Let us now consider the category of relation, to which all the foregoing remarks have been preliminary; for qualities which obviously arise from the association of another term do not appear to predicate anything concerning the substance of a subject. For instance, master and slave are relative terms; let us see whether either of them are predicates of substance. If you suppress the term slave, you simultaneously suppress the term master. On the other hand, though you suppress the term whiteness, you do not suppress some white thing, though, of course, if the particular whiteness inhere as an accident in the thing, the thing disappears as soon as you suppress the accidental quality whiteness. But in the case of master, if you suppress the term slave, the term master disappears. But slave is not an accidental quality of master, as whiteness is of a white thing; it denotes the power which the master has over the slave. Now since the power goes when the slave is removed, it is plain that power is no accident to the substance of master, but is an adventitious augmentation arising from the possession of slaves.
It cannot therefore be affirmed that a category of relation increases, decreases, or alters in any way the substance of the thing to which it is applied. The category of relation, then, has nothing to do with the substance of the subject; it simply denotes a condition of relativity, and that not necessarily to something else, but sometimes to the subject itself. For suppose a man standing. If I go up to him on my right and stand beside him, he will be left, in relation to me, not because he is left in himself, but because I have come up to him on my right. Again, if I come up to him on my left, he becomes right in relation to me, not because he is right in himself, as he may be white or long, but because he is right in virtue of my approach. What he is depends entirely on me, and not in the least on the essence of his being.
Accordingly those predicates which do not denote the essential property of a thing cannot alter, change or disturb its nature in any way. Wherefore if Father and Son are predicates of relation, and, as we have said, have no other difference but that of relation, and if relation is not asserted of its subject as though it were time subject itself and its substantial quality, it will effect no real difference in its subject, but, in a phrase which aims at interpreting what we can hardly understand, a difference of persons. For it is a canon of absolute truth that distinctions in incorporeal things are established by differences and not by spatial separation. It cannot be said that God became Father by the addition to His substance of some accident; for he never began to be Father, since the begetting of the Son belongs to His very substance; however, time predicate father, as such, is relative. And if we bear in mind all the propositions made God in the previous discussion, we shall admit that God the Son proceeded from God the Father, and the Holy Ghost from both, and that They cannot possibly be spatially different, since They are incorporeal. But since the Father is God, the Son is God, and the Holy Spirit is God, and since there are in God no points of difference distinguishing Him from God, He differs from none of the Others. But where there are no differences there is no plurality; where is no plurality there is Unity. Again, nothing but God can be begotten of God, and lastly, in concrete enumerations the repetition of units does not produce plurality. Thus the Unity of the Three is suitably established.