Consciousness and Cognition 10, 117–124 (2001)

doi:10.1006/ccog.2000.0481, available online at http://www.idealibrary.com on

What Colors? Whose Colors?

Mohan Matthen

Department of Philosophy, The University of British Columbia,Vancouver, British Columbia V6T 121 Canada

E-mail: mohan.matthen@ubc.ca

Scientists often complain that philosophers do not know what they are talkingabout. Unfortunately, this is sometimes true (though just as often the truth is thatscientists cannot fathom what philosophers are talking about). Until recently, philoso-phers took it to be obvious that the visual field corresponds closely to the pattern ofelectromagnetic energy incident upon the retina, the retinal ‘‘image,’’ as it is some-times called. They thought that everything we see—surfaces, solids, movements,faces, and so on—is decomposable into points of incident energy in much the waythat Monet’s paintings of the Cathedral at Rouen decompose into small coloredpatches. They also believed that the visible properties of these points of color corre-sponded closely to the retinal ‘‘image.’’ (At the very least, philosophers wrote andspoke as if this assumption of correspondence was harmless.) Thanks to ‘‘visualscience’’ we now know that these assumptions are false: The perception of a faceis not decomposable into perceptions of color-points, and perceived color does notcorrespond to the physical characteristics of electromagnetic waves. If philosophersdo not get their facts straight, their theories will be useless or worse: apriorism isnot an excuse for ignorance. Fortunately, many have come to recognize this (someonly slowly).

It is not only philosophers who do not know what they are talking about whenthey venture outside their area of expertise. Often ‘‘visual scientists’’ do not either.Some visual scientists are apparently apt to say things like: ‘‘Color is a property ofthe brain,’’ or ‘‘Color is a neural process,’’ or ‘‘Color is a mental event.’’ No doubtthese jeux d’esprit are not meant to be taken literally or subjected to minute philo-sophical analysis. Perhaps they are meant only to express the irritation that visualscientists feel at the a priori pronouncements of philosophers. However that mightbe, such statements are so far off the mark, metaphysically speaking, that it is ex-tremely difficult to determine what (if any) serious point they contain. Peter Ross isright to protest that hidden behind some of the more exuberant utterances of visualscientists lie unexamined philosophical assumptions. I hope that if we philosopherscan manage to inform ourselves of at least a little visual science, and if we expressourselves clearly and logically, we can bring some degree of metaphysical order tothe discourse of that science.

Commentary on P. W. Ross (2001). The location problem for color subjectivism. Consciousness andCognition, 10, 42–58.

Address correspondence and reprint requests to Mohan Matthen, Department of Philosophy, The Uni-versity of British Columbia, Vancouver, British Columbia, V6T 121 Canada.

1171053-8100/01 $35.00

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In this commentary on Ross’s article, I try to determine whether visual sciencedoes indeed establish that ‘‘our ordinary color categories in no way correspond withphysical categories.’’ I argue that there is a sense in which this is true and also asense in which it is false. The sense in which it is true does not, I contend, offer usmuch of a reason for abandoning color realism; that is, the proposition that what wesee as color really does correspond to something out there. I’ll put this in the contextof a position that I have elsewhere (Matthen, 1999) labeled ‘‘Pluralistic Realism.’’In bare outline, this position consists of two propositions: (a) color categories areoften species-specific and (b) many species-specific color categories correspond withreality, though they somewhat distort it.1

Let’s start by taking a fresh look at the problem of metamerism. Ross defines thephenomenon as follows (the italicized labels are mine): ‘‘Physical Variability: Forany determinate color, physical objects in the same viewing condition may differ inindefinitely many ways with respect to their (spectrally relevant) physical properties,and nevertheless look precisely that color.’’2 From this it follows that, as AustenClark (1993, p. 39) puts it: ‘‘Metameric Plurality: The existence of such metamersrefutes the idea that a given color can be identified with light of a particular wave-length (39).’’ Ross infers (Claim 1a) the following: ‘‘Non-Physicality of Color: Ourordinary color categories in no way correspond with, and are not explained by, physi-cal categories.’’ Ross calls this ‘‘an uncontroversial finding of visual science.’’ Thepoint that I want to make in this comment is that it ought not to be so.

