Miscellanies 7.8.51

It is enough for him, then, to add to either assent or denial “I speak truly,” to assure those who do not perceive the firmness of his answer. For I think one should have toward outsiders a life worthy of trust, so that an oath is not even asked for, and toward oneself and those who understand, fair-mindedness, which is voluntary justice. The one who knows, indeed, keeps his oath, yet he is not prone to swearing; he comes to swearing rarely, and then only in the way we have said. Speaking truth under oath comes about through agreement with what is true; keeping one's oath, then, follows from right action in one's duties. Where, then, is there any further need of an oath for one who lives at the height of truth? The one who does not even swear is far from perjuring himself; and the one who breaks none of his agreements would never swear at all, since the ratification of both breaking and fulfilling lies in deeds, just as lying and false swearing lie in speaking and swearing contrary to duty. But the one who lives justly, breaking none of his duties, where the judgment of truth is tested, keeps his oath by his deeds; the testimony of the tongue is superfluous for him. Persuaded, then, that God is everywhere always, ashamed not to tell the truth, and knowing lying to be unworthy of himself, he is content with God's knowledge and his own conscience alone. So he does not lie or do anything contrary to his agreements; so too he does not swear an oath when one is demanded of him, nor does he ever deny, lest he lie, even if he dies under torture.