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In order to better understand this article, we must distinguish between a predicament (c.f., Aristotle's Categories) and a transcendental.
In the former, there are 10 highest genera of what we may say about things. We say say something is a "man," thus designating his substance. Further, we may say that the man we have designated is "white," thus designating a quality, and so on, going on to list the ten predicaments.
On the other hand, we must say that there is something uniting these ten categories, for, they all are. Yet, this areness cannot, like the categories, be a genus. For, within a genus, we must distinguish the various members by the genus itself, it must be something besides the genus. Now, when it comes to areness, if there was something besides that which is, it would be not. Thus, it would be nothing. This areness or isness is what is called being.