Full Idea
The madman is a trustworthy criterion of the appearances which occur in madness.
Gist of Idea
Madmen are reliable reporters of what appears to them
Source
Sextus Empiricus (Against the Logicians (two books) [c.180], I.062)
Book Reference
Sextus Empiricus: 'Against the Logicians', ed/tr. Bury,R.G. [Harvard Loeb 1997], p.33
A Reaction
It is hard to conceive of an genuinely insane person deliberately misreporting their hallucinations. They are, of course, the sole witness.