Abstract: The four-category ontology is a metaphysical system recognizing two fundamental categorial distinctions — these being between the particular and the universal, and between the substantial and the non-substantial — which cut across each other to generate four fundamental ontological categories. The four categories thus generated are substantial particulars (‘objects’), non-substantial particulars (‘modes’), substantial universals (‘kinds’), and non-substantial universals (‘attributes’). This ontology has a lengthy pedigree, with many commentators attributing a version of it to Aristotle on the basis of certain passages in one of his early works, the Categories. Although it has been revived or rediscovered at various times during the history of western philosophy, it has never found widespread favour, perhaps due to its apparent lack of parsimony and its commitment to universals. In pursuit of ontological economy, metaphysicians have generally preferred to recognize fewer than four fundamental ontological categories. This book contends that the four-category ontology has an explanatory power which is unrivalled by more parsimonious systems, and that this counts decisively in its favour. It provides a uniquely powerful explanatory framework for a unified account of causation, dispositions, natural laws, natural necessity, and many other related matters, such as the semantics of counterfactual conditionals. The book is divided into four parts: the first setting out the framework of the four-category ontology, the second focusing on its central distinction between object and property, the third exploring its applications in the philosophy of natural science, and the fourth dealing with fundamental issues of truth and realism.

Keywords: categories, causation, dispositions, laws, metaphysics, necessity, ontology, realism, truth, universals