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Physicalist Theories of Color By Paul Boghossian With J. David Velleman.

THE PROBLEM OF COLOR REALISM The dispute between realists about color and anti-realists is actually a dispute about the nature of color properties. The disputants do not disagree over what material objects are like. Rather, they disagree over whether any of the uncontroversial facts about material objects-their powers to cause visual experiences, their dispositions to reflect incident light, their atomic makeup, and so onamount to their having colors. The disagreement is thus about which properties colors are and, in particular, whether colors are any of the properties in a particular set that is acknowledged on both sides to exhaust the properties of material objects. In a previous paper we discussed at length one attempt to identify colors with particular properties of material objects-namely, with their dispositions to cause visual experiences) Here we shall discuss a different and perhaps more influential version of realism, which says that the colors of material objects are microphysical properties of their surfaces.2 We shall call this theory physicalism about color (physicalism, for short). In order to evaluate this theory, however, we shall first have to clarify some methodological issues. Our hope is that we can bring some further clarity to the question of color realism, whether or not we succeed in our critique of the physicalists’ answer. Metaphysics and Semantics To say that the question of color realism is really about the nature of color properties is not yet to define the question sufficiently. One is tempted to ask, Which are the properties whose nature is at issue? Of course, the latter question may seem like an invitation to beg the former. For in order to say which properties are at issue in the debate about the nature of colors, one would have to say which properties colors are-which would seem to require settling the debate before defining it. How, indeed, can one ever debate the nature of a property? Until one knows which property is at issue, the debate cannot get started; but as soon as one knows which property is at issue, it would seem, the debate is over. Well, not quite. One can pick out a property by means of a contingent fact about it. And one can thereby specify the property whose nature is to be debated without preempting the debate. Such indirect specifications are what motivate questions about the nature of properties. One knows or suspects that there is a property playing a particular role, say, or occupying a particular relation, and one wants to know which property it is, given that playing the role or occupying the relation isn’t the property in question. The role in which colors command attention, of course, is their role as the properties attributed to objects by a particular aspect of visual experience. They are the properties that objects appear to have when they look colored. What philosophers want to know is whether the properties that objects thus appear to have are among the ones that they are generally agreed to have in reality. Yet if the question is whether some agreed-upon set of properties includes the ones that objects appear to have in looking colored, then it is partly a question about the content of visual appearances. When philosophers ask whether colors are real, they are asking whether any of the properties acknowledged to be real are the ones attributed to an object by the experience of its looking colored; and so they are asking, in part, which properties are represented in that experiencewhich is a question of its content. What is Looking Colored? The foregoing attempt to define which properties are at issue in the question of color realism may seem viciously circular. For we identified colors as the properties that things appear to have when they look colored; and how can this description help to pick out the relevant properties? It specifies the properties in terms of their being represented in a particular kind of experience, but then it seems to specify the relevant kind of experience in terms of its representing those properties. Which properties objects appear to have in looking colored depends on what counts as looking colored, which would seem to depend, in turn, on which properties colors are-which is precisely what was to be defined. This problem is not insuperable, however. The phrase “looks colored” and its determinate cousins-“looks red,” “looks blue,” and so forth-have a referential as well as an attributive use. That is, one learns to associate these phrases directly with visual experiences that are introspectively recognizable as similar in kind to paradigm instances. Paradigm cases of looking red fix a reference for the phrase “looks red,” which then refers to all introspectively similar experiences. We can therefore speak of something’s looking red and rely on the reader to know which kind of visual experience we mean, without our having to specify which property red is.3 There is no circularity, then, in identifying red as the property attributed to objects by their looking red and, more generally, in identifying colors as the properties attributed to objects by their looking colored.4 Color Experience vs. Color Discourse One might think that the references we have stipulated here are simply the references that color terms have anyway, in ordinary discourse. Surely, words like “red” and “blue” are sensory terms, designed to report what is seen. One may therefore feel entitled to presuppose that the term “red,” as used in ordinary discourse, already denotes the property that things appear to have when they look red. Yet the validity of this presupposition may depend on the answer to the question of color realism. For whether the ordinary term “red” always