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.