The only roads of enquiry there are to think of: one, that it is and that it is not possible for it not to be, this is the path of persuasion (for truth is its companion); the other, that it is not and that it must not be — this I say to you is a path wholly unknowable.

Frag. B 2.2-6, quoted by Proclus, Commentary on the Timaeus I, 345