Full Idea
Morality is capable of demonstration as well as mathematics, since the precise real essence of the things moral words stand for may be perfectly known, and so the congruity or incongruity of the things themselves be certainly discovered.
Gist of Idea
Morality can be demonstrated, because we know the real essences behind moral words
Source
John Locke (Essay Conc Human Understanding (2nd Ed) [1694], 3.11.16)
Book Reference
Locke,John: 'Essay Concerning Human Understanding', ed/tr. Nidditch,P.H. [OUP 1979], p.516
A Reaction
This is a rare case of Locke saying that we can know real essences (he apparently having said the same about triangles). I increasingly like the notion that real essences lead to true knowledge in every sphere of our activities, including the moral.