Plato, Laws 894b

λαβεῖν μετʼ ἀριθμοῦ, πλήν γε, ὦ φίλοι, δυοῖν; ποίαιν δή; σχεδόν, ὠγαθέ, ἐκείναιν ὧν ἕνεκα πᾶσα ἡμῖν ἐστιν ἡ σκέψις τὰ νῦν. λέγε σαφέστερον. ψυχῆς ἦν ἕνεκά που; πάνυ μὲν οὖν. ἔστω τοίνυν ἡ μὲν ἕτερα δυναμένη κινεῖν κίνησις, ἑαυτὴν δὲ ἀδυνατοῦσα, ἀεὶ μία τις, ἡ δὲ αὑτήν τʼ ἀεὶ καὶ ἕτερα δυναμένη κατά τε συγκρίσεις ἔν τε διακρίσεσιν αὔξαις τε καὶ τῷ ἐναντίῳ καὶ γενέσεσι καὶ φθοραῖς ἄλλη μία τις

save only two? What two? Those, my good sir, for the sake of which, one may say, the whole of our present enquiry was undertaken. Explain more clearly. It was undertaken, was it not, for the sake of soul? Certainly. As one of the two let us count that motion which is always able to move other things, but unable to move itself; and that motion which always is able to move both itself and other things,—by way of combination and separation, of increase and decrease, of generation and corruption,—let us count as another separate unit