Mind: A Brief Introduction (John R. Searle) / quote 96 of 138
Causal reduction without ontological reduction
On the dualist’s conception, consciousness is definitely something over and above its material substrate. Indeed the dualist supposes that the irreducibility of consciousness already implies that consciousness is something over and above its neurobiological base. I deny that implication. This point is so crucial for the entire argument of this book that I am going to spell it out in some detail. The fact that the causal powers of consciousness and the causal powers of its neuronal base are exactly the same shows that we are not talking about two independent things, consciousness and neuronal processes. If two things in the real empirical world have an independent existence they must have different causal powers. But the causal powers of consciousness are exactly the same as those of the neuronal substrate. This situation is exactly like the causal powers of solid objects and the causal powers of their molecular constituents. We are not talking about two different entities but about the same system at different levels. Consciousness differs from solidity, liquidity, etc., in that the causal reduction does not lead to an ontological reduction. This, as we have seen, is so for an obvious, and indeed trivial, reason. Consciousness has a first-person ontology; neuronal processes have a third-person ontology. For that reason you cannot ontologically reduce the former to the latter. Consciousness is thus an aspect of the brain, the aspect that consists of ontologically subjective experiences. But there are not two different metaphysical realms in your skull, one “physical” and one “mental.” Rather, there are just processes going on in your brain and some of them are conscious experiences.