Plato, Laws 679c

γὰρ ὕβρις οὔτʼ ἀδικία, ζῆλοί τε αὖ καὶ φθόνοι οὐκ ἐγγίγνονται. ἀγαθοὶ μὲν δὴ διὰ ταῦτά τε ἦσαν καὶ διὰ τὴν λεγομένην εὐήθειαν· ἃ γὰρ ἤκουον καλὰ καὶ αἰσχρά, εὐήθεις ὄντες ἡγοῦντο ἀληθέστατα λέγεσθαι καὶ ἐπείθοντο. ψεῦδος γὰρ ὑπονοεῖν οὐδεὶς ἠπίστατο διὰ σοφίαν, ὥσπερ τὰ νῦν, ἀλλὰ περὶ θεῶν τε καὶ ἀνθρώπων τὰ λεγόμενα ἀληθῆ νομίζοντες ἔζων κατὰ ταῦτα· διόπερ ἦσαν τοιοῦτοι παντάπασιν οἵους αὐτοὺς ἡμεῖς ἄρτι διεληλύθαμεν.

for in it there is no place for the growth of insolence and injustice, of rivalries and jealousies. So these men were good, both for these reasons and because of their simple-mindedness, as it is called; for, being simple-minded, when they heard things called bad or good, they took what was said for gospel-truth and believed it. For none of them had the shrewdness of the modern man to suspect a falsehood; but they accepted as true the statements made about gods and men, and ordered their lives by them. Thus they were entirely of the character we have just described.