UNITY IS NOT SYNONYMOUS WITH ESSENCE.
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.