But he who knows what insanity is, is sane; whereas insanity can no more be sensible of its own existence, than blindness can see itself.

[Sanus est, qui scit quid sit insania, quippe insania scire se non potest, non magis quam caecitas se videre.]

Apuleius (AD c. 124 - c. 170) Numidian Roman writer, philosopher, rhetorician [Lucius Apuleius Madaurensis]

Apologia; or, A Discourse on Magic [Apologia; seu, Pro Se de Magia], ch. 80 [tr. Bohn’s (1853)]
    (Source)

Alt. trans.: "He who knows what madness is, is ipso facto sane. For madness cannot know itself any more than blindness can see itself." [tr. Butler (1909)]