Objection 1. It would seem that in God there are not only four real relations--paternity, filiation, spiration and procession. For it must be observed that in God there exist the relations of the intelligent agent to the object understood; and of the one willing to the object willed; which are real relations not comprised under those above specified. Therefore there are not only four real relations in God.
Objection 2. Further, real relations in God are understood as coming from the intelligible procession of the Word. But intelligible relations are infinitely multiplied, as Avicenna says. Therefore in God there exists an infinite series of real relations.
Objection 3. Further, ideas in God are eternal (15, 1); and are only distinguished from each other by reason of their regard to things, as above stated. Therefore in God there are many more eternal relations.
Objection 4. Further, equality, and likeness, and identity are relations: and they are in God from eternity. Therefore several more relations are eternal in God than the above named.
Objection 5. Further, it may also contrariwise be said that there are fewer relations in God than those above named. For, according to the Philosopher (Phys. iii text 24), "It is the same way from Athens to Thebes, as from Thebes to Athens." By the same way of reasoning there is the same relation from the Father to the Son, that of paternity, and from the Son to the Father, that of filiation; and thus there are not four relations in God.
I answer that, According to the Philosopher (Metaph. v), every relation is based either on quantity, as double and half; or on action and passion, as the doer and the deed, the father and the son, the master and the servant, and the like. Now as there is no quantity in God, for He is great without quantity, as Augustine says (De Trin. i, 1) it follows that a real relation in God can be based only on action. Such relations are not based on the actions of God according to any extrinsic procession, forasmuch as the relations of God to creatures are not real in Him (13, 7). Hence, it follows that real relations in God can be understood only in regard to those actions according to which there are internal, and not external, processions in God. These processions are two only, as above explained (27, 5), one derived from the action of the intellect, the procession of the Word; and the other from the action of the will, the procession of love. In respect of each of these processions two opposite relations arise; one of which is the relation of the person proceeding from the principle; the other is the relation of the principle Himself. The procession of the Word is called generation in the proper sense of the term, whereby it is applied to living things. Now the relation of the principle of generation in perfect living beings is called paternity; and the relation of the one proceeding from the principle is called filiation. But the procession of Love has no proper name of its own (27, 4); and so neither have the ensuing relations a proper name of their own. The relation of the principle of this procession is called spiration; and the relation of the person proceeding is called procession: although these two names belong to the processions or origins themselves, and not to the relations.
Reply to Objection 1. In those things in which there is a difference between the intellect and its object, and the will and its object, there can be a real relation, both of science to its object, and of the willer to the object willed. In God, however, the intellect and its object are one and the same; because by understanding Himself, God understands all other things; and the same applies to His will and the object that He wills. Hence it follows that in God these kinds of relations are not real; as neither is the relation of a thing to itself. Nevertheless, the relation to the word is a real relation; because the word is understood as proceeding by an intelligible action; and not as a thing understood. For when we understand a stone; that which the intellect conceives from the thing understood, is called the word.
Reply to Objection 2. Intelligible relations in ourselves are infinitely multiplied, because a man understands a stone by one act, and by another act understands that he understands the stone, and again by another, understands that he understands this; thus the acts of understanding are infinitely multiplied, and consequently also the relations understood. This does not apply to God, inasmuch as He understands all things by one act alone.
Reply to Objection 3. Ideal relations exist as understood by God. Hence it does not follow from their plurality that there are many relations in God; but that God knows these many relations.
Reply to Objection 4. Equality and similitude in God are not real relations; but are only logical relations (42, 3, ad 4).
Reply to Objection 5. The way from one term to another and conversely is the same; nevertheless the mutual relations are not the same. Hence, we cannot conclude that the relation of the father to the son is the same as that of the son to the father; but we could conclude this of something absolute, if there were such between them.
Source: Summa Theologica (New Advent)