d his innocent daughter on account of his vow (Judges
11). Since, then, the slaying of an innocent person is not a better
good, but is in itself unlawful, it seems that a vow may be made not
only about a better good, but also about something unlawful.

Obj. 3: Further, things that tend to be harmful to the person, or
that are quite useless, do not come under the head of a better good.
Yet sometimes vows are made about immoderate vigils or fasts which
tend to injure the person: and sometimes vows are about indifferent
matters and such as are useful to no purpose. Therefore a vow is not
always about a better good.

_On the contrary,_ It is written (Deut. 23:22): "If thou wilt not
promise thou shalt be without sin."

_I answer that,_ As stated above (A. 1), a vow is a promise made to
God. Now a promise is about something that one does voluntarily for
someone else: since it would be not a promise but a threat to say
that one would do something against someone. In like manner it would
be futile to promise anyone something unacceptable to him. Wherefore,
as every sin is against God, and since no work is acceptable to God
unless it be virtuous, it follows that nothing unlawful or
indifferent, but only some act of virtue, should be the matter of a
vow. But as a vow denotes a voluntary promise, while necessity
excludes voluntariness, whatever is absolutely necessary, whether to
be or not to be, can nowise be the matter of a vow. For it would be
foolish to vow that one would die or that one would not fly.

On the other hand, if a thing be necessary, not absolutely but on the
supposition of an end--for instance if salvation be unattainable
without it--it may be the matter of a vow in so far as it is done
voluntarily, but not in so far as there is a necessity for doing it.
But that which is not necessary, neither absolutely, nor on the
supposition of an end, is altogether voluntary, and therefore is most
properly the matter of a vow. And this is said to be a greater good
in comparison with that which is universally necessary for salvation.
Therefore, properly speaking, a vow is said to be about a better good.

Reply Obj. 1: Renouncing the devil's pomps and keeping the faith of
Christ are the matter of baptismal vows, in so far as these things
are done voluntarily, although they are necessary for salvation. The
same answer applies to Jacob's vow: although it may also be explained
that Jacob vowed that he would have the Lord for