Our knowledge becomes more perfect by learning, and so the more imperfect an idea is, the more prior it is in our knowing. At the limit of this imperfection is a concept that St. Thomas calls being.
This concept is drawn from sense experience and so it is tied up with the actuality of bodily existence, so much so that our understanding of immaterial beings- of God or of the very mind with which we are thinking the concept-involves a negation of this first idea of being. But even in its negation it remains a referent for the negation. What remains on the other side of the negation is also called “a being” (what else could we call it?), although the word clearly has a new sense which cannot be understood part from its order to the old sense. This is one sense in which being is analogous. The other ways being is said analogously will also have reference to this first meaning.
Again, the being that is first in conception is tied up with bodily existence, and yet is no distinct body in particular. The imagination, in fact, sees it as it sees nothing. The blackness of the nothing indicates nothing, the latent dimensions of the nothing indicates being.