In the domain of Ontology there are many scholastic theories and discussions which are commonly regarded by non-scholastic writers as possessing nowadays for the student of philosophy an interest that is merely historical. This mistaken notion is probably due to the fact that few if any serious attempts have yet been made to transpose these questions from their medieval setting into the language and context of contemporary philosophy. Perhaps not a single one of these problems is really and in substance alien to present-day speculations. The author has endeavoured, by his treatment of such characteristically “medieval” discussions as those on _Potentia_ and _Actus_, Essence and Existence, Individuation, the Theory of Distinctions, Substance and Accident, Nature and Person, Logical and Real Relations, Efficient and Final Causes, to show that the issues involved are in every instance as fully and keenly debated—in an altered setting and a new terminology—by recent and living philosophers of every school of thought as they were by St. Thomas and his contemporaries in the golden age of medieval scholasticism. And, as the purposes of a text-book demanded, attention has been devoted to stating the problems clearly, to showing the significance and bearings of discussions and solutions, rather than to detailed analyses of arguments. At the same time it is hoped that the treatment is sufficiently full to be helpful even to advanced students and to all who are interested in the “Metaphysics of the Schools”. For the convenience of the reader the more advanced portions are printed in smaller type. The teaching of St. Thomas and the other great Schoolmen of the Middle Ages forms the groundwork of the book. This _corpus_ of doctrine is scarcely yet accessible outside its Latin sources. As typical of the fuller scholastic text-books the excellent treatise of the Spanish author, Urraburu,(2) has been most frequently consulted. Much assistance has also been derived from Kleutgen’s _Philosophie der Vorzeit_,(3) a monumental work which ought to have been long since translated into English. And finally, the excellent treatise in the Louvain _Cours de Philosophie_, by the present Cardinal Archbishop of Mechlin,(4) has been consulted with profit and largely followed in many places. The writer freely and gratefully acknowledges his indebtedness to these and other authors quoted and referred to in the course of the present volume. GENERAL INTRODUCTION.