“For just as sophisticated doctors and nearly everyone concerned with physical training agree that those who are to be good doctors or physical trainers need experience about nature, so too good legislators need experience of nature, perhaps even more than the former. For the former are craftsmen of only the body’s excellence; the latter who are craftsmen of the excellence of the soul and who profess to teach about the flourishing and failure of the state have in fact an even greater need of philosophy.

“For in all the other craftsmen’s arts the best tools have been discovered from nature, as in carpentry the level, straight-edge and compass (the ones, I take it, that are grasped through water and light and the rays of sunshine), relative to which when we are making a judgment we test what is adequately straight and smooth to our sensation; likewise the politician also needs to have some guidelines from nature and the truth itself relative to which he will distinguish what is just, what is noble and what is useful.”

ὥσπερ γὰρ τῶν ἰατρῶν ὅσοι κομψοὶ καὶ τῶν περὶ τὴν γυμναστικὴν οἱ πλεῖστοι σχεδὸν ὁμολογοῦσιν ὅτι δεῖ τοὺς μέλλοντας ἀγαθοὺς ἰατροὺς ἔσεσθαι καὶ γυμναστὰς περὶ φύσεως ἐμπείρους εἶναι, οὕτω καὶ τοὺς ἀγαθοὺς νομοθέτας ἐμπείρους εἶναι δεῖ τῆς φύσεως, καὶ πολύ γε μᾶλλον ἐκείνων. οἱ μὲν γὰρ τῆς τοῦ σώματος ἀρετῆς εἰσι δημιουργοὶ μόνον, οἱ δὲ περὶ τὰς τῆς ψυχῆς ἀρετὰς ὄντες καὶ περὶ πόλεως εὐδαιμονίας καὶ κακοδαιμονίας διδάξειν προσποιούμενοι πολὺ δὴ μᾶλλον προσδέονται φιλοσοφίας.

καθάπερ γὰρ ἐν ταῖς ἄλλαις τέχναις ταῖς δημιουργικαῖς ἀπὸ τῆς φύσεως εὕρηται τὰ βέλτιστα τῶν ὀργάνων, οἷον ἐν τεκτονικῇ στάθμη καὶ κανὼν καὶ τόρνος † * τὰ μὲν ὕδατι καὶ φωτὶ καὶ ταῖς αὐγαῖς τῶν ἀκτίνων ληφθέντων, πρὸς ἃ κρίνοντες τὸ κατὰ τὴν αἴσθησιν ἱκανῶς εὐθὺ καὶ λεῖον βασανίζομεν, ὁμοίως δὲ καὶ τὸν πολιτικὸν ἔχειν τινὰς ὅρους δεῖ ἀπὸ τῆς φύσεως αὐτῆς καὶ τῆς ἀληθείας, πρὸς οὓς κρινεῖ τί δίκαιον καὶ τί καλόν καὶ τί συμφέρον.

*Pistelli marks this passage with a crux; other editors have tried various solutions, none very satisfying. I’ve looked at the mss. available online, but they all preserve the same text. In their reconstruction of the Protrepticus, Doug and Monte think a line is missing (p.52 of the pdf here). Ronja is working on some compelling solutions to explain what’s going on philosophically. I think it might be an interpolation, τὰ μὲν … ληφθέντων being originally a marginal note: maybe Aristotle (or Iamblichus) didn’t bother giving examples of the kinds of tools “discovered from nature” and so someone early in the tradition wrote in some examples of the kinds of things he might have had in mind and this was later brought into the text.

Aristotle ap. Iamblichus, Protrepticus 10, 54,12–55,3 Pistelli