I answer that, Strictly speaking, a thing is one numerically in three ways. First, as something indivisible, which is neither actually nor potentially several---as a point, and unity. Secondly, as something continuous, which is actually one, but potentially several---as a line. Thirdly, as something complete, that is composed of several parts---as a house, which is, in a way, several things, even actually, although those several things go together towards making one. In this way each sacrament is said to be one thing, in as much as the many things which are contained in one sacrament, are united together for the purpose of signifying or causing one thing, because a sacrament is a sign of the effect it produces. Hence when one action suffices for a perfect signification, the unity of the sacrament consists in that action only, as may be seen in Confirmation. When, however, the signification of the sacrament can be both in one and in several actions, then the sacrament can be complete both in one and in several actions, even as Baptism in one immersion and in three, since washing which is signified in Baptism, can be completed by one immersion and by several. But when the perfect signification cannot be expressed except by means of several actions, then these several actions are essential for the perfection of the sacrament, as is exemplified in the Eucharist, for the refreshment of the body which signifies that of the soul, can only be attained by means of meat and drink. It is the same in this sacrament, because the healing of the internal wounds cannot be perfectly signified save by the application of the remedy to the various sources of the wounds. Hence several actions are essential to the perfection of this sacrament.