes a vocabulary which is physicalistic, platonistic, and

particularistic. However, in this paper I will show that constructions from the latter basehave the wherewithal to ensnare qualia. If one accepts a functional definition of

discrimination capacities (as is plausible), and the physicalist analogs of (N) and (S) (as are

also plausible), a consequence is a constructional definition of qualia.

1.To begin, we allow primitives referring to things, to parts of things, and to the relations'simultaneous with' and 'later than'. A slice or stage can be defined using those primitives:x

is a slice ifx is a part of some thingy, and all parts ofx are simultaneous. Our variables

range over space-time regions of finite extent, so we count things and thing-slices as theirvalues, but not space-time points.

We need certain predicates to describe relationships between slices. The first is that of a

'generalizable slice sequence' which obtains between part of a slicex and part of some later

slicey just in case there is a sequence of slices fromx toy which instantiates a law. Supposebetween subslicesx andy there is a chain of subslices related to one another by contiguity

and temporal succession. For example,x is contiguous to some u and some successor ofu

has a part contiguous toy. If that sequence is an instance of a law, then betweenx andy

there is a 'generalizable slice sequence.' Each later stagezin the sequencebetweenx andy is an instance of some generalizable principle of association: some

function projectingx toz.

We need a predicate to describe the information-theoretic notion of one slicex signalling or

encoding occurrence of some other slicey. This notion requires specification of an inputensemble, an output ensemble, and a collection of contingent probabilities relating inputs to

outputs. An ensemble can be considered a class of event classes. To signaly, slicex must

fall in a class of slice types which is one member of the output ensemble--a class of such
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classes. Furthermore, there must be relations of dependence between ensembles, so that

(roughly) the conditional probability of some input eventy, givenx, is different from the apriori probability fory alone. One can give a fully explicit definition of 'signal' in terms of

the conditional probabilities relating input events to outputs.

Finally, the basis will include the resources of second order logic, so that we can quantify

not just over things and slices, but also over sets of things and slices.2.The key psychological predicate needed in order to define qualia is "indiscriminable," orI(x,y,p), holding between stimulix andy for personp whenp cannot discriminate between

x andy. It means something different than "matches" or "looks the same," and the

differences are important to specify at the outset.

By "indiscriminable" I mean that the subjectp literally cannot tell the difference betweenx

andy. It does not mean that on casual or even scrupulous inspection, the subject avers thatx andy 'look the same,' for the subject may aver that even if, in some situation, the subject

could tellx fromy. "Match" is closer in meaning, but still does not carry the implication

that there is no way in which the subject can tell the difference between stimuli x andy.

How does one assess indiscriminability? One method uses a forced-choice task. Label onestimulus the 'target', and present both simultaneously, altering the placement of target from

left to right in a random way. Require the subject to identify the target each time. If, over a

sufficiently large number of trials, the subject's identification of the target is not statisticallydistinct from a random distribution, then the two stimuli are indiscriminable for the subject.

If, however, the subject can pick out the target at a better than chance level, then he or she

can discriminate the two stimuli.

Discriminability is a purely physical notion. It is a term which can immediately be admitted

into a physicalist vocabulary, as it merely ascribes a certain statistical relation amongclasses of stimulus events and choice behavior.

Information theory provides a useful perspective on discriminability. Consider stimulix andy the input ensemble, and choice behaviors ('left' vs. 'right') the output. Now ifx andy are

discriminable, then there is a statistically significant difference between the distribution ofchoices and a random distribution. There is a higher contingent probability that the target is

on the left, given the choice 'left', then the a priori probability that the target is on the left.

Hence the choice behavior signals the input, and a channel exists between eventensembles. The information concerning the difference betweenx andy is retained by the

system and is reflected statistically in behavior.

In order to explain information transfer, one will naturally posit some generalizable slice

sequence between the stimulix andy and behavior. Suppose there is such a

sequence. Ifx andy are discriminable, then in each slice in the sequence the effects ofx and

y must have some different properties, which retain the information that they are distinct.For suppose that at some slice effects ofx had all the information bearing properties of

effects ofy. Then no successive slice could yield different effects forx than fory, and theinformation concerning their distinctness could never reliably be regained. So ifx andy are

discriminable, then at every stage between presentation of stimuli and behavior,

information bearing properties of the effects ofx must differ (in some way) from those ofy.
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Is the converse true as well? That is, ifx andy are indiscriminable, must there be some

stage at which effects ofx andy are identical with respect to all their information bearingproperties?

