According to functionalism, the same belief can be instantiated in individuals whose cognitive processes occur in different substrata (think of how there might be a silicon-based life form capable of reasoning, including, for example, mathematical calculation).
The central claim of reductionism, on the other hand, is that cognitive processes are identical with physical processes. If that is true, then it would seem at first glance that wherever there is a difference in physical substrata there will different physical processes and hence different cognitive processes. At a certain level of physical description, for example, a computation done by a carbon-based life form differs from a computation done by a silicon-based life form. Since their computational processes differ at the level of physics, their cognitive processes do as well. They do not, therefore, think in the same way. In this way, it seems that reductionism is inconsistent with functionalism.
I think the functionalist reply is that functionalism is holistic, whereas the above objection is focused on the parts. Think of two car jacks. One is made of one metal, the other of another metal. They have the same shape so that I can use them interchangeably. Their inner mechanisms may be correspondingly different in order to compensate for differences in their substrata in such a way that they end up yielding the same output. Or their inner mechanisms may be configured in the same way. It doesn't matter how they are configured: what does matter is that they work the same way. That is, if I use precise tools to measure their "outputs" (i.e., the mechanical advantage that they give me) without peeking inside, I get the same numbers. Holistically considered, they function in exactly the same way.
Holism of this sort might seem helpful to a behaviorist, who is not interested in in the inner workings of human nature. For other psychologists, as well as cognitive scientists and many philosophers, however, this sort of holism just won't do. For they would still ask how cognition, appetite, and physical effort work together to produce speech acts that have the same meaning. To answer that question, one must consider how the parts interact. And if those parts differ at the level of physical description, then how is it that they interact in the same way?
The answer to this question must keep in mind the sort of argument introduced by Dennett and Carroll to justify their (reductive) brand of materialism: nothing other than physical forces will do, because anything else will violate the law of conservation. So any answer to the question must work in terms of physics: it can't appeal to the meaning that I attribute to the as if it were a standard guiding us in our estimation of how to describe those forces, for according to reductionism, meaning is not a physical force.
That's a requirement that I think cannot be met.
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