Reply Obj. 4: When the intention of the swearer is not the same as the intention of the person to whom he swears, if this be due to the swearer's guile, he must keep his oath in accordance with the sound understanding of the person to whom the oath is made. Hence Isidore says (De Summo Bono ii, 31): "However artful a man may be in wording his oath, God Who witnesses his conscience accepts his oath as understood by the person to whom it is made." And that this refers to the deceitful oath is clear from what follows: "He is doubly guilty who both takes God's name in vain, and tricks his neighbor by guile." If, however, the swearer uses no guile, he is bound in accordance with his own intention. Wherefore Gregory says (Moral. xxvi, 7): "The human ear takes such like words in their natural outward sense, but the Divine judgment interprets them according to our inward intention." _______________________
EIGHTH ARTICLE [II-II, Q. 89, Art. 8]
Whether an Oath Is More Binding Than a Vow?
Objection 1: It would seem that an oath is more binding than a vow. A vow is a simple promise: whereas an oath includes, besides a promise, an appeal to God as witness. Therefore an oath is more binding than a vow.
Obj. 2: Further, the weaker is wont to be confirmed by the stronger. Now a vow is sometimes confirmed by an oath. Therefore an oath is stronger than a vow.
Obj. 3: Further, the obligation of a vow arises from the deliberation of the mind, a stated above (Q. 88, A. 1); while the obligation of an oath results from the truth of God Whose testimony is invoked. Since therefore God's truth is something greater than human deliberation, it seems that the obligation of an oath is greater than that of a vow.
On the contrary, A vow binds one to God while an oath sometimes binds one to man. Now one is more bound to God than to man. Therefore a vow is more binding than an oath.
I answer that, The obligation both of vow and of an oath arises from something Divine; but in different ways. For the obligation of a vow arises from the fidelity we owe God, which binds us to fulfil our promises to Him. On the other hand, the obligation of an oath arises from the reverence we owe Him which binds us to make true what we promise in His name. Now every act of infidelity includes an irreverence, but not conversely, because the infidelity of a subject to his lord would seem to be the greatest irreverence. Hence a vow by its very nature is more binding than an oath.
Reply Obj. 1: A vow is not any kind of promise, but a promise made to
God; and to be unfaithful to God is most grievous.
Reply Obj. 2: An oath is added to a vow not because it is more stable, but because greater stability results from "two immutable things" [*Heb. 6:18].