If indiscriminability ofx andy implied that at some stage effects ofx andy had such

identical properties, then indiscriminability would be transitive. Supposex andy are

indiscriminable, so that (by hypothesis) encodings ofx and ofy at some stage u haveidentical information bearing properties. Suppose also thaty andzare indiscriminable, sothat encodings ofy andzhave identical information bearing properties at stage u. Then

there would be a stage at whichx andzare encoded identically, and sox andzwould be

indiscriminable.

But indiscriminability is intransitive. Two pairs may each be indiscriminable, but theendpoints (x andz) sufficiently different to be discriminated. The difference betweenx and

y may be just small enough that the subject cannot reliably tell them apart. Likewise, the

difference betweeny andzmay be below a threshold for discrimination of differences. Yetthe accumulated differences betweenx andzmay allow reliable discrimination. A simple

example is hue discrimination, where the difference in wavelengths of monochromatic

stimulix andy is below the threshold for hue difference, as is the difference betweeny andz, but the sum of the differences (fromx toz) is not.

Since indiscriminability is intransitive, it is consistent with there being no stage at whicheffects ofx and ofy have identical information bearing properties. They may have distinct

properties at every stage of processing yet remain indiscriminable. Indiscriminability is not

co-extensive with identity of properties of encodings at some stage of processing. Presenceof distinct encodings is a necessary condition for discriminability, but not a sufficient

condition. To develop the latter we need to consider further the channel between stimuli and

behavior.

3.A stimulus is an event occurring at one or another sensory transducer: retinal rods, musclestretch detectors, cochlear membrane cells, and so on. A thing or slice within the proximity

of a person is no stimulus unless it affects a sensory neuron in some way, and rather than

attempt to identify which element of the causal path leading to that effect is the stimulus(the illumination, the reflectance properties of the surface, the reflected light, the light

entering the eye, or the light absorbed by light-sensitive cells), one can simply specify

stimuli as the furthermost afferent events in the nervous system.

This causal path can be extended into the brain. For example, events in retinal rods arerelated to later events in retinal bipolar cells by generalizable functions, and thence to

events in retinal ganglion cells. At each stage there is some later subsectionzsuch that the

relation betweenx andzis an instance of some generalizable principle of association: some

function projectingx toz. These functions are determined by the biophysical workings ofthe cells. We need not assume that the function is deterministic; it may, for a given slice,

yield no unique successor, but only a probability distribution for a range of potential

successors. Nevertheless, a stochastic function satisfies the definition for generalizablesequences.

I shall call the function mapping stimulus events to properties of sections of later stages an

encoding function. There is an encoding function fromx toy if and only ifx is a stimulus

event and there is both a signal relation and a generalizable slice sequence betweenx andy.


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Fory to be a 'code' forx, both must be members of classes found in ensembles, and

between the ensembles there must be an information channel. Furthermore, thisinformational relationship must be instantiated in a physical channel (a generalizable slice

sequence) to rule out the possibility that it is a mere 'ghost' channel arising from some

common cause of bothx andy. No particular anatomical claims are implied

concerning encoding: we do not require that the function always be subserved by the samebiological structures.

4.Now we are ready to describe a necessary condition for indiscriminability. Recall that a

sufficient condition forx andy to be indiscriminable is that at some stage the information

bearing properties of encodings ofx andy be identical, so that the system loses theinformation thatx andy are distinct. However, the converse does not hold, since there can

be indiscriminable differences between encodings ofx andy at every stage. Since a

necessary condition for indiscriminability is just a sufficient condition for discriminability,we ask: what general definition can be given for the conditions under whichx andy can be

discriminated?

