Accordingly those predicates which do not denote the essential nature 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 the 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, the predicate father, as such, is relative. And if we bear in mind all the propositions made concerning 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. [23] _Dominus_ and _seruus_ are similarly used as illustration, _In Cat._ (Migne, _P.L._ lxiv. 217). [24] i.e. which is external to the master. [25] i.e. which is external to the whitened thing. VI.