Full Idea
It is no harder to conceive how thinking should exist without matter, than how matter should think.
Gist of Idea
Thinking without matter and matter that thinks are equally baffling
Source
John Locke (Essay Conc Human Understanding (2nd Ed) [1694], 2.23.32)
Book Reference
Locke,John: 'Essay Concerning Human Understanding', ed/tr. Nidditch,P.H. [OUP 1979], p.314
A Reaction
This kind of aporia is at the heart of modern 'mysterianism', exemplified by Colin McGinn, and I find that Locke fully endorses such an attitude, and should be seen as the first Mysterian.