An informal presentation will be given first. To discriminate is to compare. At some stage

of processing there must be a 'discriminal process' in which properties of encodings arecompared. Some differences between properties of encodings of stimuli are

insufficient to surpass the threshold of the discriminal process, in which case the respective

stimuli are indiscriminable. Other differences are sufficient to reject a match. Properties ofencodings sometimes sufficient to reject a match and assure discriminability of the

respective stimuli will be called 'critical' properties. For example, color sensations

presumably have critical properties corresponding to hue, saturation, and brightness. If

encodings share all critical properties, then there is no discrimination between their stimuli,and they match. If encodings differ in a single critical property, then their comparison does

not yield a match, and the stimuli they encode are discriminable. A sufficient condition for

discriminability is that encodings ofx andy differ in at least one critical property.

This would all be thoroughly circular unless one could give a noncircular way of specifyingthe critical properties, but the latter can be done. Roughly, a critical property is any property

which if not shared by both encodings makes their respective stimuli discriminable. Slices

u and v encode indiscriminable stimuli if they both share any property which, if missing ineither, would make those stimuli discriminable. Note that this is not a mere tautology. Let

us suppose u is an encoding of some stimulusx (for which I will write u = e(x)), and v is

one ofy. PropertyPis a critical property ofu just in case there is a v such that a difference

between u and v inPis alone sufficient to assure the discriminability of the stimuli u and vencode. One way to capture the force ofPbeing alone sufficient to assure discriminability

is ifu and v match in all properties Q other thanPwhich can assure discriminability among

any stimuli. SoPis a critical property ofu if and only if there are stimulix,y and encodingv such that:

(i) u = e(x) & v = e(y)

(ii) ~(Pu Pv)

(iii) ~(Pu Pv) ~Ixy

(iv) ( Q)(s,t,w,z)( (s = e(w) & t= e(z) & (~(Qs Qt) ~Iwz) & ~(Q = P) ) (Qu Qv) )


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What makesPcritical is that encodings u and v fail to match in it (ii), that failure is

sufficient to make their respective stimuli discriminable (iii), and furthermore that it alonesuffices to make them discriminable, since u and v match in all other properties Q which

could account for that discriminability (iv). Using 'Pis critical' to abbreviate that condition,

our minimal theory of discrimination is:

Ixy (P)( (Pis critical & (~(Pu Pv) ~Ixy) ) (Pu Pv) )and

(P)( (Pis critical) (Pu Pv) ) Ixy

The first says that ifx andy are indiscriminable, then their encodings match in all those

critical properties which if not matched would suffice to makex andy discriminable. Thesecond says that if their encodings match in all critical properties (not merely those that

would makex andy discriminable, but all of them), thenx andy are indiscriminable.

Neither conditional is a logical truth since both relate properties of encodings todiscriminability of stimuli. But the first is a truism, while the second is not. It is a truism

that ifx andy match, then if the failure of encodings ofx andy to share some critical

property would lead to a mismatch, then encodings ofx andy share that critical property.But the second conditional is not a truism. The mere absence of any guarantee of

discriminability does not alone guarantee indiscriminability.

It would, however, if we assume that stimuli are indiscriminable unless proven otherwise.

That is, the discriminal process initially presumes no difference between stimuli, and

proceeds on that basis until a difference in critical properties is found. Absence of evidencethat the stimuli are distinct is then sufficient for them to be indiscriminable. What are the

critical properties? Just the ones which, if present, constitute sufficient evidence that the

two stimuli differ. If there is some way of defining what makes a property 'critical' whichdoes not rely on its being sufficient for discrimination, then this construction is absolved

from the threat of circularity. Such a definition will be provided in the next section.

The presumption favoring indiscriminability explains several characteristics of qualia. Oneconsequence is the intransitivity of matching. That both pairs (u,v) and (v,w) share allproperties which if absent in one would lead to a detectable difference does not show that

the pair (u,w) also share all such properties. Clearly v may have some propertyPwhich

differs from any found in u but which does not renderu and v discriminable, and w may

have some further property P'distinct in the same way from properties ofu, whileP'maysuffice to renderu and w discriminable.

