How many books can claim to be so influential as to inspire the development of a whole school of thought? Metaphysics did exactly that, laying the foundations for a new branch of philosophy concerned with the cause and nature of being. Plato's renowned Theory of Forms argued that everything in the world is merely an imperfect model of its perfect form, which exists elsewhere in the universe. But Aristotle focused on the reality of the material world, and argued that if something has a form, it exists; we should be talking about objects, not their abstractions. Metaphysics also introduced important philosophical ideas such as the "unmoved mover," potentiality, and actuality.