We have trancendental reasons for supposing that our intentionality, normativity, purposiveness, and subjectivity is real. If our thought are not about anything determinate, then the thoughts of scientists are not about anything determinate, and the thoughts of atheistic philosophers are not about anything determinate. If there is no mental causation; if mental states do not cause other mental states, then scientists can't say that they believe what they believe on the basis of the scientific evidence. If there is no mental causation, then you can't say that you reject the existence of God because of the evil in the world.
I love the Fodor quote: if it isn't literally true that my wanting is causally responsible for my reaching, and my itching is causally responsible for my scratching, and my believing is causally responsible for my saying. . . . if none of that is literally true, then practically everything I believe about anything is false and it's the end of the world.
( Fodor, A Theory of Content and Other Essays, Cambridge, Mass, Bradford Book/MIT Press, 1990, p. 156; quoted in Stich, Deconstructing the Mind, New York, 1996, OUP, p. 169)