Plato, Laws 650b

γε τούτων οὔτʼ ἂν Κρῆτας οὔτʼ ἄλλους ἀνθρώπους οὐδένας οἰόμεθα ἀμφισβητῆσαι, μὴ οὐ πεῖράν τε ἀλλήλων ἐπιεικῆ ταύτην εἶναι, τό τε τῆς εὐτελείας καὶ ἀσφαλείας καὶ τάχους διαφέρειν πρὸς τὰς ἄλλας βασάνους. ἀληθὲς τοῦτό γε. τοῦτο μὲν ἄρʼ ἂν τῶν χρησιμωτάτων ἓν εἴη, τὸ γνῶναι τὰς φύσεις τε καὶ ἕξεις τῶν ψυχῶν, τῇ τέχνῃ ἐκείνῃ ἧς ἐστιν ταῦτα θεραπεύειν· ἔστιν δέ που, φαμέν, ὡς οἶμαι, πολιτικῆς. ἦ γάρ; πάνυ μὲν οὖν.

I imagine, nor any other people would dispute the fact that herein we have a fair test of man by man, and that for cheapness, security and speed it is superior to all other tests. That certainly is true. This then—the discovery of the natures and conditions of men’s souls—will prove one of the things most useful to that art whose task it is to treat them; and that art is (as I presume we say) the art of politics: is it not so? Undoubtedly.