IV. To give validity to an oath, the obligation, which it imposes ought to be lawful. Therefore a sworn promise, to commit an illegal act, to do any thing in violation of natural or revealed law, will be of no effect. V. Indeed if a thing promised upon oath be not actually illegal, but only an obstruction to some greater moral duty, in that case also the oath will not be valid. Because it is a duty which we owe to God not to deprive ourselves of the freedom of doing all the good in our power. VI. Oaths may differ in form, and yet agree in substance. For they all ought to include an appeal to God, calling upon him to witness the truth, or to punish the falsehood of their assertions, both of which amount to the same thing. For an appeal to the testimony of a superior, who has a right to punish, is the same as requiring him to avenge an act of perfidy. Now the omniscience of God gives him power to punish, as well as to witness every degree of falsehood. VII. It was a custom with the ancients to swear by persons or beings expressly distinct from the supreme creator, either imprecating the wrath of those by whom they swore, whether it were the sun, the heavens, or the earth; or swearing by their own heads, by their children, their country or their prince, and calling for destruction upon THEM, if there were any falsehood in their oaths. Nor was this practice confined to Heathen nations only, but, as we are informed by Philo, it prevailed among the Jews. For he says that we ought not, in taking an oath upon every occasion, to have recourse to the maker and father of the universe, but to swear by our parents, by the heavens, the earth, the universe. Thus Joseph is said to have sworn by the life of Pharaoh, according to the received custom of the Egyptians. Nor does our Saviour, in the fifth chapter of St. Matthew's Gospel, intend, as it is supposed by some, to consider these oaths to be less binding than those taken expressly by the name of God. But as the Jews were too much inclined to make use of, and yet disregard them, he shews them that they are real oaths. For, as Ulpian has well observed, he who swears by his own life, seems to swear by God, bearing a respect and reference to his divine power. In the same manner Christ shews that he, who swears by the temple, swears by God who presides in the temple, and that he who swears by Heaven, swears by God, who sits upon the Heavens. But the Jewish teachers of that day thought that men were not bound by oaths made in the name of created beings, unless some penalty were annexed, as if the thing, by which they swore, were consecrated to God. For this is the kind of oath implied in the word κορβᾶν {korban} as BY A GIFT. And it is this error of theirs, which Christ refutes.