As to Joseph's oath by the health of Pharaoh this may be understood in both ways: either by way of a curse, as though he pledged Pharao's health to God; or by way of contestation, as though he appealed to the truth of God's justice which the princes of the earth are appointed to execute.
Reply Obj. 1: Our Lord forbade us to swear by creatures so as to give them the reverence due to God. Hence Jerome adds that "the Jews, through swearing by the angels and the like, worshipped creatures with a Divine honor."
In the same sense a cleric is punished, according to the canons (22, qu. i, can. Clericum, Obj. 2), for swearing by a creature, for this savors of the blasphemy of unbelief. Hence in the next chapter, it is said: "If any one swears by God's hair or head, or otherwise utter blasphemy against God, and he be in ecclesiastical orders, let him be degraded."
This suffices for the Reply to the Second Objection.
Reply Obj. 3: Religious worship is shown to one whose testimony is invoked by oath: hence the prohibition (Ex. 23:13): "By the name of strange gods you shall not swear." But religious worship is not given to creatures employed in an oath in the ways mentioned above. _______________________
SEVENTH ARTICLE [II-II, Q. 89, Art. 7]
Whether an Oath Has a Binding Force?
Objection 1: It would seem that an oath has no binding force. An oath is employed in order to confirm the truth of an assertion. But when a person makes an assertion about the future his assertion is true, though it may not be verified. Thus Paul lied not (2 Cor. 1:15, seqq.) though he went not to Corinth, as he had said he would (1 Cor. 16:5). Therefore it seems that an oath is not binding.
Obj. 2: Further, virtue is not contrary to virtue (Categ. viii, 22). Now an oath is an act of virtue, as stated above (A. 4). But it would sometimes be contrary to virtue, or an obstacle thereto, if one were to fulfil what one has sworn to do: for instance, if one were to swear to commit a sin, or to desist from some virtuous action. Therefore an oath is not always binding.
Obj. 3: Further, sometimes a man is compelled against his will to
promise something under oath. Now, "such a person is loosed by the
Roman Pontiffs from the bond of his oath" (Extra, De Jurejur., cap.
Verum in ea quaest., etc.). Therefore an oath is not always binding.