Aristotle's Metaphysics, 994a,26-7:
ἀεὶ γάρ ἐστι μεταξύ, ὥσπερ τοῦ εἶναι καὶ μὴ εἶναι γένεσις, οὕτω καὶ τὸ γιγνόμενον τοῦ ὄντος καὶ μὴ ὄντος
for there is always an intermediate, so that as between being and not being there is coming to be, so too the thing that is coming to be is between the thing that is and the thing that is not
How can an adverb function as a noun? I have seen this function in some other instances, too.