Signs are given to men, to whom it is proper to discover the unknown by means of the known. Consequently a sacrament properly so called is that which is the sign of some sacred thing pertaining to man; so that properly speaking a sacrament, as considered by us now, is defined as being the "sign of a holy thing so far as it makes men holy."
[St. Thomas Aquinas, Summa Theologiae 3.60.2.]
