Miscellanies 7.8.50

The one who is proved in such piety is far from being ready either to lie or to swear. For an oath is a decisive declaration with the added invocation of the divine. How could the one who is once faithful show himself unfaithful, so as to need an oath, and not have his life itself be a firm and decisive oath? He lives and conducts himself, and shows the trustworthiness of his word in a life and speech that are unchanging and steadfast. And if wrongdoing lies in the judgment of the one who acts and speaks, and not in what the one wronged suffers, he will neither lie nor perjure himself as though wronging the divine, knowing that it is by nature beyond harm. Nor will he lie or break any agreement because of his neighbor, whom he has learned to love, even if the neighbor is no acquaintance of his. And still less, for his own sake, will he lie or perjure himself, since he would never willingly be found unjust toward himself. He will not even swear, choosing to use for assent only “yes” and for denial “no.” For to swear is to bring forth an oath, or … as an oath, from the mind by way of affirmation.