It may be objected that the unity which is in essence, in movement,
and the remainder of the genera, is common to all of them, and that
one might therefore identify unity with essence.[309] It must then be
answered that, just as essence was not made a genus of other things
because they were not what was essence, but that they were called
essences in another sense, here likewise unity could not be a common
attribute of other things, because there must be a primary Unity,
and a unity taken in a secondary sense. If, on the other hand, it
be said that unity should not be made a genus of all things, but
something which exists in itself like the others, if afterwards unity
be identified with essence, then, as essence has already been listed
as one of the genera, we would be merely uselessly introducing a
superfluous name.[310] Distinguishing between unity and essence is an
avowal that each has its separate nature; the addition of "something"
to "one" makes a "certain one"; addition of nothing, on the other
hand, allows unity to remain absolute, which cannot be predicated of
anything. But why could this unity not be the First Unity, ignoring
the absolute Unity? For we use "first Unity" as a designation of the
essence which is beneath the "absolute Unity." Because the Principle
anterior to the first Essence (that is, the first and absolute Unity)
is not essence; otherwise, the essence below Him would no longer be
the first Essence; here, on the contrary, the unity which is above
this unity is the absolute Unity. Besides, this unity which would
be separated from essence only in thought, would not admit of any
differences.
Besides, there are three alternatives. Either this unity alleged to
inhere in essence will be, just like all other essences, a consequence
of the existence of essence; and consequently, would be posterior
to it. Or, it will be contemporaneous with essence and the other
(categories); but a genus cannot be contemporaneous with the things of
which it is the genus. The third possibility is that it may be anterior
to essence; in which case its relation to Essence will be that of a
principle, and no longer a genus containing it. If then unity be not a
genus in respect to essence, neither can it be a genus in respect of
other things; otherwise, we would have to say of essence also that it
was a genus embracing everything else.