I do not trust my eyes to tell me what a man is: I have a better and more trustworthy light by which I can distinguish what is true from what is false: let the mind find out what is good for the mind.
Oculis de homine non credo, habeo melius et certius lumen quo a falsis uera diiudicem: animi bonum animus inueniat.
Original text (Latin)
Notes from Wikiquote
De Vita Beata (On the Happy Life): cap. 2, line 2
Variants
I do not distinguish by the eye, but by the mind, which is the proper judge of the man. (translator unknown).