This is Brentano's doctoral dissertation and his first book. In it he contemplates the several senses of "being", using Aristotle as his guide. He finds that (in Aristotle's view) being in the sense of the categories, in particular substantial being, is the most basic; all other modes, potential and actual being, being in the sense of the true, etc., stand to it in a relation of well-founded analogy. Many of his mature views are prepared in this work. For exemple his discussion of being in the sense of being true appears to be the foundation of his later nonpropositional theory of judgment.