 A vow denotes a binding to do or omit some particular thing. Now one man binds himself to another by means of a promise, which is an act of the reason to which faculty it belongs to direct. For just as a man by commanding or praying, directs, in a fashion, what others are to do for him, so by promising he directs what he himself is to do for another. […] Now a promise is the outcome from a purpose of doing something: and a purpose presupposes deliberation, since it is the act of a deliberate will. Accordingly three things are essential to a vow: the first is deliberation; the second is a purpose of the will; and the third is a promise, wherein is completed the nature of a vow. Sometimes, however, two other things are added as a sort of confirmation of the vow, namely, pronouncement by word of mouth, according to Psalm 65:13, “I will pay Thee my vows which my lips have uttered”; and the witnessing of others. Hence the Master says (Sent. iv, D, 38) that a vow is “the witnessing of a spontaneous promise and ought to be made to God and about things relating to God”: although the “witnessing” may strictly refer to the inward protestation. (Summa, II-II, q.88)