Second, identity of critical properties of encodings parallels Goodman's definition of

identity of qualia: not merely that the two stimuli are indiscriminable, but rather that they

both match (are indiscriminable with) the same sets of stimuli. They may differindiscriminably in certain ways, and identity is only guaranteed by indiscriminability with

the same collection of other terms.5.How can one define the critical properties of encodings of stimuli? Thus far our only

criterion is that a critical property is any property of encodings which if not shared by bothmembers of a pair suffices to make their respective stimuli discriminable. While it logically

follows that indiscriminable stimuli lead to encodings which share all critical properties,

unless some independent definition for 'critical' is given, the sufficient condition fordiscriminability is circular.


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Critical properties will reveal themselves indirectly in the data concerning matches. If there

were just one critical property relevant to the matching of a set of stimuli, then allencodings of those stimuli could be ordered in terms of that property. Any two encodings

sharing it will encode matching stimuli: there is no other difference which could occasion

discriminability. If there are two critical properties, then even though encodings u and v

may be alike in one of them, they can still differ in the other, and therefore still bediscriminable. For example, things matched in brightness may differ in hue. Similarly, if

there are three critical properties, then encodings u and v can match in two critical

properties and still be discriminable, since they may differ in some third respect.

The dimensionality of discrimination pair lists clearly gives a lower bound for the numberof critical properties of encodings of stimuli. Does it give an upper bound as well? To say

thatPis a critical property of encodings is to say that there are stimulix andy such that

mismatch inPof encodings u and v alone suffices to makex andy discriminable. Can sucha property fail to appear as a dimension of the discrimination pair list? Suppose that u and v

share n critical properties but fail to shareP, and thatx andy are discriminable. In terms of

the n dimensions provided by critical properties ofu and v,x andy are in the same 'place';

but the fact that they are discriminable implies they cannot be in the same place. Such asupposition implies a failure in n-dimensionality of the pair list. Since this is true of every

critical property, there are no critical properties not revealed by the dimensionality of

discriminations; and so the latter provides an upper bound for the former.

Critical properties are therefore revealed as dimensions along which encodings can differ. Ifthere are n + 1 critical properties among encodings, then u and v can match in n properties

and their stimuli remain discriminable. The key to avoiding circularity is that

dimensionality will reveal itself in the structure of the list of pairs which are judgedto be indiscriminable. By examining that list, the dimensionality of the similarity judgments

can be determined; and the latter provides the means to identify the critical properties of

encodings.

How is a two dimensional structure revealed in the pair list? Suppose we have three stimulisuch thatIxy,Iyz, and ~Ixz. Encodings ofx andzare discriminable, andy is 'between' them. If such encodings have but one critical property, then any stimulus u

indiscriminable from bothx andzwill also be indiscriminable fromy. (Ifu is 'between'x

andz, then it must be indiscriminable from that pointy which is indiscriminable from both.)But with two dimensions, this condition fails. Then there can be some u indiscriminable

from bothx andzand discriminable fromy. Intuitively, u does not lie on the line betweenx

andz(as it must, if the encodings have one critical property) but rather somewhere else inthe (two dimensional) plane. So the encodings are shown to be two dimensional.

Two dimensionality is demonstrated by the failure of some point to be colinear with others.

Failure of colinearity is shown by the discriminability of two stimuli which are both

indiscriminable from the same pair of points. Three dimensionality will be shown by afailure of some point to be coplanar with others, by its failure to lie within a square.

These definitions generalize in an easy way. We show discriminations areN+ 1dimensional by showing that the pair list could not beNdimensional. Failure inN

dimensions is demonstrated by finding anNdimensional structure (line, plane, cube, and so

on) and a point which is not co-n-dimensional. Each corner is indiscriminable fromadjacent corners and discriminable from all non-adjacent corners. Each edge represents the

relation of indiscriminability, and any distance longer than an edge represents a relation of


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discriminability. We find a point contained 'within' the n dimensional structure (i.e.,

indiscriminable from its far corners) yet failing to match any other corners. Since it cannotboth be within the structure and fail to match some other corner, the discriminations cannot

beNdimensional.

