Plato, Laws 676c

λόγον οὐκ ἐλάττους ἐφθαρμέναι; πεπολιτευμέναι δʼ αὖ πάσας πολιτείας πολλάκις ἑκασταχοῦ; καὶ τοτὲ μὲν ἐξ ἐλαττόνων μείζους, τοτὲ δʼ ἐκ μειζόνων ἐλάττους, καὶ χείρους ἐκ βελτιόνων γεγόνασι καὶ βελτίους ἐκ χειρόνων; ἀναγκαῖον. ταύτης δὴ πέρι λάβωμεν, εἰ δυναίμεθα, τῆς μεταβολῆς τὴν αἰτίαν· τάχα γὰρ ἂν ἴσως δείξειεν ἡμῖν τὴν πρώτην τῶν πολιτειῶν γένεσιν καὶ μετάβασιν. εὖ λέγεις, καὶ προθυμεῖσθαι δεῖ, σὲ μὲν ὃ διανοῇ περὶ αὐτῶν ἀποφαινόμενον, ἡμᾶς δὲ συνεπομένους.

And have they not in each case exhibited all kinds of constitutions over and over again? And have they not changed at one time from small to great, at another from great to small, and changed also from good to bad and from bad to good? Necessarily. Of this process of change let us discover, if we can, the cause; for this, perhaps, would show us what is the primary origin of constitutions, as well as their transformation. You are right; and we must all exert ourselves,—you to expound your view about them, and we to keep pace with you.