There are many who claim as theirs the dignity of the Christian
religion; but that form of faith is valid and only valid which, both on
account of the universal character of the rules and doctrines affirming
its authority, and because the worship in which they are expressed has
spread throughout the world, is called catholic or universal. The belief
of this religion concerning the Unity of the Trinity is as follows: the
Father is God, the Son is God, the Holy Spirit is God. Therefore Father,
Son, and Holy Spirit are one God, not three Gods. The principle of this
union is absence of difference[10]: difference cannot be avoided by
those who add to or take from the Unity, as for instance the Arians,
who, by graduating the Trinity according to merit, break it up and
convert it to Plurality. For the essence of plurality is otherness;
apart from otherness plurality is unintelligible. In fact, the
difference between three or more things lies in genus or species or
number. Difference is the necessary correlative of sameness. Sameness is
predicated in three ways: By genus; e.g. a man and a horse, because of
their common genus, animal. By species; e.g. Cato and Cicero, because of
their common species, man. By number; e.g. Tully and Cicero, because
they are numerically one. Similarly difference is expressed by genus,
species, and number. Now numerical difference is caused by variety of
accidents; three men differ neither by genus nor species but by their
accidents, for if we mentally remove from them all other accidents,[11]
still each one occupies a different place which cannot possibly be
regarded as the same for each, since two bodies cannot occupy the same
place, and place is an accident. Wherefore it is because men are plural
by their accidents that they are plural in number.
[10] The terms _differentia, numerus, species,_ are used expertly, as
would be expected of the author of the _In Isag. Porph. Commenta._ See
S. Brandt's edition of that work (in the Vienna _Corpus_, 1906), s.v.
_differentia,_ etc.
[11] This method of mental abstraction is employed more elaborately in
_Tr._ iii. (_vide infra_, p. 44) and in _Cons._ v. pr. 4, where the
notion of divine foreknowledge is abstracted in imagination.