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.