It seems to me immensely unlikely that mind is a mere by-product of matter. For if my mental processes are determined wholly by the motions of atoms in my brain, I have no reason to suppose that my beliefs are true. They may be sound chemically, but that does not make them sound logically. And hence I have no reason for supposing my brain to be composed of atoms.
Possible Worlds, p. 209
My own suspicion is that the Universe is not only queerer than we suppose, but queerer than we can suppose.
Possible Worlds and Other Papers (1927), p. 286