This book contains the definitive statement of Franz Brentano's views on meta physics. It is made up of essays which were dictated by Brentano during the last ten years of his life, between 1907 and 1917. These dictations were assembled and edited by Alfred Kastil and first published by the Felix Meiner Verlag in 1933 under the title Kategorienlehre. Kastil added copious notes to Brentano's text. These notes have been included, with some slight omissions, in the present edition; the bibliographical references have been brought up to date. Brentano's approach to philosophy is unfamiliar to many contemporay readers. I shall discuss below certain fundamental points which such readers are likely to find the most difficult. I believe that once these points are properly understood, then what Brentano has to say will be seen to be of first importance to philosophy. THE PRIMACY OF THE INTENTIONAL To understand Brentano's theory of being, one must realize that he appeals to what he calls inner perception for his paradigmatic uses of the word "is". For inner perception, according to Brentano, is the source of our knowledge of the nature of being, just as it is the source of our knowledge of the nature of truth and of the nature of good and evil. And what can be said about the being of things that are not apprehended in inner perception can be understood only by analogy with what we are able to say about ourselves as thinking subjects.