As the Philosopher says in V Metaphysicae cap. 7 (1017a22-35), being has two senses. In one sense, being signifies that which is divided into the ten categories; in another sense, that which signifies the truth of propositions. The difference between these is that, in the second sense, anything can be called a being about which an affirmative proposition can be formed, even if the thing posits nothing in reality. In this way, privations and negations are called beings, as when we say that affirmation is opposed to negation, or that blindness is in the eye. But in the first sense, nothing can be called a being unless it posits something in reality, and thus in this first sense blindness and similar things are not beings (St. Thomas, De Esse et Essentia Chapter I).The fact that we use the same words for different purposes - to speak of things that really exist, and to speak of things that exist only in thought or idea - is an unfortunate source of confusion. If a thing "posits nothing in reality," then it does not actually exist: it is not a being.