Of God and His Creatures

Rather a wide definition, as it might be taken to include the institution of a clergyman to a benefice. It is usually insisted upon, as an essential note of a Sacrament, that the corporeal sign must have been 'ordained by Christ,' -- ex institutione ipsius Christi, as St Thomas himself says (Chap. LVI ad fin.) Christ's ordinance may have been more or less indeterminate, e.g., the imposition of hands in Ordination, an indeterminate prescription afterwards determined by the Church, adding rites and formularies, which it would be unsafe now to set aside as unessential because not primitive.

Of God and His Creatures: 4.74