Full Idea
There cannot any one moral rule be proposed, whereof a man may not justly demand a reason.
Gist of Idea
There couldn't be a moral rule of which a man could not justly demand a reason
Source
John Locke (Essay Conc Human Understanding (2nd Ed) [1694], 1.03.04)
Book Reference
Locke,John: 'Essay Concerning Human Understanding', ed/tr. Nidditch,P.H. [OUP 1979], p.68
A Reaction
I am more with Locke than Williams here. One can demand reasons up to the point where no one can provide them. Then we look for plan B, which probably ought to be conservative.