Objection 1. It would seem that there are not seven Orders. For the Orders of the Church are directed to the hierarchical acts. But there are only three hierarchical acts, namely "to cleanse, to enlighten, and to perfect," for which reason Dionysius distinguishes three Orders (Eccl. Hier. v). Therefore there are not seven.
Objection 2. Further, all the sacraments derive their efficacy and authenticity from their institution by Christ, or at least by His apostles. But no mention except of priests and deacons is made in the teaching of Christ and His apostles. Therefore seemingly there are no other Orders.
Objection 3. Further, by the sacrament of Order a man is appointed to dispense the other sacraments. But there are only six other sacraments. Therefore there should be only six Orders.
Objection 4. On the other hand, It would seem that there ought to be more. For the higher a power is, the less is it subject to multiplication. Now the hierarchical power is in the angels in a higher way than in us, as Dionysius says (Eccl. Hier. i). Since then there are nine Orders in the angelic hierarchy, there should be as many, or more, in the Church.
Objection 5. Further, the prophecy of the Psalms is the most noble of all the prophecies. Now there is one Order, namely of readers, for reading the other prophecies in the Church. Therefore there ought to be another Order for reading the Psalms, especially since (Decretals, Dist. xxi, cap. Cleros) the "psalmist" is reckoned as the second Order after the doorkeeper.
I answer that, Some show the sufficiency of the orders from their correspondence with the gratuitous graces which are indicated 1 Corinthians 12. For they say that the "word of wisdom" belongs to the bishop, because he is the ordainer of others, which pertains to wisdom; the "word of knowledge" to the priest, for he ought to have the key of knowledge; "faith" to the deacon, for he preaches the Gospel; the "working of miracles" to the subdeacon, who sets himself to do deeds of perfection by the vow of continency; "interpretation of speeches" to the acolyte, this being signified by the light which he bears; the "grace of healing" to the exorcist; "diverse kinds of tongues" to the psalmist; "prophecy" to the reader; and the "discerning of spirits" to the doorkeeper, for he excludes some and admits others. But this is of no account, for the gratuitous graces are not given, as the Orders are, to one same man. For it is written (1 Corinthians 12:4): "There are distributions [Douay: 'diversities'] of graces." Moreover the episcopate [Cf. 40, 5] and the office of psalmist are included, which are not Orders. Wherefore others account for the Orders by likening them to the heavenly hierarchy, where the Orders are distinguished in reference to cleansing, enlightening, and perfecting. Thus they say that the doorkeeper cleanses outwardly, by separating even in the body the good from the wicked; that the acolyte cleanses inwardly, because by the light which he bears, he signifies that he dispels inward darkness; and that the exorcist cleanses both ways, for he casts out the devil who disturbs a man both ways. But enlightening, which is effected by teaching, is done by readers as regards prophetic doctrine; by subdeacons as to apostolic doctrine; and by deacons as to the gospel doctrine; while ordinary perfection, such as the perfection of Penance, Baptism, and so forth is the work of the priest; excellent perfection, such as the consecration of priests and virgins, is the work of the bishop; while the most excellent perfection is the work of the Sovereign Pontiff in whom resides the fulness of authority. But this again is of no account; both because the orders of the heavenly hierarchy are not distinguished by the aforesaid hierarchical actions, since each of them is applicable to every Order; and because, according to Dionysius (Eccl. Hier. v), perfecting belongs to the bishops alone, enlightening to the priests, and cleansing to all the ministers. Wherefore others suit the orders to the seven gifts, so that the priesthood corresponds to the gift of wisdom, which feeds us with the bread of life and understanding, even as the priest refreshes us with the heavenly bread; fear to the doorkeeper, for he separates us from the wicked; and thus the intermediate Orders to the intermediate gifts. But this again is of no account, since the sevenfold grace is given in each one of the Orders. Consequently we must answer differently by saying that the sacrament of Order is directed to the sacrament of the Eucharist, which is the sacrament of sacraments, as Dionysius says (Eccl. Hier. iii). For just as temple, altar, vessels, and vestments need to be consecrated, so do the ministers who are ordained for the Eucharist; and this consecration is the sacrament of Order. Hence the distinction of Orders is derived from their relation to the Eucharist. For the power of Order is directed either to the consecration of the Eucharist itself, or to some ministry in connection with this sacrament of the Eucharist. If in the former way, then it is the Order of priests; hence when they are ordained, they receive the chalice with wine, and the paten with the bread, because they are receiving the power to consecrate the