If unity were to be considered a genus, it could be that only in
respect to the things of which each is said to be one;[309] as if,
for instance, one should, from "being," deduce the unity contained
within it. Unity would then be the genus of certain things; for just
as essence is a genus, not in respect to all things, but in respect
to those species that possess essence, so unity would be a genus
in respect to the species that possess unity. This, however, is
impossible; for things do not differ in respect to unity, as they do in
respect to essence.
It might further be objected that if the same divisions which were
applied to essence were applied to unity, and if essence be a genus
because it divides itself, and manifests itself as the same in a
number of things, why then should unity also not be a genus, since it
appears in as many things as essence, and similarly divides itself?
Mere recurrence of something in several essences is no proof it is a
genus; whether in respect to the essences in which it occurs, or to
others. Merely being common to several essences by no means constitutes
a genus. No one will claim that a point is a genus for lines or for
anything else, though points be found in all lines. As said, unity
is found in every number, and nevertheless it is not a genus for
any number, or for anything else. The formation of a genus demands
that what is common to several things show specific differences,
constituting species, and be predicated of what exists. But what are
the specific differences within unity? What species does it form? If to
this it be answered that it forms the same species as essence, then it
blends with essence, and (unity) is (as said above), only another name
for essence; and essence, as category, suffices.