Before we go on, I want to make a point that may seem picky, but which assumessome importance later on. Imagine a light-detecting cell rigged up to buzz wheneverit detected light or, better, a spectrometer that detects light between 500 and 550 nm.A version of the Metameric Plurality proposition is true of such a device. It will beactivated by monochromatic light of 500, 501, 502, . . . . 550 nm: it will, in otherwords, detect light that ‘‘differs in indefinitely many ways with respect to its (spec-trally relevant) physical properties.’’ What it detects cannot be ‘‘identified with lightof a particular wavelength.’’ Are we to say, then, that what the spectrometer detects‘‘in no way corresponds with, and is not explained by, physical categories?’’ Presum-ably not. A continuous range of wavelengths strikes most people as a physical cate-gory. The light-detecting cell detects visible light, and this is a physical category,even though it is defined by the sensitivity range of our own eyes. Similarly thespectrometer.

This shows what ought to have been obvious all along, but is nevertheless worthnoting explicitly: the inference from Metameric Plurality to Non-Physicality involvesadditional principles about what counts, and what does not count, as a ‘‘physical

1 I am following Ross here in my use of the term ‘‘category’’: I do not mean it to evoke the phenomenonof ‘‘categorical perception.’’ I just mean that what Ross calls ‘‘the psychological color space,’’ i.e. forhumans, corresponds to something in physical reality. And so does the psychological color space ofmany other animals.

2 There is a flaw in Ross’s statement of this principle. From the fact that things with different spectralproperties may look the same color, nothing follows about color categories. What is needed is that thingswith different spectral properties must actually be the same color. The claim that visual scientists makeis that lights with different spectral composition actually are the same color.

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category.’’ The ‘‘uncontroversial finding of visual science’’ above is difficult to un-derstand and evaluate because these additional principles, which in any case falloutside the domain of visual science, have not been stated.

With this in mind, let us turn to the case of human color vision. In Matthen (1999),I protested that when we ask whether perceived categories ‘‘correspond’’ or ‘‘corre-late’’ with physical categories, our question is too crude to accommodate the nuancesthat lurk in the general area. A perceptual state like that of seeing purple seemssimple, and so it seems that its content can be explicated by means of a simple corre-spondence relation. But since color perception (and indeed any perception) is con-structed from a plurality of distinct processes, such an explication is far too laconic.In actual fact, even a simple state like that of seeing a purple light carries a complexmessage that needs to be expressed in terms of a composite proposition.

I want now to explore what that message is. In doing so, I shall the followingsimplifying moves: (1) I speak of lights only, not transparencies or reflecting surfaces;(2) I deal only with the hue-components, not their relative strength in particular hues;and (3) I restrict myself to the information carried by a perceptual state, not themessage introspectively available to the perceiver. (In other words, I stick to thequestion of what external condition ‘‘corresponds’’ to the state or ‘‘explains’’ it,disregarding, for now, the role that the state plays in internal information-processing.)Later, I comment briefly on how we can reintroduce the more complicated consider-ations that arise out of real-world human color vision.

Let me recapitulate briefly the neurocomputational basis of early human color vi-sion. It is hypothesized that humans possess three types of photoreceptor, or ‘‘visualpigment,’’ each sensitive to distinct but overlapping wavebands of the visual spec-trum.3 Call these wavebands long (L), middle (M), and short (S). Information aboutspectral distribution is extracted from the outputs of these cells by assessing the rela-tive strength of the outputs of these receptors. This process, called ‘‘opponent pro-cessing,’’ (in effect) computes two functions: (L 1 S) 2 M and (L 1 M) 2 S. Thefirst function tells us the relative strength of the source at the ends of the visualspectrum (L and S) compared to the middle. If this function takes a positive value,that is, if the source is stronger at one end or another of the spectrum, it will lookreddish. If, on the other hand, it is stronger in the middle of the spectrum, and thevalue of the function is negative, it looks greenish. The second function tells us howmuch of the source-light’s strength is in the middle and long wavebands. If it isstronger here than in the short waveband, the source looks yellowish; if it is strongerin the short waves, it looks bluish.

