The comparisons with lightning and water, however, carry what many philosophers have thought to be an implausible implication. Although every instance of lightning is an instance of the same type of physical state—electrical discharge—it is doubtful that every instance of believing that grass grows, for example, is also an instance of the same type of physical state—i.e., the excitation of specific neurons in the brain. This is because it seems possible for two people to have brains composed of slightly different substances and yet to share the same belief or other mental state. Likewise, it could be that there are extraterrestrials who believe that grass grows, though their brains are composed of materials very different from those that make up human brains. Why should reductionists rule out this possibility?
This unwanted implication can be avoided by noticing an ambiguity in identity statements between types and tokens. According to a “type-identity” theory, every type of mental phenomenon is some (naturally specifiable) type of physical phenomenon. This is quite a strong claim, akin to saying that every letter of the alphabet is identical to a certain type of physical shape (or sound). But this seems clearly wrong: there is quite a diversity of shapes (and sounds) that can count as a token of the letter a. A more reasonable claim would be that every token of the letter a is identical to a token of some type of physical shape (or sound). Accordingly, many materialist philosophers have retreated to a “token-identity” theory, according to which every mental phenomenon is identical to some physical phenomenon.
The distinction between types and tokens of mental phenomena may afford a way for the reductive physicalist to concede a point to the traditional dualist without giving up anything important. This is because distinguishing between types of phenomena can be regarded as a way of distinguishing between different ways of classifying them, and there may be any number of ways of classifying a given phenomenon that are not reducible to each other. For example, every piece of luggage is presumably a physical object, but no one believes that “luggage” is a classification that can be expressed in—or reduced to—physics, and no one has ever seriously proposed a “luggage-physics” dualism. If this is the kind of dualism that the mental involves, it would therefore seem to be quite innocuous.
Even if the identity theory is restricted to token-identity claims, however, there are still problems. One simple example concerns the relation of many mental phenomena to physical space. As noted earlier (see above Abstract and concrete), it is ordinarily quite unclear exactly where such things as beliefs and desires are located. They are often said to be “in the head”—but where in the head, exactly? Or, to take a harder example, mental images seem to have certain physical properties, such as being oval and vividly coloured. But if such images are to be identified with physical things, then it would seem to follow that those things should have the same physical properties—there should be oval, vividly coloured objects in the brains of people who experience such images. But this is absurd. So it would seem that a mental image cannot be a physical thing. (Arguments of this sort are sometimes called “Leibniz-law arguments,” after a metaphysical principle formulated by the German philosopher Gottfried Wilhelm Leibniz [1646–1716]: if x = y, then whatever is true of x must also be true of y). Other, more technical problems with the identity theory (pressed most vigorously by the American philosopher Saul Kripke) are beyond the scope of this article. The cumulative effect of these difficulties has been to make philosophers wary of couching reductionism in terms of identity.
An alternative is to say not that mental phenomena are identical to physical phenomena but rather that they are “constituted” by them. Consider a porcelain vase. Suppose someone were to break the vase and make a statue out of all of the pieces. If both the vase and the statue are identical to the pieces, it would follow that the statue is identical to the vase, which is absurd. So the vase and the statue are not identical to the pieces but merely constituted by (or composed of) them.
Physicalists think that it is possible to say more than this. Not only is every mental phenomenon constituted by physical phenomena, but every property of the mental crucially depends upon some physical property. Physicalists think that mental properties “supervene” on the physical, in the sense that every change or difference in a mental property depends upon some change or difference in a physical property. It follows that it is impossible for there to be two universes that are physically identical throughout their entire history but that differ with respect to whether a certain individual is in pain at a particular time. Explanatory gaps
The thesis of supervenience has called attention to a particularly striking difficulty about how to integrate talk about minds into a general scientific understanding of the world, a difficulty that arises both in the case of conscious states and in the case of intentional ones. Although mental properties may well supervene on physical properties, it is surprisingly difficult to say exactly how they might do so.
Consider how most ordinary nonmental phenomena are explained. It is one of the impressive achievements of modern science that it seems to afford in principle quite illuminating explanations of almost every nonmental phenomenon one can think of. For example, most adults who want to understand why water expands when it freezes, why the Sun shines, why the continents move, or why fetuses grow can easily imagine at least the bare outlines of a scientific explanation. The explanation would consider the physical properties of trillions of little particles, their spatial and temporal relations, and the physical (e.g., gravitational and electrical) forces between them. If these particles exist in these relations and are subject to these forces, it follows that water expands, the Sun shines, and so on. Indeed, if one knew these physical facts, one would see in each case that these phenomena must happen as they do. As the American philosopher Joseph Levine nicely put it, the microphysical phenomena “upwardly necessitate” the macrophysical phenomena: water could not but expand when it freezes, given the properties of its physical parts.
But it is precisely this upward necessitation that seems very difficult to even imagine in the case of the mental, particularly in the case of the two phenomena discussed above—consciousness and intentionality. The easiest way to see this is to consider a simple puzzle called the “inverted spectrum.” How is it possible to determine whether two people’s colour experiences are the same? Or, to put the question in terms of physicalism: what physical facts about a person determine that he must be having red experiences and not green ones when he looks at ordinary blood? This problem is made especially acute by the fact that the three-dimensional colour solid (in which every hue, saturation, and tone of every colour can be assigned a specific location) is almost perfectly symmetricaclass="underline" the reds occupy positions on one side of the solid that are nearly symmetrical with the positions occupied by the greens. This suggests that with a little tinkering—e.g., secretly implanting colour-reversal lenses in a child at birth—one could produce someone who used colour vocabulary just as other people do but had experiences that were exactly the reverse of theirs. Or would they be? Perhaps the effect of the tinkering would be to ensure not that the person’s experiences were the reverse of others’ experiences but that they were the same.
The problem is that it seems impossible to imagine how one could discover which description is correct. Unlike the case of the expansion of water, knowing the microphysical facts does not seem to be enough. One would like somehow to get inside other people’s minds, in something like the way each person seems to be able do in his own case. But mere access to the physical facts about other people’s brains does not enable one to do this. (An analogous problem about intentionality was raised by Quine: What physical facts about someone’s brain would determine that he is thinking about a rabbit as opposed to “rabbithood” or “undetached rabbit parts”?)