9. The above considerations- to which others, doubtless, might be added- suffice to show that these five are primary genera. But that they are the only primary genera, that there are no others, how can we be confident of this? Why do we not add unity to them? Quantity? Quality? Relation, and all else included by our various forerunners?
As for unity: If the term is to mean a unity in which nothing else is present, neither Soul nor Intellect nor anything else, this can be predicated of nothing, and therefore cannot be a genus. If it denotes the unity present in Being, in which case we predicate Being of unity, this unity is not primal.
Besides, unity, containing no differences, cannot produce species, and not producing species, cannot be a genus. You cannot so much as divide unity: to divide it would be to make it many. Unity, aspiring to be a genus, becomes a plurality and annuls itself.
Again, you must add to it to divide it into species; for there can be no differentiae in unity as there are in Substance. The mind accepts differences of Being, but differences within unity there cannot be. Every differentia introduces a duality destroying the unity; for the addition of any one thing always does away with the previous quantity.
It may be contended that the unity which is implicit in Being and in Motion is common to all other things, and that therefore Being and unity are inseparable. But we rejected the idea that Being is a genus comprising all things, on the ground that these things are not beings in the sense of the Absolute Being, but beings in another mode: in the same way, we assert, unity is not a genus, the Primary Unity having a character distinct from all other unities.
Admitted that not everything suffices to produce a genus, it may yet be urged that there is an Absolute or Primary Unity corresponding to the other primaries. But if Being and unity are identified, then since Being has already been included among the genera, it is but a name that is introduced in unity: if, however, they are both unity, some principle is implied: if there is anything in addition [to this principle], unity is predicated of this added thing; if there is nothing added, the reference is again to that unity predicated of nothing. If however the unity referred to is that which accompanies Being, we have already decided that it is not unity in the primary sense.
But is there any reason why this less complete unity should not still possess Primary Being, seeing that even its posterior we rank as Being, and "Being" in the sense of the Primary Being? The reason is that the prior of this Being cannot itself be Being- or else, if the prior is Being, this is not Primary Being: but the prior is unity; [therefore unity is not Being].
Furthermore, unity, abstracted from Being, has no differentiae.
Again, even taking it as bound up with Being: If it is a consequent of Being, then it is a consequent of everything, and therefore the latest of things: but the genus takes priority. If it is simultaneous with Being, it is simultaneous with everything: but a genus is not thus simultaneous. If it is prior to Being, it is of the nature of a Principle, and therefore will belong only to Being; but if it serves as Principle to Being, it is not its genus: if it is not genus to Being, it is equally not a genus of anything else; for that would make Being a genus of all other things.
In sum, the unity exhibited in Being on the one hand approximates to Unity-Absolute and on the other tends to identify itself with Being: Being is a unity in relation to the Absolute, is Being by virtue of its sequence upon that Absolute: it is indeed potentially a plurality, and yet it remains a unity and rejecting division refuses thereby to become a genus.