33.  For our treatment does not correspond with virtue and vice, one of which is most excellent and beneficial at all times and in all cases, and the other most evil and harmful; and, instead of one and the same of our medicines invariably proving either most wholesome or most dangerous in the same cases—be it severity or gentleness, or any of the others which we have enumerated—in some cases it proves good and useful, in others again it has the contrary effect, according, I suppose, as time and circumstance and the disposition of the patient admit.  Now to set before you the distinction between all these things, and give you a perfectly exact view of them, so that you may in brief comprehend the medical art, is quite impossible, even for one in the highest degree qualified by care and skill:  but actual experience and practice are requisite to form77    Are requisite to form, lit., by ‘actual…they become clear to.’ a medical system and a medical man.

ΛΓʹ. Οὐ γὰρ ὥσπερ ἐπὶ τῆς ἀρετῆς ἔχει καὶ τῆς κακίας, τὴν μὲν καλλίστην εἶναι καὶ ὠφελιμωτάτην ἀεὶ καὶ πᾶσι, τὴν δὲ χειρίστην τε καὶ βλαβερωτάτην: οὕτω καὶ τῆς φαρμακείας τῆς ἡμετέρας, ἕν τι καὶ τὸ αὐτὸ ὑγιεινότατον, ἢ ἐπισφαλέστατον ἀεὶ καὶ τοῖς αὐτοῖς ἀποδέδεικται: οἷον τὸ αὐστηρὸν, ἢ τὸ πρᾶον, ἢ τῶν ἄλλων ὧν ἀπηριθμησάμην ἕκαστον. Ἀλλὰ τοῖς μὲν τοῦτο καλὸν καὶ χρήσιμον, τοῖς δὲ τοὐναντίον πάλιν, ὅπως ἂν, οἶμαι, συμπίπτωσιν οἵ τε καιροὶ καὶ τὰ πράγματα, καὶ ὁ τῶν θεραπευομένων ἐπιδέχηται τρόπος. Ἃ πάντα μὲν διελέσθαι λόγῳ, καὶ συνιδεῖν ἐπὶ τὸ ἀκριβέστατον, ὥστε καὶ κεφαλαίῳ τὴν θεραπείαν περιλαβεῖν ἀμήχανον, κἂν ἐπὶ πλεῖστον ἐξίκηταί τις ἐπιμελείας τε καὶ συνέσεως: ἐπὶ δὲ τῆς πείρας αὐτῆς καὶ τῶν πραγμάτων τῷ θεραπευτῇ λόγῳ καὶ ἀνδρὶ καταφαίνεται.