Plato, Laws 735b

πᾶσαν ἀγέλην ποιμὴν καὶ βουκόλος τροφεύς τε ἵππων καὶ ὅσα ἄλλα τοιαῦτα παραλαβών, οὐκ ἄλλως μή ποτε ἐπιχειρήσει θεραπεύειν ἢ πρῶτον μὲν τὸν ἑκάστῃ προσήκοντα καθαρμὸν καθαρεῖ τῇ συνοικήσει, διαλέξας δὲ τά τε ὑγιῆ καὶ τὰ μὴ καὶ τὰ γενναῖα καὶ ἀγεννῆ, τὰ μὲν ἀποπέμψει πρὸς ἄλλας τινὰς ἀγέλας, τὰ δὲ θεραπεύσει, διανοούμενος ὡς μάταιος ἂν ὁ πόνος εἴη καὶ ἀνήνυτος περί τε σῶμα καὶ ψυχάς, ἃς φύσις

In dealing with a flock of any kind, the shepherd or cowherd, or the keeper of horses or any such animals, will never attempt to look after it until he has first applied to each group of animals the appropriate purge—which is to separate the sound from the unsound, and the well-bred from the ill-bred, and to send off the latter to other herds, while keeping the former under his own care; for he reckons that his labor would be fruitless and unending if it were spent on bodies and souls which nature and