Contemporary wisdom holds that all physical events have entirely physical causes -- causes cashed out in terms of physical processes that we can potentially measure. This includes everything we ever say and do. This might be taken to suggest that our reasoning, our intentions, plans and so on, don't actually do anything.
However, it seems clear to me that our reasoning must have causal powers. It cannot merely be physical processes that have causal powers. Otherwise thinking something through, and reaching an understanding, would be illusory since any conclusion at the end of a chain of reasoning would not be caused by the chain of reasoning itself, but rather by the neural correlates of the chain of reasoning. If this is so then we can have no more reason to think that our reasoning processes will lead to true conclusions, than false conclusions (see my Can consciousness be causally inefficacious? for a more comprehensive defence of this point).
Materialists often agree with this but assert that materialism does not have this consequence. It does not have this consequence because, so they claim, conscious experiences, such as our reasoning processes, are literally identical to physical processes in the brain. If a train of thought is literally identical to some physical processes, and these physical processes have causal powers, then it necessarily follows that the train of thought itself has causal powers too. So we have no problem here.
I beg to differ.
Let's suppose that in the brain we have a physical causal chain:
1. A → B → C → D → E
And we have a mental chain representing a chain of reasoning:
2. a → b → c → d → e
Now, of course, the materialist claims that “A” is identical to “a”, “B” is identical to “b” etc.
But nevertheless, we have 2 different accounts of how A/a progresses to E/e. In "1" we have the interactions of molecules as mathematically described by the laws of physics. In "2" we have a train of reasoning which, when we attain an understanding of something, will have involved rational connections between thoughts.
Now if materialism/physicalism is true, then everything has the ability to be explained in terms of the physical as exemplified in "1". Account "2" is simply not required, since physical laws, which describe physical processes, make no reference to reasoning, nor indeed do they make any reference to intentions, desires, plans, or any other aspect of consciousness. Indeed, reasoning only comes into the picture for a vanishingly medium part of the world; namely brain processes, and a minority of brain processes at that. And it is held by materialists that physical laws provide a sufficient explanation for these minority of brain processes just as much as they provide a sufficient explanation for the rest of the Universe.
But it then follows that reasoning something through is causally irrelevant. Hence identifying reasoning, and the rest of our mental life, with physical processes, doesn't allow us to escape an epiphenomenalist position. I regard this as a reductio ad absurdum of materialism.
