, "his sin shall be upon
him: and if he dissemble it," by swearing falsely, and with
dissimulation, "he offendeth double," (because, to wit, "pretended
equity is a twofold iniquity," as Augustine [*Enarr. in Ps. lxiii, 7]
declares): "and if he swear in vain," i.e. without due cause and
necessity, "he shall not be justified."
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SIXTH ARTICLE [II-II, Q. 89, Art. 6]

Whether It Is Lawful to Swear by Creatures?

Objection 1: It would seem that it is not lawful to swear by
creatures. It is written (Matt. 5:34-36): "I say to you not to swear
at all, neither by heaven . . . nor by the earth . . . nor by
Jerusalem . . . nor by thy head": and Jerome, expounding these words,
says: "Observe that the Saviour does not forbid swearing by God, but
by heaven and earth," etc.

Obj. 2: Further, punishment is not due save for a fault. Now a
punishment is appointed for one who swears by creatures: for it is
written (22, qu. i, can. Clericum): "If a cleric swears by creatures
he must be very severely rebuked: and if he shall persist in this
vicious habit we wish that he be excommunicated." Therefore it is
unlawful to swear by creatures.

Obj. 3: Further, an oath is an act of religion, as stated above (A.
4). But religious worship is not due to any creature, according to
Rom. 1:23, 25. Therefore it is not lawful to swear by a creature.

_On the contrary,_ Joseph swore "by the health of Pharaoh" (Gen.
42:16). Moreover it is customary to swear by the Gospel, by relics,
and by the saints.

_I answer that,_ As stated above (A. 1, ad 3), there are two kinds of
oath. One is uttered as a simple contestation or calling God as
witness: and this kind of oath, like faith, is based on God's truth.
Now faith is essentially and chiefly about God Who is the very truth,
and secondarily about creatures in which God's truth is reflected, as
stated above (Q. 1, A. 1). In like manner an oath is chiefly referred
to God Whose testimony is invoked; and secondarily an appeal by oath
is made to certain creatures considered, not in themselves, but as
reflecting the Divine truth. Thus we swear by the Gospel, i.e. by God
Whose truth is made known in the Gospel; and by the saints who
believed this truth and kept it.

The other way of swearing is by cursing and in this kind of oath a
creature is adduced that the judgment of God may be wrought therein.
Thus a man is wont to swear by his head, or by his son, or by some
other thing that he