Epictetus, Works, book 1, Περὶ τῶν ἐφ’ ἡμῖν καὶ οὐκ ἐφ’ ἡμῖν. 4:

τίσ οὖν ἐρεῖ; ἡ καὶ αὑτὴν θεωροῦσα καὶ τἆλλα πάντα. αὕτη δ’ ἐστὶ τίσ; ἡ δύναμισ ἡ λογική· μόνη γὰρ αὕτη καὶ αὑτὴν κατανοήσουσα παρείληπται, τίσ τέ ἐστι καὶ τί δύναται καὶ πόσου ἀξία οὖσα ἐλήλυθεν, καὶ τὰσ ἄλλασ ἁπάσασ.