E. J. Lowe, a prominent figure in contemporary metaphysics, sets out and defends his theory of what there is. His four-category ontology is a metaphysical system which recognizes four fundamental categories of beings: substantial and non-substantial particulars and substantial and non-substantial universals. Lowe argues that this system has an explanatory power which is unrivaled by more parsimonious theories and that this counts decisively in its favor. He shows that it provides a powerful explanatory framework for a unified account of causation, dispositions, natural laws, natural necessity and many other related matters, thus constituting a full metaphysical foundation for natural science.