Plato, Letters 322d

μέγιστα ὠφελεῖν. Ἑρμείᾳ μὲν γὰρ οὔτε ἵππων πλῆθος οὔτε ἄλλης πολεμικῆς συμμαχίας οὐδʼ αὖ χρυσοῦ προσγενομένου γένοιτʼ ἂν μείζων εἰς τὰ πάντα δύναμις, ἢ φίλων βεβαίων τε καὶ ἦθος ἐχόντων ὑγιές· Ἐράστῳ δὲ καὶ Κορίσκῳ, πρὸς τῇ τῶν εἰδῶν σοφίᾳ τῇ καλῇ ταύτῃ, φήμʼ ἐγώ, καίπερ γέρων ὤν, προσδεῖν σοφίας τῆς περὶ τοὺς πονηροὺς καὶ ἀδίκους

so that you can help one another in the things of greatest importance. For Hermeias will find in his multitude of horses or of other military equipment, or even in the gaining of gold itself, no greater source of power for all purposes than in the gaining of steadfast friends possessed of a sound character; while Erastus and Coriscus, in addition to this fair Science of Ideas, need also—as I, old though I am, assert—the science which is a safeguard in dealing with the wicked and unjust, and a kind of self-defensive power.