The unity of a being is simply a certain negation of distinction or division. As was said in the last post, distinction consists in the fact that this thing is not that thing. To say that a thing is one is to say that it is “this thing” rather than “this thing which is not that thing, and that thing which is not this thing”. Thus saying that the thing is one does not deny all distinction, since “this thing” remains “not that thing.” But it denies the distinction within “this thing and that thing,” since this is not one thing.

Or with a concrete example, if I am talking about an apple and an orange, the apple is not the orange, and the orange is not the apple. By reason of this mutual distinction, “the apple and the orange” does not constitute something one. But the apple is one precisely because it is not something like this; “apple” does not name a distinct something and something else. Likewise the orange is one, for the same reason, despite the fact that the apple is not the orange.

St. Thomas explains that it follows that one and being are in some way the same:

“One” does not add any reality to “being”; but is only a negation of division; for “one” means undivided “being.” This is the very reason why “one” is the same as “being.” Now every being is either simple or compound. But what is simple is undivided, both actually and potentially. Whereas what is compound, has not being whilst its parts are divided, but after they make up and compose it. Hence it is manifest that the being of anything consists in undivision; and hence it is that everything guards its unity as it guards its being.

In other words, if you cut the apple into two halves, there is no longer an apple, but one half and another half. And just as you no longer have an apple, you no longer have the being that you had, since that being was an apple. The apple is always one apple; and one apple is always an apple. In this way being and unity are convertible.

On the other hand, just as there are many ways of being, there are many ways to be one. Thus although the two halves are not an apple, and consequently not one apple, they are a pair of apple halves. And being a pair of something is being something at least in some way; and consequently they are also one pair.