What is the property of “rationality”? Let us say that it is a property of a rational being such that that being can make general use of an understanding of logical and mathematical functions and relationships. Rationality is a necessary component of agency (Kant stresses this): the reasons for the action can only be described with respect to the logical implications that obtain between them (and thus Kant offers a “coherence” theory of truth). Of course ultimately there are physical reasons for everything: one has to eat. “Reason is the slave of the passions,” wrote Hume, and he is right. However, the ability to grasp logical entailments is, as Chomsky, a self-styled “Cartesian rationalist” would say, “generative,” just as logical relationships are themselves non-specific. “If X, then Y.” “X.” The inference to be made from the coincidental truth of these two propositions is the same regardless of what “X” and “Y” are. Humans are able to formalize math and logic, abstracting from concrete incidents in order to be able to study these formal relationships as such. And, although my sympathy with Hume is great and my antipathy towards human exceptionalism equally so, the fact is that empiricism has a hard time dealing with mathematical and logical thought (I will use the term “rational” to refer to this kind of formal thinking for the sake of economy; in any event I think that logic and mathematics are the same thing). The kind of knowledge generated by rational thought (for example the proof of the infinity of prime numbers) appears to go beyond anything that could be explained as the product of interaction with the environment.
The idea that the propositional contents of intentional states are the bearers of logical relationships with each other is an expression of this problem: physical states and processes don’t appear to have any logical relationships whatever, whether they are “in the head” or not. However, this problem is a separate problem from the problem of mental representation, for whether one endorses a representational model of the mind or not one must still acknowledge the supervenient nature of rationality.
In fact the multiple realizability of rationality is the core metaphysical problem here. The problem can be stated this way: there do not appear to be any physical criteria that fix the extension of the set of rational beings. Flipper the dolphin, Max the Martian, Hal 2000 the intelligent artifact and I all take intentional predicates that entail the rationality assumption even though we’re not all made of the same sort of stuff. Although all four of us have physical properties sufficient to instantiate rationality, none of these physical properties are necessary for rationality (since we don’t share them). The extension of our set is indefinitely large.
A common popular view is that emotional experience and feeling are what make the naturalization of psychology so difficult, but philosophers and psychologists from the ancient Greeks on have more frequently taken humans’ rational capacity as the principal warrant for dualism. In fact both of the two greatest rationalists, Plato and Kant, saw emotions as fundamentally physical in origin, “passions” of the soul (like hunger) that sprang from our contingent natures as physical things. Plato and Kant also agreed that the capacity for rational thought was the key to human freedom, which they defined as freedom from the coercion of physical cause-and-effect relations. Both thought that qua embodied beings humans were mere material things, but through participation in transcendent rationality humans became (or could become) more than mere things.
I doubt that being motivated by purely logical thoughts (whatever that would be like; I suspect it’s inconceivable: and see Chapter Four) would result in anything recognizably like our usual conception of “freedom.” After all logical entailments follow necessarily from their antecedent propositions, so that to the extent that one is motivated by purely logical considerations one does not experience choice (or possess any psychological individuality for that matter). But putting the question of freedom aside, it does look like emotional experience is part of consciousness (emotions are essentially phenomenal) whereas rational thought is part of intentionality. Anyway that view, and the view that phenomenal “properties” of mind are, ontologically speaking, identical to physical properties of the body, will be defended in Chapter Three.
It turns out that understanding a difference between Plato and Kant is the key to the resolution of the problem of rationality when viewed as one of the mind-body problems. Before elaborating that difference, however, it will be useful to lay some groundwork by considering the views of two other great rationalists.