A slightly easier construction employs relative similarity. First we define relative similarity

in terms of indiscriminability. A stimulusx is more similar toy than toz(which I will write'Sxyz') if and only if the power ofI(indiscriminability) holding betweenx andy is less thanthe power ofIholding betweenx andz. Intuitively,x is closer toy than tozbecause the

number of steps required to get fromx toy is less than the number required to get fromx to

z.

Relative similarity then gives a easy definition of dimensionality. Topologically, relativesimilarity corresponds to distance, so that ifx is more similar toy than toz, thenx is 'closer'

toy than toz. Dimensionality is revealed in the structure of the triples list of relative

similarity. Once again the definition is recursive and proceeds by setting a minimumdimensionality to the structure. Certain combinations of triples are impossible if the

structure is n dimensional, and show it to be at least n + 1 dimensional. The construction

proceeds by defining when one point is co-n-dimensional with other points (co-linear, co-planar, co-3-dimensional, and so on), and then defining the dimensionality of the list as thesmallestNsuch that all terms are co-N-dimensional.

6.Before proceeding it is worthwhile to show that the definitions in the last section are

workable, and provide the basis for psychophysical techniques determining the critical

properties of encodings.

Judgments of relative similarity provide the basis for a family of procedures used in so-called 'multidimensional scaling.' Subjects are given a list of objects and asked to judge

relative similarities: whetherx is more similar toy than toz. One can then determine the

number of dimensions required to describe the data, and plot the objects in a space definedby those dimensions using matrix algebra. Distances in that plot correspond to relative

similarity. In this way our intuitions on the attributes of sensation can be subjected to

empirical test. This has been done, for instance, with color. Numerous triples of color chipswere presented to subjects, and the only data collected were judgments of relative

similarity: whetherx was more similar toy than toz. The resulting structure was found to

be three dimensional, and calculated distances between chips were in close agreement withmore direct psychophysical approaches.

The somewhat simpler two-place predicate 'indiscriminable' or 'matches' provides the

foundation for the classical psychophysical techniques, including the method of limits, the

method of adjustment, and the method of paired comparisons. In the method of limits the

experimenter creates ascending and descending series of stimuli proceeding inindiscriminably small steps, and repeatedly assesses the point at which the subject says the

series matches or fails to match the target. In the method of adjustment the subject is

provided with a target and a method of adjusting physical parameters of a second stimulus,and is asked to adjust those parameters until the stimuli match. Finally, in paired

comparisons the experimenter simply presents the subject with many pairs of stimuli and

asks the subject whether they match or not.


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The payoff from all these techniques is a topology of quality: an ordering (in terms of

physical parameters) of all the potentially detectable differences between stimuli. One givesa physical description of just those conditions under which stimuli are discriminable.

In short, the dimensionality of discriminations can be determined relatively directly via

multidimensional scaling, or less directly via a definition of distance and subsequent model

fitting. Since dimensionality determines the number of critical properties, anddimensionality can be empirically established by various psychophysical proceduresapplied to the discriminations, it is clear that the definition of 'critical property' is not only

noncircular but also empirically useful.

7.With necessary and sufficient conditions for 'critical property' defined, we can now present

analyses of a host of notions including qualitative similarity, perceptual qualities, andqualia.

a. The first provides physicalist analogs for Goodman's principles (N) and (S) above. The

relationship 'presents the same qualia' has (at least in one standard sense of 'qualia') exactly

the extension of 'has the same critical properties', as given in the account above.

First we should clarify the meaning of the relationship 'presents the same qualia,' which Iwill also call 'qualitative' or 'phenomenal' identity. It is the relation which obtains ifu

presents just the same phenomenal appearance as v. Visually such a relation is that of

'looking phenomenally the same'; in bodily sensations it is that of 'feeling the same'. I shalluse 'seems(ph)' to stand for the general notion. It is a relation obtaining

between encodings of stimuli, not between stimuli.

Clearly qualitative differences bear some relation to discriminability, and I shall argue that

the relation they bear to discriminability is exactly that satisfied by critical properties of

encodings.

