Plato, Laws 838a

ἐν τῷ νῦν παρόντι τὴν μὲν ῥᾳδίαν ἔχω, τὴν δʼ αὖ τινα τρόπον παντάπασιν ὡς οἷόν τε χαλεπωτάτην. πῶς δὴ λέγεις; ἴσμεν που καὶ τὰ νῦν τοὺς πλείστους τῶν ἀνθρώπων, καίπερ παρανόμους ὄντας, ὡς εὖ τε καὶ ἀκριβῶς εἴργονται τῆς τῶν καλῶν συνουσίας οὐκ ἄκοντες, ὡς οἷόν τε δὲ μάλιστα ἑκόντες. πότε λέγεις; ὅταν ἀδελφὸς ἢ ἀδελφή τῳ γένωνται καλοί. καὶ

which is in one way easy, but in another quite the hardest possible. Explain your meaning. Even at present, as we are aware, most men, however lawless they are, are effectively and strictly precluded from sexual commerce with beautiful persons,—and that not against their will, but with their own most willing consent. On what occasions do you mean? Whenever any man has a brother or sister who is beautiful. So too in the case of a son or daughter, the same unwritten law