body and blood of Christ. The co-operation of the ministers is directed either to the sacrament itself, or to the recipients. If the former, this happens in three ways. For in the first place, there is the ministry whereby the minister co-operates with the priest in the sacrament itself, by dispensing, but not by consecrating, for this is done by the priest alone; and this belongs to the deacon. Hence in the text (Sent. iv, D, 24) it is said that it belongs to the deacon to minister to the priests in whatever is done in Christ's sacraments, wherefore he dispenses Christ's blood. Secondly, there is the ministry directed to the disposal of the sacramental matter in the sacred vessels of the sacrament. and this belongs to subdeacons. Wherefore it is stated in the text (Sent. iv, D, 24) that they carry the vessels of our Lord's body and blood, and place the oblation on the altar; hence, when they are ordained, they receive the chalice, empty however, from the bishop's hands. Thirdly, there is the ministry directed to the proffering of the sacramental matter, and this belongs to the acolyte. For he, as stated in the text (Sent. iv, D, 24), prepares the cruet with wine and water; wherefore he receives an empty cruet. The ministry directed to the preparation of the recipients can be exercised only over the unclean, since those who are clean are already apt for receiving the sacraments. Now the unclean are of three kinds, according to Dionysius (Eccl. Hier. iii). For some are absolute unbelievers and unwilling to believe; and these must be altogether debarred from beholding Divine things and from the assembly of the faithful; this belongs to the doorkeepers. Some, however, are willing to believe, but are not as yet instructed, namely catechumens, and to the instruction of such persons the Order of readers is directed, who are therefore entrusted with the reading of the first rudiments of the doctrine of faith, namely the Old Testament. But some are believers and instructed, yet lie under an impediment through the power of the devil, namely those who are possessed: and to this ministry the order of exorcists is directed. Thus the reason and number of the degrees of Orders is made clear.
Reply to Objection 1. Dionysius is speaking of the orders not as sacraments, but as directed to hierarchical actions. Wherefore he distinguishes three Orders corresponding to those actions. The first of these Orders, namely the bishop, has all three actions; the second, namely the priest, has two; while the third has one, namely to cleanse; this is the deacon who is called a minister: and under this last all the lower Orders are comprised. But the Orders derive their sacramental nature from their relation to the greatest of the sacraments, and consequently the number of Orders depends on this.
Reply to Objection 2. In the early Church, on account of the fewness of ministers, all the lower ministries were entrusted to the deacons, as Dionysius says (Eccl. Hier. iii), where he says: "Some of the ministers stand at the closed door of the Church, others are otherwise occupied in the exercise of their own order; others place the sacred bread and the chalice of benediction on the altar and offer them to the priests." Nevertheless all the power to do all these things was included in the one power of the deacon, though implicitly. But afterwards the Divine worship developed, and the Church committed expressly to several persons that which had hitherto been committed implicitly in one Order. This is what the Master means, when He says in the text (Sent. iv, D, 24) that the Church instituted other Orders.
Reply to Objection 3. The orders are directed to the sacrament of the Eucharist chiefly, and to the other sacraments consequently, for even the other sacraments flow from that which is contained in that sacrament. Hence it does not follow that the orders ought to be distinguished according to the sacraments.
Reply to Objection 4. The angels differ specifically [Cf. I, 50, 4]: for this reason it is possible for them to have various modes of receiving Divine things, and hence also they are divided into various hierarchies. But in men there is only one hierarchy, because they have only one mode of receiving Divine things, which results from the human species, namely through the images of sensible objects. Consequently the distinction of orders in the angels cannot bear any relation to a sacrament as it is with us, but only a relation to the hierarchical actions which among them each Order exercises on the Orders below. In this respect our Orders correspond to theirs; since in our hierarchy there are three Orders, distinguished according to the three hierarchical actions, even as in each angelic hierarchy.
Reply to Objection 5. The office of psalmist is not an Order, but an office annexed to an Order. For the psalmist is also styled precentor because the psalms are recited with chant. Now precentor is not the name of a special Order, both because it belongs to the whole choir to sing, and because he has no special relation to the sacrament of the Eucharist. Since, however, it is a particular office, it is sometimes reckoned among the Orders, taking these in a broad sense.
Source: Summa Theologica (New Advent)