Full Idea
To ask whether man's will be free is as improper as to ask whether sleep be swift, or virtue square.
Gist of Idea
Asking whether man's will is free is liking asking if sleep is fast or virtue is square
Source
John Locke (Essay Conc Human Understanding (2nd Ed) [1694], 2.21.14)
Book Reference
Locke,John: 'Essay Concerning Human Understanding', ed/tr. Nidditch,P.H. [OUP 1979], p.240
A Reaction
Beautiful illustrations of category mistakes, long before the actual phrase was coined.