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Full Idea
We may never be able to know whether any material being thinks; it being impossible for us, by contemplation of our own ideas to discover whether Omnipotency has not given to some systems of matter fitly disposed, a power to perceive and think.
Gist of Idea
For all we know, an omnipotent being might have enabled material beings to think
Source
John Locke (Essay Conc Human Understanding (2nd Ed) [1694], 4.03.06)
Book Ref
Locke,John: 'Essay Concerning Human Understanding', ed/tr. Nidditch,P.H. [OUP 1979], p.540
A Reaction
Leibniz attacked this vigorously, but I have to agree with Locke. We now see that it is just as mysterious for 'mental' substance to think as it is for physical substance. If in doubt, apply the Razor, and stick with the substance you know.