We can summarize the hues in terms of a system of signals. In the context ofspeech perception, Liberman et al. (1967) make a distinction between a cipher, whichsubstitutes one symbol for every unit of the original message (however those unitsmay be conceived), and a code, which does not make a one-to-one correspondencebetween original and coded units. To this taxonomy, I add the term ‘‘signal system’’

3 This theory may stand in need of revision. Abramov (1997) locates the trichromacy of human colorvision in processes in the lateral geniculate nucleus. This would not seriously affect the content of myexplication.

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to denote a special kind of code: one in which the original message is a full sentence4

and the encoded version is a single unit such as a flag or light. For example, a yellowflag might stand for the (indicative) sentence ‘‘There is infectious disease on board,’’a green running light for ‘‘This is the starboard side,’’ and a flower pot in the windowfor ‘‘Stay away.’’

As summarized above, the opponent processing of the outputs of human photo-receptors can be tabulated in terms of a signal system as follows: red: the source isstronger at the ends of the visible spectrum than in the middle; green: the source isstronger in the middle of the visible spectrum than at the ends; yellow: the sourceis stronger in the middle to long region of the visible spectrum than in the short; andblue: the source is stronger in the short wave end of the visible spectrum than in themiddle to long.

Every perceived hue consists of up to two such signals, one from red and greenand the other from yellow and blue. (A few, the so-called unique hues, have one nullcomponent.) The conjoined message contained in the two-membered signals can bedisplayed in the form of a two-by-two matrix:

Ends (L 1 S) dominate Middle dominatesover middle (reddish) over ends (greenish)

Long-middle (L 1 M) domi- The source consists dominantly The source consists dominantlynates over short (yellowish) of wavelengths in the mid- of wavelengths in the long-

dle-long to long part of the middle part of the visiblevisible spectrum. (Example: spectrum. (Examples: greens,orange) yellows, and mixtures.)

Short dominates over long-mid- The source consists dominantly The source consists dominantlydle (bluish) of wavelengths at the short of wavelengths in the short-

end of the visual spectrum. middle part of the visible(Example: violet). spectrum. (Example: blue-

green.)

Notice the following consequences of making the hue system a signal system in-stead of a code: (A) we completely bypass the question of the kinds of categoriesto which perceived hue-components correspond, (B) the propositional informationthe hue components carry is completely physical, and (C) metameric plurality makesabsolutely no difference to (B).

Of course, this system of signals can be translated into a code—just collect togetherthe sources that are described by the above propositions.5 Many philosophers allegethat such a collection of sources does not constitute a physical category. I am not

4 It is more usual to posit an ontological (rather than a linguistic) category to correlate with perceptualstates. Thus, signal systems are more usually defined in terms of one-to-one correspondence with situa-tions (see Lewis, 1967, for an example). I use a linguistic category here for two reasons: (a) to emphasizethe continuity with Liberman et al. and (b) to accommodate imperative signals (like those used bybaseball coaches and the like).

5 Clark (1993, pp. 37–39) does something equivalent: He represents each wavelength mixture to athree vector consisting of the outputs of the three receptors.

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sure that I understand why. If the propositions in the above table convey physicalinformation about a source, why do the categories corresponding to their truth notcount as physical? Be that as it may: construing the hues as a system of signals ratherthan as a code allows us to sidestep this tricky question. Even if the hue code invokescategories from outside of physics, the hue signal system encodes propositions thatcan be expressed in a purely physical vocabulary.

Now, of course, the above analysis is oversimplified. Color perception revealsmany thousands of hues, not four. But before we get to that, let’s note one respectin which the analysis is not oversimplified. The variability of perceptual appearanceis not taken into account in the above analysis, but that is not a simplification. Dif-ferent objects look the same in different conditions of illumination and contrast,the same object looks different. This has nothing to do with Metameric Plurality.The proposition that we are examining here is a consequence of Physical Variability.The latter proposition does not say merely that different objects look the same colorin certain conditions (see footnote 2). That would not tell us anything about the natureof colors because all sorts of things appear different though they are the same andappear the same though they are different. For example, it might well be that (viewedthrough an appropriately placed peephole or painted on a ceiling) a circle and anellipse look the same in certain conditions and that a circle looks like an ellipse undercertain conditions. This does not imply (by itself ) that the property of being a circle‘‘in no way corresponds with’’ any physical category. The claim that needs to bemade about Physical Variability (pace Ross) is that there are things which differ intheir spectral properties, though they actually are different colors. This is why Iused lights in my exposition above rather than surfaces. There is no question hereof appearance: lights with different spectral properties do actually have the samecolor if they evoke the same output from the cone cells.6