First suppose that u presents exactly the same qualitative content as v, so that u and v are

qualitatively identical. The subject then could not tell apart the stimuli occasioning u and v,and they would be indiscriminable. Put another way: if the subject can discriminatex from

y, then there must be some difference in the qualia they present. So qualitative identity issufficient for indiscriminability.

Suppose conversely thatx andy are indiscriminable. Mustx andy then present the same

qualitative content? While indiscriminability is intransitive, qualitative identity presumably

is not; hence we must allow indiscriminable qualitative differences. What assuresqualitative identity is not merely thatx andy be indiscriminable, but that they each be

indiscriminable from the same set of stimuli. This is precisely Goodman's definition of

identity of qualia, and it is precisely coextensive with identity of critical

properties as developed above. That is, ifu and v differ in some critical property, then there

is some w indiscriminable from one but not the other; so ifu and v are indiscriminable fromthe same set, then they share all critical properties. Qualitatively identical encodings offer

no proof to the discriminal process that their respective stimuli are different.

To defeat this identification one must either show qualitatively identical experiences whichare nevertheless discriminable, or experiences which both are qualitatively identical to the

same sets yet which are qualitatively distinct. Since neither seems plausible, Goodman's

definition survives interpretation on a physicalist basis.


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b. Since qualitative identity is transitive, we can form equivalence classes using it. Sensory

qualities correspond to classes of objects whose encodings fall into an equivalence classwith respect to qualitative identity. A color predicate, for example, is a predicate applied to

stimuli which are encoded in such an equivalence class. Two things are in the same color

class if the visual encoding function maps both into the same equivalence class with respect

to seems(ph).Color cannot be defined as a physical attribute of things or a psychological state ofobservers. A relational account is required, which identifies physical attributes in terms of

the relations they bear to human visual processing. A color is a power of a thing to affect

your visual system in a certain way. This is similar to the classical definition of secondaryqualities, but differs in that the similarity of effects which two things of the same color have

is not described as 'leading to experiences of the same kind', but rather as 'falling in the

same equivalence class with respect to phenomenal identity'. Two things have the samecolor if they are encoded so as to match in all critical properties. Since critical properties

can be identified from the dimensionality of discrimination pair lists, this account does not

rest on any circular definitions of 'same kind of experience'.

c. When it comes to defining a particular perceptual quality (such as red), the relation ofqualitative identity does not enable one to name any particular equivalence class. To attachnames to classes some further step is required. This can be done indexically. We define 'red'

by (at some time) picking out some red exemplar, and allowing 'red' to characterize the

class of things whose encodings fall in its equivalence class with respect to qualitativeidentity. The paradigm and all other red things cause sensory codes which match in all

critical properties. Let 'p' name the ostended paradigm and 'Suv' mean that u and v are

qualitatively identical. Then

red(x) (u)( u = e(x) & S(u,e(p)) )

That is, the encoding ofx is within the same equivalence class relative to phenomenal

identity as is the encoding of the ostended paradigm. So

red(x) (u)( u = e(x) & ( P)(Pis critical (Pu Pe(p)) ) )

To use color names, the structure of color similarity must be attached to things at several

points, and this can only be done by ostension. The use of indexicals within definitions is

perfectly legitimate, however, as shown by recent work on natural kinds.

One way in which the analysis is idealized is that a thing can be red without being literally

indiscriminable from any red exemplar ever ostended, as long as it is relatively moresimilar to the red exemplars than to others. The analysis also requires as many paradigms as

there are discriminable colors. A simple way to avoid both difficulties is to employ the

relationship of relative similarity. We pick out five or six exemplars or paradigms of colors.Then 'red' is the class of things which are more similar to the red exemplar than to any of

the other exemplars. The occurrence of the term 'red' in the definiens is again eliminated byostension. The judgments of relative similarity need to be judgments of similarity in respectto hue (and not shape, for example), but that can be guaranteed by identifying the

appropriate dimensions in the discrimination pair list.

d. Qualia are properties of sensations; and if one asks "which properties?" the immediate

answer is that they are those properties which enable one to differentiate different

sensations and identify similar ones. Similarities and differences among sensations arerelated to judgments of similarity and difference among things. If two things present
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identical qualia then the things are judged to look the same or match. If two things present

different qualia, then the two things do not match the same set of things. In short, qualia canbe identified with critical properties of encodings of stimuli.

