Full Idea
It seems not improbable that if we had sufficient knowledge we could infer the state of a man's mind from the state of his brain, or the state of his brain from the state of his mind.
Gist of Idea
We could probably, in principle, infer minds from brains, and brains from minds
Source
Bertrand Russell (The Ultimate Constituents of Matter [1915], p.131)
Book Reference
Russell,Bertrand: 'Mysticism and Logic' [Unwin 1989], p.131
A Reaction
This strikes me as being a very good summary of the claim that mind is reducible to brain, which is the essence of physicalism. Had he been born a little later, Russell would have taken a harder line with physicalism.