Now let’s consider the fact that human color vision discerns many thousands ofcolors, not just four. What are we to make of this? In principle, nothing. The manythousands of colors arise from the fact that color sensations are not just a productof two-valued states, as in the matrix above, but the product of many-valued states.A particular hue is not just reddish, but reddish to such and such a degree. However,these values still reflect the opponent functions mentioned above. A hue is reddishif it reflects a positive value of (L 1 S) over M. It is very red if the difference islarge and not very red if the difference is small. Thus if we substitute a many-valuedsignaling system for the four-valued system above, we get an interpretation for theinformation carried by hues that still makes them equivalent to difference statementsabout the spectral properties of the source, statements like ‘‘The source consists of(a lot more, a little more, about the same amount of) energy in the long and shortwavebands relative to the medium.’’

When we split up a ray of white light into its components by means of a prism,we get a linear array of monochromatic beams. These beams create in us an arrayof perceived hues. This phenomenon might have led some philosophers (though I

6 There are metameric surfaces as well as metameric lights. However, metameric surfaces look thesame only under a highly circumscribed set of illumination conditions. Metameric lights look the sameunder a very wide set of conditions.

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am not sure which ones) to suppose that the (saturated) hues constituted a cipher (aone unit to one unit coding) for spectral wavebands. Clearly this is a mistake. How-ever, we would be wrong to conclude that the hue system is not any kind of codefor the spectral properties of lights. I have suggested that one can interpret it as asignaling system that carries information about such properties. Such a signalingsystem is easily translated into a code.

Of course, a major part of the controversy about the physical reality of the colorsarises not from lights or incident energy, which are not usually objects of vision, butfrom surfaces and reflectances. I do not try to get into a thorough discussion of thetwo cases here, since Ross moves between them as if from his point of view this isa distinction without a significant difference. It is, however, relevant to rememberthat if two light mixtures have the same effect on the cone cells, then they are thesame color. On the other hand, if two reflecting surfaces have the same effect onthe cone cells, they only look the same color. Why? Because of color ‘‘constancy,’’the phenomenon that there is less variation in the color appearance of surfaces thanwould be predicted by the variation in the light they send to the eye in differentconditions. There is considerable controversy in the literature about how best to un-derstand the significance of color constancy. Some think that it shows that the visualsystem is capable of distinguishing, to some extent, between appearance and reality.Others think that it is an example of how the visual system confabulates. I have mademy case above without appealing to this additional feature.

Having thus made the case for physicalism with respect to what is encoded by thehue signaling system, let me move to a rather different sort of consideration in supportof a certain kind of subjectivism. The question might be asked: What is the signifi-cance of the propositions signaled by that system. Consider the proposition signaledby a reddish hue: ‘‘The source is stronger at the ends of the visible spectrum than inthe middle.’’ Does this proposition have any significance in physics? Does it signifyanything of importance to other animals? And here, clearly, the answer is ‘‘Not neces-sarily.’’ The only reason why this proposition is significant to us is that it plays arole in internal processing in us. And the reason why it plays an important role inus is that (a) it is easy to extract this information from cone cell outputs by opponentprocessing; and (b) this information is usable for the further processing functions,whatever they are, of the human visual system. If an animal had different kinds ofcone cells or if its processing needs were different, it might not attach any significanceto the proposition signaled by our reddish hues. We use reddishness as a means forclassifying objects in the world. Given what we have just said about the significanceof this hue feature, we may conclude that the classification is, to some degree andto some extent, anthropocentric.