To say that u is a red sensation is to say that u is the encoding of somex and that u has all

those properties which are such that ifu lacked one,x would be discriminable from some

paradigm red thing. So u and the encoding of the paradigm share all properties which ifabsent in either encoding would makex and the paradigm discriminable. Letp name someostended paradigm of a red thing. Then u is a sensation of red if and only if

(x)( u = e(x) & (P)( (Pe(p) & (~ (Pu Pe(p)) ~Ixp) Pu ) )

Red qualia are universals instantiated in red sensations: the propertiesPascribed to u in

virtue of which u is qualitatively identical to some v which is the encoding of a redexemplar:

Red quale(Q) Q = {u: (x) (P)( u=e(x) & (Pe(p) & (~(Pu Pe(p)) ~Ixp)) Pu )}

A red quale is just a set of red sensations u as defined above. Intuitively, qualia are

whatever properties are such that a code has those properties if and only if its referent

seems the same as some red exemplar.

Qualia are properties of Sellarsian 'sensa' or 'impressions' in that they are properties ofinternal states involved in perception, which help to explain 'looks', and which have

similarities and differences that are structurally similar to similarities and differences

among colored things. They are properties of sensation which can play a certainrole in the discrimination of objects.

The properties of sensa are not literally sensory qualities: colors, smells, and so on.

Sensations of color are not themselves colored, but rather underly discriminations between

things which are colored. Since dimensionality of the pair list yields the critical propertiesof encodings, those properties can be identified by their place within a network of

discriminations and differences. A structural definite description of the form 'the set of all u

such that u is an encoding of somex and u bears Sto ...' can yield the extension of such aproperty.

8.The account here developed in effect identifies qualia with dimensions of the

discriminability pair list. It purports to give an objective characterization of phenomenal

properties. It seems an easy target for the kinds of criticisms Nagel raises in 'What is it like

to be a bat?', and as a summary it will be useful to contrast its treatment of bat qualia withphenomenalistic constructions, and see how well it deals with Nagel's worries.

Whatever their differences, one thing is common to all accounts of qualia: namely that

qualia describe what something looks like, feels like, or in general, seems like from a given

creature's point of view. Nagel argues that there are intractable difficulties in giving any

objective characterization of facts of this sort, and hence that qualia--as descriptions of theimmediate subjective character of experience--cannot be characterized objectively. The

difficulty is that facts concerning what it is like to beB "embody a particular point of view"--namely that ofB--and they are inaccessible except from the point of view ofB.

It seems that no objective characterization can capture facts of this sort, since the 'real

nature' of experience cannot be described by leaving behind the creature's point of view, but

only by retaining it, and in fact, by experiencing things from that point of view.
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So we cannot expect to be able to describe bat phenomenology with any current methods.

That task requires a new discipline: one of "objective phenomenology."

How would the theory developed above attempt to construct a bat phenomenology--for batsonar, say? By studying discrimination thresholds and the structure of the discrimination

pair list; in other words, by constructing bat psychophysics. One can test discrimination

capacities among non-verbal creatures by making receipt of food (for example) dependentupon choosing the correct stimulus in a choice task and successively decreasing differencesbetween the choices. To what features is the sense organ sensitive? To what differences is

the creature differentially sensitive? While technically difficult, experiments to answer

these questions (even for bat sonar) are possible.

And, I would argue, they would give an answer to "what is it like to be a bat?" If batB candiscriminatex fromy (in the behavioral sense) then even from the bat's point of view,x

cannot seem toB to be just the same asy. So an ability to discriminate shows that what it is

like to be a bat experiencingx is not phenomenally the same as what it is like to be a batexperiencingy. If there are qualitative differences between bat experiences ofx and ofy,

then there will be somezfor the bat indiscriminable fromx but not fromy, and so the

structure of the pair list will reveal sufficient conditions as well for qualitative similarityfrom the bat's point of view. In short, all the qualitative, phenomenal, or subjectivelikenesses and differences among experiences of the bat could be identified extensionally

from the structure of its discriminations. So we get an 'objective' characterization of what it

is like to be a bat. Of course such a description does not enable us to experience the worldthe way the bat does, so in that sense it does not answer the question "what is it like to be a

bat?", but it does suffice to defeat Nagel's claim that no objective characterization is

possible.

