This book revives a neglected but important topic in philosophy: the nature of substance. The belief that there are individual substances, for example, material objects and persons, is at the core of our common-sense view of the world yet many metaphysicians deny the very coherence of the concept of substance. The authors develop a novel account of what an individual substance is in terms of independence from other beings. In the process many other important ontological categories are explored: property, event, space, time. The authors show why alternative theories of substance fail, and go on to defend the intelligibility (though not the existence) of interacting spiritual and material substances.