Spinoza on why there can only be one substance


It is just possible to discern some points beneath the heated rhetoric in which Patricia Churchland indulges. But none of these points is right. If you hold that “mental processes are actually processes in the brain,” to quote Churchland, then you are committed to the thesis that it is sufficient to understand the mind that one understands the brain, and not merely necessary. This is just the well-known “identity theory” of mind and brain: mental processes are identical to brain processes; and the identity of a with b entails the sufficiency of a for b. To hold the weaker thesis that knowledge of the brain is merely necessary for knowledge of the mind is consistent even with being a heavy-duty Cartesian dualist, since even such a dualist accepts that mind depends causally on brain.

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