The current Philosophia Christi (14:1) includes Michael Tooley's “strongest argument for materialism,” specifically the belief that there is no soul, no immaterial mind, nothing to explain human mental and behavioral conditions except the physical brain. His explanation of that argument begins here:
One can treat the proposition that humans have immaterial, rational minds as a scientific hypothesis, and then subject that hypothesis to scientific investigation.
Following that he lists several well-known facts of neuroscience showing that our actions and behaviors in the physical world are strongly related to the health of various parts of our brains. This demonstrable physicality of the brain-mental state-behavior connection, he takes it, demonstrates that there is no immaterial, non-physical mind influencing our mental states and our behaviors.
But there is a problem with that. Before I explain it, I want to point out that Tooley has no problem with supposing that an immaterial mind could interact causally with the material world:
Nor do I think that there is any serious a priori objection to the idea of causal connections between physical states of affaris and states of an immaterial mind, since virtually every analysis of causation that has ever been advance is compatible with the possibility of such a connection.
Some readers think such connections are impossible; Tooley does not. His problem with an immaterial mind lies elsewhere, in what I summarized above.
Unless I'm reading him wrong, though, it seems to me as if he is saying that the reason it's hard to believe immaterial mind exists and has causal influence in the physical world is because there is evidence that the brain has causal influence in the physical world. But that's tantamount to saying that we should reject immaterial mind's causal influence in the physical world if it exercises that influence by way of the brain. Why would that be?
Here's the same question, expanded:
1. Any genuine scientific hypothesis includes a differentiating factor: “if condition X, then x; if condition not-X, then y.”
2. Among all the people I know who believe in an immaterial mind, everyone agrees that when an immaterial mind causes something to happen in the physical world, it causes something to happen in the physical world. If I ever meet someone who denies that, I promise I will pay them no further attention. Most believers in immaterial mind take it for granted that when Smith (with his immaterial mind) mind decides to lift an arm, the effect will generally be that Smith will lift his arm.
3. But I do not know of anyone who thinks that Smith's immaterial mind acts directly upon Smith's arm. We all agree that what happens to the arm depends on what happens in the brain.
4. Thus (2 & 3) it is perfectly consistent with belief in an immaterial mind acting on the physical world to suppose that its actions are processed through the brain.
5. But the effect of Smith's decision to lift his arm not always be the lifting of his arm. Perhaps Smith's arm has fallen asleep (once both my arms fell so totally asleep they were temporarily paralyzed). Perhaps his arm is under a heavy rock. Perhaps Smith is an amputee, and doesn't have that arm. Quite obviously Smith's intentions can be thwarted by conditions in the physical world.
6. Immaterial mind's effects are processed through the brain (4), which of course exists in the physical world; so (5) the physical-world effect of Smith's decisions, made in his immaterial mind, can be affected by physical conditions in Smith's brain. If there is a lesion in a particular point in his brain, his decision to lift his arm will have no affect on his arm.
7. Now, whether any of this (2-6) is the case or not is debatable. What's not debatable is that it means that immaterialism predicts that what happens in the physical brain will affect what Smith's immaterial mind can make happen in the physical world. Materialism and immaterialism both predict the same effect. Therefore (1) Tooley's hypothesis does not differentiate materialism from immaterialism; his observations support both theories quite nicely.
If that's his strongest argument for materialism, then the immaterialism has little to fear.