Questions about the existence and nature of minds, bodies, god, space, time, causality, unity, identity, and the world are all metaphysical issues. From Plato  onwards, many philosophers have tried to determine what kinds of things(and how many of each) exist. But Kant argued that this task is impossible; he proposed instead that we consider the general structure of our thought about the world. Strawson calls the former activity revisionary, and the latter descriptive, metaphysics.