Plato, Laws 950a

καινοτομίας ἀλλήλοις ἐμποιούντων ξένων ξένοις· ὃ δὴ τοῖς μὲν εὖ πολιτευομένοις διὰ νόμων ὀρθῶν βλάβην ἂν φέροι μεγίστην πασῶν, ταῖς δὲ πλείσταις πόλεσιν, ἅτε οὐδαμῶς εὐνομουμέναις, οὐδὲν διαφέρει φύρεσθαι δεχομένους τε αὐτοῖς ξένους καὶ αὐτοὺς εἰς τὰς ἄλλας ἐπικωμάζοντας πόλεις, ὅταν ἐπιθυμήσῃ τις ἀποδημίας ὁπῃοῦν καὶ ὁπότε, εἴτε νέος εἴτε καὶ πρεσβύτερος ὤν. τὸ δʼ αὖ μήτε ἄλλους δέχεσθαι μήτε αὐτοὺς ἄλλοσε ἀποδημεῖν ἅμα μὲν οὐκ ἐγχωρεῖ τό γε

and this result would cause immense damage to peoples who enjoy a good polity under right laws; but the majority of States are by no means well governed, so that to them it makes no difference if their population is mixed through the citizens admitting strangers and through their own members visiting other States whenever any one of them, young or old, at any time or place, desires to go abroad. Now for the citizens to refuse altogether either to admit others or to go abroad themselves is by no means a possible policy,