In the previous paragraph, Thomas discussed how an essence cannot be considered either one or many. In paragraph 3 of chapter 3 of De Ente et Essentia, Thomas discusses the implications of this.
The nature considered in this way, however, has a double existence. It exists in singulars on the one hand, and in the soul on the other, and from each of these there follow accidents.
By “nature”, Thomas means the essence of a being. By “singulars”, Thomas means individual beings that have the given essence. You and I, for example, have the essence of human beings. Clearly, the essence of being human exists in me; and I’ll give you the benefit of the doubt.
Second, when I apprehend another human being, that essence exists in my soul, which is to say, as a concept in my intellect.
In singulars, furthermore, the essence has a multiple existence according to the multiplicity of singulars.
And then, of course, there are many human beings, and all of them possess human nature (though some of them I wonder about).
Nevertheless, if we consider the essence in the first, or absolute, sense, none of these pertain to the essence. For it is false to say that the essence of man, considered absolutely, has existence in this singular, because if existence in this singular pertained to man insofar as he is man, man would never exist outside this singular.
In other words, if it was part of the essence of man that I, personally, am a man, then I’d by definition be the only man–because no one else is me. So while the essence of man exists in me, this fact is not part of the essence of man.
Similarly, if it pertained to man insofar as he is man not to exist in this singular, then the essence would never exist in the singular.
So the essence of man cannot preclude the existence of that essence in individual men.
But it is true to say that man, but not insofar as he is man, has whatever may be in this singular or in that one, or else in the soul. Therefore, the nature of man considered absolutely abstracts from every existence, though it does not exclude the existence of anything either. And the nature thus considered is the one predicated of each individual.
So, then, the essence of man neither requires nor denies the existence of any individual man. And it is the essence understood in this way that is predicated of each individual man.
Given that existence is not a predicate, that makes perfect sense. (Or perhaps I’m fooling myself. But at the very least, I’m clueless at a higher level than I used to be.)