Laudator Temporis Acti

Athenaeus 6.273d-e (tr. S. Douglas Olson):

Because it is a mark of intelligent men to maintain the ancient practices that allowed them to overcome other nations in war, while simultaneously adopting anything good or useful that their defeated enemies had worth imitating. This is what the Romans did in ancient times: they preserved their traditional practices, while at the same time taking over from the people they conquered any remnant of good behavior they discovered there, leaving them what was worthless, so they would never be able to recover their losses.

συνετῶν γάρ ἐστιν ἀνδρῶν ἐμμένειν τοῖς παλαιοῖς ζηλώμασιν δι᾽ ὧν στρατευόμενοι κατεστρέφοντο τοὺς ἄλλους, λαμβάνοντες ἅμα τοῖς δοριαλώτοις καὶ εἴ τι χρήσιμον καὶ καλὸν ὑπῆρχε παρ᾽ ἐκείνοις εἰς μίμησιν: ὅπερ ἐν τοῖς πάλαι χρόνοις ἐποίουν οἱ Ῥωμαῖοι. διαφυλάττοντες γὰρ ἅμα καὶ τὰ πάτρια μετῆγον παρὰ τῶν χειρωθέντων εἲ τι λείψανον καλῆς ἀσκήσεως εὕρισκον, τὰ ἀχρηστα ἐκείνοις ἐῶντες, ὅπως μηδ᾽ εἰς ἀνάκτησιν ὧν ἀπέβαλον ἐλθεῖν ποτε δυνηθῶσι.

Cf. Sallust, The War with Catiline 51.37-38 (reporting a speech by Caesar; tr. J.C. Rolfe):

Our ancestors, Fathers of the Senate, were never lacking either in wisdom or courage, and yet pride did not keep them from adopting foreign institutions, provided they were honourable. They took their offensive and defensive weapons from the Samnites, the badges of their magistrates for the most part from the Etruscans. In fine, whatever they found suitable among allies or foes, they put in practice at home with the greatest enthusiasm, preferring to imitate rather than envy the successful.

maiores nostri, patres conscripti, neque consili neque audaciae umquam eguere; neque illis superbia obstat, quo minus aliena instituta, si modo proba erant, imitarentur. arma atque tela militaria ab Samnitibus, insignia magistratuum ab Tuscis pleraque sumpserunt. postremo, quod ubique apud socios aut hostis idoneum videbatur, cum summo studio domi exsequebantur: imitari quam invidere bonis malebant.