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,
