Objection 1. It would seem that the divine relations are not really distinguished from each other. For things which are identified with the same, are identified with each other. But every relation in God is really the same as the divine essence. Therefore the relations are not really distinguished from each other.
Objection 2. Further, as paternity and filiation are by name distinguished from the divine essence, so likewise are goodness and power. But this kind of distinction does not make any real distinction of the divine goodness and power. Therefore neither does it make any real distinction of paternity and filiation.
Objection 3. Further, in God there is no real distinction but that of origin. But one relation does not seem to arise from another. Therefore the relations are not really distinguished from each other.
On the contrary, Boethius says (De Trin.) that in God "the substance contains the unity; and relation multiplies the trinity." Therefore, if the relations were not really distinguished from each other, there would be no real trinity in God, but only an ideal trinity, which is the error of Sabellius.
I answer that, The attributing of anything to another involves the attribution likewise of whatever is contained in it. So when "man" is attributed to anyone, a rational nature is likewise attributed to him. The idea of relation, however, necessarily means regard of one to another, according as one is relatively opposed to another. So as in God there is a real relation (1), there must also be a real opposition. The very nature of relative opposition includes distinction. Hence, there must be real distinction in God, not, indeed, according to that which is absolute--namely, essence, wherein there is supreme unity and simplicity--but according to that which is relative.
Reply to Objection 1. According to the Philosopher (Phys. iii), this argument holds, that whatever things are identified with the same thing are identified with each other, if the identity be real and logical; as, for instance, a tunic and a garment; but not if they differ logically. Hence in the same place he says that although action is the same as motion, and likewise passion; still it does not follow that action and passion are the same; because action implies reference as of something "from which" there is motion in the thing moved; whereas passion implies reference as of something "which is from" another. Likewise, although paternity, just as filiation, is really the same as the divine essence; nevertheless these two in their own proper idea and definitions import opposite respects. Hence they are distinguished from each other.
Reply to Objection 2. Power and goodness do not import any opposition in their respective natures; and hence there is no parallel argument.
Reply to Objection 3. Although relations, properly speaking, do not arise or proceed from each other, nevertheless they are considered as opposed according to the procession of one from another.
Source: Summa Theologica (New Advent)