“A thing is one in perfection, when it is complete through the presence of all that is needed for its end; as a man is complete by having all the members required for the operation of his soul, and a house by having all the parts needful for dwelling therein. And so this sacrament is said to be one. Because it is ordained for spiritual refreshment, which is conformed to corporeal refreshment. Now there are two things required for corporeal refreshment, namely, food, which is dry sustenance, and drink, which is wet sustenance. Consequently, two things concur for the integrity of this sacrament, to wit, spiritual food and spiritual drink, according to John: ‘My flesh is meat indeed, and My blood is drink indeed.’ Therefore, this sacrament is materially many, but formally and perfectively one.” -St. Thomas Aquinas