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 an 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.