expresses the property that things appear to have in looking red may depend on whether that appearance is veridical or illusory. Suppose that an error theory of color experience is correct, in that the property that things appear to have when they look red is a property that they do not (and perhaps could not) have. In that case, the meaning of “red” in ordinary discourse will be subject to conflicting pressures. The term may still be used to express the property that objects are seen as having when they look red. Yet statements calling objects red in that sense will be systematically false, even if such statements tend to be made, and to garner assent, in reference to objects that have some physical property in common. In the interest of saying what’s true, rather than what merely appears true, speakers may then be inclined to shade the meaning of “red” toward denoting whatever property is distinctive of red-looking objects.5 The pressure towards speaking the truth will thus conflict with the pressure towards reporting the testimony of vision. How the meaning of “red” will fare under these conflicting pressures is hard to predict; it may even break apart, yielding two senses of the term, one to express the content of color experience and another to denote the property tracked by color attributions. We are not here proposing or defending such an account of color language. We are merely pointing it out as a possibility and suggesting that this possibility shouldn’t be excluded at the outset of inquiry about color. To assume that color terms denote the properties represented in color experience is to assume that terms used to attribute those properties to objects wouldn’t come under pressure from the systematic falsity of such attributions-something that may or may not turn out to be the case but shouldn’t be assumed at the outset. One should begin as an agnostic about whether color terms ordinarily denote the properties that are represented in color experience. If they are to be used in a debate about those properties, their reference to them must be explicitly stipulated. VERSIONS OF PHYSICALISM If physicalism is to settle the debate over color realism, it must be formulated as a thesis about the properties at issue in that debate. When the physicalist says that colors are microphysical properties, he must mean that microphysical properties are the ones attributed to objects by their looking colored. Otherwise, his claim will not succeed in attaching the uncontroversial reality of microphysical properties to the properties whose reality is in question-that is, the properties represented in color experience. Physicalism must therefore be, in part, a thesis about which properties color experience represents. The Naive Objection When the physicalist thesis is so interpreted, however, it tends to elicit the following, naive objection. The microphysical properties of an object are invisible and hence cannot be what is represented when the object looks colored. One can tell an object’s color just by looking at it, but one cannot tell anything about its molecular structure-nor, indeed, that it has such a structurewithout the aid of instruments or experimentation. How can colors, which are visible, be microphysical properties, which are not? Physicalists regard this objection as obviously mistaken, although different physicalists regard it as committing different mistakes. A particular physicalist’s response to the objection will be conditioned by his brand of physicalism, on the one hand, and his conception of visual representation, on the other. We therefore turn our attention, in the next two sections, to these potential differences among proponents of physicalism. Colors vs. Ways of Being Colored The claim that red is a microphysical property can express either of two very different theses. On the one hand, the claim may state a strict identity between properties. In that case, it means that having a particular microphysical configuration is one and the same property as being red. On the other hand, the claim may mean that having this microphysical configuration is a way of being red and, in particular, the way in which things are red in actuality. In that case, the relation drawn between these properties is not identity. Rather, red is envisioned as a higher-order property-the property of having some (lower-order) property satisfying particular conditions-and the microphysical configuration is envisioned as a lower-order property satisfying those conditions, and hence as a realization or embodiment of red. The difference between these two views is analogous to that between typephysicalism and functionalist materialism in the philosophy of mind. Physicalism says that pain is one and the same state as a configuration of excited neurons. Functionalist materialism says that pain is the higherorder state of occupying some state that plays a particular role, that this role is played in humans by a configuration of excited neurons, and hence that having excited neurons is the way in which humans have pain. Both views can be expressed by the claim that pain is a neural state, but this claim asserts a strict identity only when expressing the former view. We shall distinguish between the corresponding views of color by referring to them as the physical identity view and the physical realization view, or identityphysicalism and realizationphysicalism.6 To repeat, only identity-physicalism says that red is one and the same property as a microphysical configuration; realization-physicalism says that the microphysical configuration is merely a way of being red. Adherents of both identity-physicalism and realization-physicalism will dismiss the naive objection mooted above, but they will dismiss it on different grounds. A realization-physicalist can say that the naive objection confuses