Ontology is concerned with the objects of knowledge, with reality considered in the widest, deepest, and most fundamental aspects under which it is conceived by the human mind: with the being and becoming of reality, its possibility and its actuality, its essence and its existence, its unity and plurality ; with the aspects of truth, goodness, perfection, beauty, which it assumes in relation with our minds; with the contingency of finite reality and the grounds and implications both of its actual existence and of its intelligibility; with the modes of its concrete existence and behaviour, the supreme categories of reality as they are called: substance, individual nature, and personality; quantity, space and time, quality and relation, causality and purpose. These are the principal topics investigated in the present volume.