Colour and qualia

3. The identity theory and its problems

This latter line of reasoning has led many to what has been called the ‘central state identity theory’. On this view, an experience is identifiable with a neural state, and its (intrinsic) properties are ultimately neural in character (see Mind, identity theory of). Three sorts of challenges have been mounted against this identification: (1) the conceivability argument, (2) the multiple-realizability argument, and (3) the knowledge argument.

According to the first, since it seems conceivable that one could be having an experience of a reddish sort but not be in the neural state in question, the experience cannot be the same as the neural state. However, put so baldly, this argument is clearly fallacious. It is conceivable that water might not have been H₂O, but that is no reason to deny that it is in fact identical to it (see Intentionality; Propositional attitude statements). However, the conceivability argument seems to many to be getting at something not so easily dismissed, and this is brought out by considering the second argument, about multi-realizability: subjective red cannot be identical to a neural state because it seems possible that there could be creatures with a different sort of physical constitution (say, Martians, or robots) that could nevertheless experience reddish visual sensations like ours. This seems more interesting than mere conceivability, since the possibility seems to survive the discovery that, among all the instances we have observed, subjective red always happens to be realized by a certain neural state. This does not seem to be the case with, for example, water and H₂O: if all the water we have observed is H₂O, that seems to be a good reason to think that all water is (see Kripke 1980; Essentialism).

The last argument, the knowledge argument, is due to Frank Jackson (1982). If subjective red is a physical property, then if one knows all the relevant physical facts pertaining to colour vision one will know all there is to know about subjective red. Imagine Mary, a vision scientist who possesses the complete physical theory of colour vision, but who has lived in a totally achromatic environment her entire life; she has only seen the world in black and white. Upon release from this restricted environment, she encounters a ripe tomato, and exclaims, ‘So that’s what red looks like!’. It seems she has learned something new, and yet, Jackson argues, if physicalism is true she should already have known all there was to know about colour experience. Hence, subjective red is not a physical property.

Now, one reply to this argument is the same as to the conceivability argument. ‘Knowledge’, another intentional term, is referentially opaque: the fact that Mary knows all the physical facts and yet cannot recognize subjective red from its physical description does not show that subjective red is not identical to a physical property; after all, the fact that she might know all the physical facts about H₂O without recognizing that it is water does not show that water is not H₂O. Mary may, in one sense, know everything there is to know when she learns all the physical facts, just not under every possible description. But that constitutes no problem for physicalism.

Still, however, many philosophers feel that these replies are missing the point. What provides plausibility to the three arguments is that appeal to the neural properties of one’s visual state does not seem to explain its qualitative properties. The fact that these neurons are firing in this way does not really explain the shirt’s looking ‘reddish’. This is known as the ‘explanatory gap’ argument (see Levine 1983, 1993; Explanation).

It could be put this way: once we discover the chemical composition of water, and know various chemical laws governing the states and interactions of physical substances, we can explain the features of water that we initially used in picking it out. For instance, we can explain why it freezes and boils at the temperatures it does, why it is necessary for life, and a host of its other properties. In fact, it is not really conceivable that something could be H₂O and yet fail to manifest these various properties, at least so long as we fix basic chemical laws. This seems to be what at least the conceivability and knowledge arguments are getting at: that the physical facts do not ‘upwardly necessitate’ facts about qualitative experience in the way that chemical facts necessitate facts about water. No matter how detailed our knowledge of the physical mechanisms by which neurons transmit their impulses from one to the other, we are still left with the question of why all this electrochemical activity should constitute an experience with a reddish (as opposed to a greenish, say) qualitative character. It still seems quite conceivable that such electrochemical activity should occur and yet not the experience of subjective red, and this residual conceivability is a manifestation of the explanatory gap. To put the matter in terms of the knowledge argument, the fact that Mary would learn something new upon emerging from her achromatic world is evidence that the physical theory of colour vision does not really explain the qualitative character. Even if she knew the meaning of ordinary ‘red’ she could not deduce from the rest of her knowledge that tomatoes look red (in the way that she could deduce that water freezes at 32 °F).

One might object that appeals to neurophysiology can indeed provide an explanation of subjective red. For one thing, if we were to find a stable correlation between subjects’ reports of reddish sensations and their occupying certain neurophysiological states, then we could predict the occurrence of these sensations from knowledge of their brain states. For another, knowledge of the neurophysiological properties explains the mechanisms by which subjective red performs its cognitive functions. Colour perception involves selective sensitivity to fairly complicated properties of the light reflected from physical surfaces, and we now know a lot about how that sensitivity is implemented in neural hardware.

The problem with the first consideration is that in place of a genuine explanation all we are given is a brute correlation. I may be able to predict with perfect confidence that when someone occupies this particular neurophysiological state they are having a reddish sensation, but my ability to make this prediction does not manifest an understanding of why this brain state should go with reddish, and not bluish or greenish sensations. Brute facts are just that; they are not explicable. Moreover, they do not support counterfactuals: how are we to judge whether something lacking a specific physical property we possess really would not be having a reddish experience? What could satisfy us about the modal claim that not only are reddish experiences correlated with a certain neural state in fact, but must be so correlated?

Of course sometimes we admit that certain facts are brute facts; for instance, the value of the gravitational constant, the basic laws of physics, and the like. But notice it is a methodological assumption of current scientific practice, a corollary of physicalism, that only at the basic physical level are brute facts to be found. The idea that something as complicated as the brain should just give rise to reddish experience, and there be no explanation of this fact, is inconsistent with fundamental Physicalist assumptions.

If there is any hope for explaining subjective colour within a Physicalist framework, it would seem to depend on the second consideration: showing how the neurophysiological states of the brain provide the mechanisms by which colour sensations perform their cognitive functions. But to make this work it is necessary to analyse subjective colour in terms of its function, for otherwise explaining that function would not count as a full explanation of subjective colour itself. In other words, we need to abandon the treatment of subjective colour as an intrinsic property of sensation, and treat it instead as a relational, or functional, property.