As I see it, materialism has three components. One is the causal close of the physical. The second is the mechanism of the physical, that there are no mental states in the supervenience base. Causation occurs without purpose at the basic level. The thrd is the supervenience of all other states on the physical. The second is the tripper for materialist who wants to be a compatibilist. An action caused by a desire is not mechanistically caused. The brain state that is the desire might cause the brain state that begins the action, but not because of the content of the desire. Think of this. Suppose an opera singer were to tell someone 'Look at that glass window. I will tell it to shatter, and it will listen to me and shatter. She sings 'Shatter now" and it shatters. But the shattering has nothing to do with the content of the words she sings. Hence compatibilist free will, which requires causation by desire, is impossible on materialism.
