Plato, Laws 736b

τὰ δὲ χειμάρρων εἰς μίαν λίμνην, ἀναγκαῖον προσέχοντας τὸν νοῦν φυλάττειν ὅπως ὅτι καθαρώτατον ἔσται τὸ συρρέον ὕδωρ, τὰ μὲν ἐξαντλοῦντας, τὰ δʼ ἀποχετεύοντας καὶ παρατρέποντας. πόνος δʼ, ὡς ἔοικεν, καὶ κίνδυνός ἐστιν ἐν πάσῃ κατασκευῇ πολιτικῇ. τὰ δʼ ἐπείπερ λόγῳ γʼ ἐστὶν τὰ νῦν ἀλλʼ οὐκ ἔργῳ πραττόμενα, πεπεράνθω τε ἡμῖν ἡ συλλογὴ καὶ κατὰ νοῦν ἡ καθαρότης αὐτῆς ἔστω συμβεβηκυῖα· τοὺς γὰρ κακοὺς

when there is a confluence of floods from many sources—some from springs, some from torrents—into a single pool we have to take diligent precautions to ensure that the water may be of the utmost possible purity, by drawing it off in some cases, and in others by making channels to divert its course. Yet toil and risk, it would appear, are involved in every exercise of statecraft. Since, however, our present efforts are verbal rather than actual, let us assume that our collection of citizens is now completed, and its purity secured to our satisfaction; for we shall test thoroughly by every kind of test and by length of time the vicious among those