Plato, Epinomis 990c

τῶν αὐτῶν διαλεγώμεθα, τὰς ἄλλας ὅσας ἐν τῷ πρόσθεν διεξήλθομεν ὁδοὺς τούτων οὐ ῥᾴδιον συννοεῖν, ἐπὶ δὲ ταῦτα παρασκευάζοντας φύσεις οἵας δυνατὸν εἶναι, χρεὼν πολλὰ προδιδάσκοντα καὶ ἐθίζοντα ἀεὶ διαπονήσασθαι παῖδα ὄντα καὶ νεανίσκον. διὸ μαθημάτων δέον ἂν εἴη· τὸ δὲ μέγιστόν τε καὶ πρῶτον καὶ ἀριθμῶν αὐτῶν ἀλλʼ οὐ σώματα ἐχόντων, ἀλλὰ ὅλης τῆς τοῦ περιττοῦ τε καὶ ἀρτίου γενέσεώς τε καὶ δυνάμεως, ὅσην παρέχεται πρὸς τὴν τῶν ὄντων φύσιν.

in our discussion, the other courses of these bodies that we have previously described are not easily understood: we must rather prepare our faculties, such as they may possibly be, for these matters; and so one must teach the pupil many things beforehand, and continually strive hard to habituate him in childhood and youth. And therefore there will be need of studies: the most important and first is of numbers in themselves; not of those which are corporeal, but of the whole origin of the odd and the even, and the greatness of their influence on the nature of reality.