Full Idea
It is about the strongest proof one could offer of a proposition being evident, that even he who contradicts it finds himself having to make use of it.
Gist of Idea
Self-evidence is most obvious when people who deny a proposition still have to use it
Source
Epictetus (The Discourses [c.56], 2.20.01)
Book Reference
Epictetus: 'The Discourses, The Handbook, Fragments', ed/tr. Gill,C [Everyman 1995], p.126
A Reaction
Philosophers sometimes make fools of themselves by trying, by the use of elaborate sophistry, to demolish propositions which are self-evidently true. Don't be one of these philosophers!