Chapter 6, Footnote #10
Mind-body problem irrelevant
Those who maintain, that the mind and the body are one substance may here object, that upon that supposition the distinction between frame of mind and frame of body is but nominal, and that accordingly there is no such thing as a frame of mind distinct from the frame of body. But granting, for argument-sake, the antecedent, we may dispute the consequence. For if the mind be but a part of the body, it is at any rate of a nature very different from the other parts of the body.
A man's frame of body cannot in any part of it undergo any considerable alteration without its being immediately indicated by phenomena discernible by the senses. A man's frame of mind may undergo very considerable alterations, his frame of body remaining the same to all appearance; that is, for any thing that is indicated to the contrary by phenomena cognizable to the senses: meaning those of other men.