ST Suppl., q. 34
Of the Sacrament of Order as to its Essence and its Parts
Supplement — compiled posthumously from Aquinas's early Sentences commentary
In the next place we must consider the sacrament of Order: (1) Order in general; (2) the difference of Orders; (3) those who confer Orders; (4) the impediments to receiving Orders; (5) things connected with Orders.
Concerning Order in general three points have to be considered: (1) Its essence, quiddity, and parts; (2) Its effect; (3) The recipients of Orders.
Under the first head there are five points of inquiry:
(1) Whether there should be Order in the Church?
(2) Whether it is fittingly defined?
(3) Whether it is a sacrament?
(4) Whether its form is expressed properly?
(5) Whether this sacrament has any matter?