Discourses: Book 2 26 : 1

Every error involves a contradiction. For since he who is in error does not wish to err, but to be right, ​it is clear that he is not doing what he wishes. For what does the thief wish to achieve? His own interest. Therefore, if thievery is against his interest, he is not doing what he wishes. Now every rational soul is by nature offended by contradiction; and so, as long as a man does not understand that he is involved in contradiction, there is nothing to prevent him from doing contradictory things, but when he has come to understand the contradiction, he must of necessity abandon and avoid it, just as a bitter necessity compels a man to renounce the false when he perceives that it is false; but as long as the falsehood does not appear, he assents to it as the truth.