color properties with the properties that embody them. The ability to see which color an object instantiates is perfectly compatible, in his view, with an inability to see the particular way in which it instantiates that color. For in his view, seeing that an object is red consists in seeing that it has some property satisfying particular conditions; and seeing that an object has some such property need not entail seeing which such property it has. The invisibility of microphysical properties therefore doesn’t preclude them from realizing or embodying colors. This refutation of the naive objection is not available to the identityphysicalist, of course, since he doesn’t draw any distinction between colors and their realizations. The identity-physicalist can still fend off the objection, however, by claiming that it misconstrues the use to which he puts the phrase “microphysical properties.” The objection construes this phrase, he says, as articulating a mode of presentation under which colors are represented in visual experience-as expressing what colors are seen as-whereas the phrase is actually intended only to identify the nature of color properties. The physicalist points out that although one never sees anything as a layer of moleculesnever sees anything under the characterization “layer of molecules”-one nevertheless sees things that are, in fact, layers of molecules, since that’s precisely what the visible surfaces of objects are. Similarly, the physicalist argues, seeing nothing under the mode of presentation “microphysical property” doesn’t prevent one from seeing things that are microphysical properties. And that colors are such properties is all that any physicalist means to say. The Propositional Content of Visual Experience Thus, the suggestion that physicalism requires colors to be seen under microphysical modes of presentation will be rejected by physicalists of all stripes. Some physicalists will go further, however, by denying that colors are seen under any modes of presentation at all. Whether a physicalist makes this further denial depends on his views about the propositional content of color experience. On the one hand, a physicalist may take a Fregean view of the visual representation of color. According to that view, the experience of seeing something as red has that content by virtue of the subject’s relation to a proposition containing a concept, characterization, or (as we have put it) mode of presentation that is uniquely satisfied by instances of red. The property itself is not an element of the propositional content, as the Fregean conceives it; rather, it is represented by an element of the content, namely, a characterization. On the other hand, a physicalist might take a completely different view of how color is visually represented, a view that we shall call Russellian. According to that view, the experience of seeing something as red has that content by virtue of the subject’s relation to a proposition containing the property red-the property itself, not a conception, characterization, or presentation of it. A Russellian believes that the property is introduced into the content of experience by something that directly refers to it. This item may be an introspectible, qualitative feature of visual experience, for example, or a word of mentalese tokened in some visual-experience “box.” Whatever it is, it must be capable of referring to the property red directly-say, by virtue of a correlation or causal relation with it7 -rather than by specifying it descriptively, in the sense of having a meaning uniquely satisfied by red objects.8 A strict Russellian may believe that the mental symbol for red has no descriptive meaning at all-just a reference. A more liberal Russellian may believe that it has a meaning, but that its meaning is not sufficient to specify the property red or to determine a complete proposition about redness, and hence that the content of seeing something as red must still be completed by the property itself, introduced via direct reference. The difference between these two variants of Russellianism is analogous to that between two variants of the familiar causal theory about naturalkind terms. On the one hand, the word “gold” can be viewed as a name that has no descriptive meaning over and above its reference to gold (although this reference may have been fixed, of course, with the help of a description). On the other hand, “gold” can be viewed as having a descriptive meaning such as “a kind of matter,” which is not sufficient to specify a particular kind of matter and must therefore be supplemented by a causally mediated relation of reference to gold. According to the latter view, “gold” and “silver” share the meaning “a kind of matter” but refer to different kinds of matter; and their contributions to the content of sentences must include not only their shared meaning but also their distinct referents. According to the corresponding view about the visual representation of color, there are mental symbols for red and orange that may contribute a shared meaning to the content of visual experiences-say, “a surface property”-while introducing different properties as their referents. A proponent of this liberal Russellianism will acknowledge that visual experience contains some characterization of colors, but his stricter colleague will not, since the strict Russellian believes that red is introduced into visual content by an item possessing no descriptive meaning at all. The strict Russellian will therefore deny that colors are seen under any modes of presentation. And he will consequently think of the naive objection to physicalism as doubly mistaken-not only in suggesting that he uses the phrase “microphysical property” to articulate such modes of presentation but also in suggesting that he acknowledges their existence.