Does the anthropocentricity of a category show that it is ‘‘a property of the brain?’’Human faces are a parallel case. Humans are able to identify them with considerableaccuracy, but they are mainly of interest to us because they encode the identity ofconspecifics. Human faces are of little interest to other animals or to physicists (intheir professional capacity). But this does not make them objectively, or physically,unreal. There are lots of facts that can be described in the vocabulary of physics, butwhich are of no interest to physicists. An organism that belongs to a social speciesis interested in identifying other organisms of the same species, but likely not organ-

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isms of other species. Even if it does use physical characteristics in doing this, thecategories that it thus captures have no universal significance.

Color is a case like this: I have argued elsewhere (Matthen, 1999) that wavelength-specific discrimination is used in different animals to detect different things. Thecolors we detect are not necessarily the target of avian or apian color vision. Thephysicalist can, and should, take this variability on board. Physical properties arenumerous, and it is possible that different color perceivers fasten on different proper-ties or even different property structures. The case that we have been discussingoffers us a very simple example of this. The red-yellow dimension of hue tracks therelative strength of the ends of the visual spectrum relative to the middle. If themiddle-wave receptor is shifted slightly toward the short-wave end, the middle wave-band will also move. Consequently the opponent function (L 1 S) 2 M will signala slightly different state of affairs. It is entirely possible that in some other organismthis shift actually occurs. This would not mean that the situation being signaled isany less real. But it does mean that the reason it is signaled is perceiver-centered.Another kind of example involves the things detected with the help of color vision.There is evidence, for example, that in primates color information goes to and is usedby noncortical processors in figure and motion discrimination.7 There is no reason tothink that this use of color information is universal. Again, honeybees use polarizedcolor to identify flowers, and birds use in directional navigation. Such uses of colorare unavailable to us. The color categories generated by such activities are, again,species-specific.

Color physicalism comes in naive as well as nuanced forms. The naive form imag-ines that the discriminable colors correspond one-to-one with invariant physical situa-tions. In other words, it posits a universal cipher in which discriminable colors standfor fixed values of some environmental variables. I have been recommending a morenuanced form of color physicalism. My position argues, first of all, that the environ-mental targets of color vision are determined by evolutionary history and reflectedin the current organization of a given species’ visual system. In short, the range ofsituations signaled is determined by contingent features of the organism, ‘‘propertiesof the brain,’’ if you will. Second, taking the visual system of a particular organismas given, perceived colors will correlate more or less well with actual physical situa-tions. And here I mean to emphasize not merely the correlations of ‘‘early colorvision’’ that I have dealt with in some detail above, but also correlations involvingmore processed states. However you interpret the ontological significance of ‘‘colorconstancy’’—whether you take it as evidence that color vision is after the truth ortake it, on the contrary, as evidence of its mendacity—there will be undeniable corre-lations between perceived color states and environmental situations involving reflec-tance, luminance, figure, and motion.

The first element of ‘‘nuanced physicalism’’—evolutionary pluralism—may bethought to constitute a kind of subjectivism. However, in view of the other element,the position does not support the claim that ‘‘Color is a property of the brain.’’ (This

7 See Weiskrantz (1997, p. 31) and the references to Heywood and the references to C.A. Heywoodand co-workers to be found there. I am grateful to Kathleen Akins and Martin Hahn for helpful discussionof this kind of example.

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is why I call it ‘‘Pluralistic Realism.’’) The proposition that color originates in thebrain might run into the kinds of difficulty that Ross patiently outlines and attacksin the central part of his article. However, there is not, as far as I can see, any empiricaljustification for such a position.

REFERENCES

Abramov, I. (1997). Physiological mechanisms of color vision. In C. L. Hardin & L. Maffi (Eds.), Colorcategories in thought and language. Cambridge, UK: Cambridge Univ. Press.

Clark, A. (1993). Sensory qualities. Oxford, UK: Clarendon Press Oxford Univ. Press.Liberman, A. M., Cooper, F. S., Shankweiler, D. P., & Studdert-Kennedy, M. (1967). Perception of the

speech code. Psychological Review, 74, 431–461.

Matthen, M. (1999). The disunity of color. The Philosophical Review, 108(1), 47–84.

Weiskrantz, L. (1997). Consciousness lost and found. Oxford, UK: Oxford Univ. Press.