Plutarch, De Iside et Osiride 4

ἐπεὶ τούς γε πολλοὺς καὶ τὸ κοινότατον τοῦτο καὶ σμικρότατον λέληθεν, ἐφʼ ὅτῳ τὰς τρίχας οἱ ἱερεῖς ἀποτίθενται καὶ λινᾶς ἐσθῆτας φοροῦσιν· οἱ μὲν γὰρ οὐδʼ ὅλως φροντίζουσιν εἰδέναι περὶ τούτων, οἱ δὲ τῶν μὲν ἐρίων ὥσπερ τῶν κρεῶν, σεβομένους τὸ πρόβατον, ἀπέχεσθαι λέγουσι, ξυρεῖσθαι δὲ τὰς κεφαλὰς διὰ τὸ πένθος, φορεῖν δὲ τὰ λινᾶ διὰ τὴν χρόαν, ἣν τὸ λίνον ἀνθοῦν ἀνίησι τῇ περιεχούσῃ τὸν κόσμον αἰθερίῳ χαροπότητι προσεοικυῖαν. ἡ δʼ ἀληθὴς αἰτία μία πάντων ἐστί· καθαροῦ γάρ, ᾗ φησιν ὁ Πλάτων, οὐ θεμιτὸν ἅπτεσθαι μὴ καθαρῷ· περίσσωμα δὲ τροφῆς καὶ σκύβαλον οὐδὲν ἁγνὸν οὐδὲ καθαρόν ἐστιν· ἐκ δὲ περιττωμάτων ἔρια καὶ λάχναι καὶ τρίχες καὶ ὄνυχες ἀναφύονται καὶ βλαστάνουσι. γελοῖον οὖν ἦν τὰς μὲν αὑτῶν τρίχας ἐν ταῖς ἁγνείαις ἀποτίθεσθαι ξυρουμένους καὶ λειαινομένους πᾶν ὁμαλῶς τὸ σῶμα, τὰς δὲ τῶν θρεμμάτων ἀμπέχεσθαι καὶ φορεῖν· καὶ γὰρ τὸν Ἡσίοδον οἴεσθαι δεῖ λέγοντα μηδʼ ἀπὸ πεντόζοιο θεῶν ἐν δαιτὶ θαλείῃ αὖον ἀπὸ χλωροῦ τάμνειν αἴθωνι σιδήρῳ διδάσκειν ὅτι δεῖ καθαροὺς τῶν τοιούτων γενομένους ἑορτάζειν, οὐκ ἐν αὐταῖς ταῖς ἱερουργίαις χρῆσθαι καθάρσει καὶ ἀφαιρέσει τῶν περιττωμάτων. τὸ δὲ λίνον φύεται μὲν ἐξ ἀθανάτου τῆς γῆς καὶ καρπὸν ἐδώδιμον ἀναδίδωσι, λιτὴν δὲ παρέχει καὶ καθαρὰν ἐσθῆτα καὶ τῷ σκέποντι μὴ βαρύνουσαν, εὐάρμοστον δὲ πρὸς πᾶσαν ὥραν, ἥκιστα δὲ φθειροποιόν, ὡς λέγουσι· περὶ ὧν ἕτερος λόγος.

It is true that most people are unaware of this very ordinary and minor matter: the reason why the priests remove their hair and wear linen garments. Some persons do not care at all to have any knowledge about such things, while others say that the priests, because they revere the sheep, abstain from using its wool, as well as its flesh; and that they shave their heads as a sign of mourning, and that they wear their linen garments because of the colour which the flax displays when in bloom, and which is like to the heavenly azure which enfolds the universe. But for all this there is only one true reason, which is to be found in the words of Plato; for the Impure to touch the Pure is contrary to divine ordinance. No surplus left over from food and no excrementitious matter is pure and clean; and it is from forms of surplus that wool, fur, hair, and nails originate and grow. So it would be ridiculous that these persons in their holy living should remove their own hair by shaving and making their bodies smooth all over, and then should put on and wear the hair of domestic animals. We should believe that when Hesiod said, Cut not the sere from the green when you honour the gods with full feasting, Paring with glittering steel the member that hath the five branches, he was teaching that men should be clean of such things when they keep high festival, and they should not amid the actual ceremonies engage in clearing away and removing any sort of surplus matter. But the flax springs from the earth which is immortal; it yields edible seeds, and supplies a plain and cleanly …