Indeed, such a feat must be possible for us even to recognize that bat sonar (or in general,an alien sensory modality) would be phenomenologically different from any of our sensory

modalities, and so it must be possible even for us to recognize that there is a problem about

bat sonar. The reason we know that what it is like to be a bat is not like what it is like to bea person is simply that the bat can discriminate stimuli which to us are indiscriminable(sizes, distances, and shapes when your eyes are closed) and fails to discriminate stimuli

which to us look different (e.g. colors). Since we can know that, we have a purchase on

alien phenomenology; and if the constructions above are sound then in principle nothingbars an extensional identification of its sensory phenomenology.

In short, then, there is no need for a new discipline of 'objective phenomenology'--of

objective characterization of the modes of appearance of the world--for psychophysics

already is that discipline.

Back to Austen Clarkonline papers.

Back to Uconn Philosophy home page.

Endnotes1. Nelson Goodman, The Structure of Appearance, 3rd edition (Boston: Dordrecht Reidel,1977).

2. See J.J.C. Smart, "Sensations and brain processes" and James Cornman, "The identity of

mind and body" both reprinted in C.V. Borst (ed) The Mind-Brain Identity Theory (London:

Macmillan, 1970). < Back.>
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3. This is similar to Reichenbach's space-time 'signal' relation. See Rudolf Carnap,

Introduction to Symbolic Logic and its Applications, New York, Dover, 1958, pp. 201-203.I reserve the term 'signal' for the sense deriving from information theory.

4. See Fred Dretske,Knowledge and the Flow of Information, Cambridge Massachusetts,

MIT Press, 1981, pp. 12-26.

5. If the choice distribution is non-random then there is a statistically significantrelationship between target presentation and choice; and hence there is a non-zero

contingent probability linking events in the two ensembles.

6. That is, one posits a physical channel to explain the information channel. Note that thelatter may exist without the former in cases of 'ghost' channels where both input and output

ensembles have a common cause. See Dretske, loc. cit., pp. 38-39.

7. Note that a generalizable slice sequence alone is insufficient to establish a signal relation

between its endpoints. See Dretske, loc. cit., pp. 33-38.

8. The term derives from L.L. Thurstone, 'A Law of Comparative Judgment',PsychologicalReview, 34 (1927), pp. 273-286. See also W.S. Torgerson, Theory and Methods of Scaling,

New York, John Wiley & Sons, 1958, pp. 156-158.

9. Goodman, loc. cit., pp. 196-197. 10. This utilizes Goodman's 'rule of order' to the effect that "every quale between two

matching qualia matches both". See Goodman, loc. cit., p. 213.

11. See Warren S. Torgerson, Theory and Methods of Scaling, New York, 1958, pp. 291-

292.

12. See Torgerson, op. cit., pp. 41-60; J.P. Guilford,Psychometric Methods, 2nd edition,New York, 1954.

13. The subscript emphasizes that the similarity in question is not a matter of judged

similarity in the properties of objects (the epistemic sense of 'seems'), but rather of the

immediate appearances they present. See C.W.K. Mundle,Perception: Facts and Theories,London, Oxford University Press, 1971, p. 20.

14. Goodman, loc. cit., p. 196.

15. See Hilary Putnam, 'Meaning and Reference', Journal of Philosophy 70 (1973), pp.

699-711.

16. See Wilfrid Sellars, 'Empiricism and the Philosophy of Mind' in his Science,

Perception, and Reality, London, Routledge & Kegan Paul, 1963, p. 193.

17. In Ned Block, (ed.)Readings in the Philosophy of Psychology, vol. 1, CambridgeMassachusetts, Harvard University Press, 1980, pp. 159-168.

18. Ibid., p. 163.

19. Ibid., p. 164.

20. Ibid., p. 162.

Return to Austen Clark's online papers .

Return to the Philosophy Department home page.
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