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[23] are relative terms; let us see whether either of them are
predicates of substance. If you suppress the term slave,[24] you
simultaneously suppress the term master. On the other hand, though you
suppress the term whiteness, you do not suppress some white thing,[25]
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
essence 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 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.