Full Idea
There cannot any one moral Rule be proposed, whereof a Man may not justly demand a Reason.
Gist of Idea
We can demand a reason for any moral rule
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
There may be some things which are indisputable duties or prohibitions, and yet people be quite bewildered when asked for the reason behind the rule. Loyalty, incest, courage.