What is known to us as metaphysics is what Aristotle called "first philosophy." Metaphysics involves a study of the universal principles of being, the abstract qualities of existence itself. Perhaps the starting point of Aristotle's metaphysics is his rejection of Plato's Theory of Forms. In Plato's theory, material objects are changeable and not real in themselves; rather, they correspond to an ideal, eternal, and immutable Form by a common name, and this Form can be perceived only by the intellect. Thus a thing perceived to be beautiful in this world is in fact an imperfect manifestation of the Form of Beauty. Aristotle's arguments against this theory were numerous. Ultimately he rejected Plato's ideas as poetic but empty language; as a scientist and empiricist he preferred to focus on the reality of the material world.
Metaphysics, or the parts still in existence, spans fourteen books. The early books give background information and survey the field before Aristotle's time. He also describes the nature of wisdom: it begins with sense perceptions, which must be translated into scientific expertise. Such knowledge requires the understanding of both facts and causes, and wisdom comes only with an understanding of the universal principles and primary causes built on this science. Aristotle's work in metaphysics is therefore motivated by this desire for wisdom, which requires the pursuit of knowledge for its own sake.
By the fourth book he begins to attack some of the sophistry that has contaminated the field. One point that he dwells on is the law of contradictions, which essentially asserts that something cannot both be and not be at the same time. In particular, he is concerned with the relativism and even nihilism that would result from a metaphysics that allowed contradictions. The relationship between form and matter is another central problem for Aristotle. He argues that both are substances, but matter is potential, while form is actual. The two are not separate but intertwined, and actuality precedes potentiality. Although the actual is produced from the potential, it is the actual that makes the production possible.
Several of the books covering topics like contrariety, unity, the nature of mathematical objects, and others are usually neglected, as they show less originality compared with the key points of the Metaphysics. Book XII, on the other hand, is usually considered the culmination of Aristotle's work in metaphysics, and in it he offers his teleological system. Before he draws any grand conclusions, he begins with the idea of substance, of which there are three kinds: changeable and perishable (e.g., plants and animals), changeable and eternal (e.g., heavenly bodies), and immutable. If all substances are perishable, then ultimate destruction of everything is inevitable. But Aristotle asserts two imperishable entities: motion and time. If time were created, then there must have been no time before the creation, but the very concept of "before" necessitates the concept of time. On the other hand, as he argued in his works of natural philosophy, the only continuous motion must be circular. Thus he returns to the idea of the Unmoved Mover, for only such a being could generate eternal circular motion. The Unmoved Mover is the ultimate cause of the universe, and it is pure actuality, containing no matter since it is the very cause of itself. In order for the Mover to be unmoved itself, it must move in a non-physical way, by inspiring desire.
Aristotle gives the Mover the name of God, but this figure is unlike most standard conceptions of a divine being. Though Aristotle asserts that it is a living creature and represents the pinnacle of goodness, it also has no interest in the world and no recognition of man, for it exists in a completely transcendent and abstract state. The activity of God–if it can be called such–is simply knowledge, and this knowledge is purely a knowledge of itself, because an abstracted being is above sense and experience and can know only what is best. Some have interpreted this to mean that God, in knowing itself, implicitly knows everything else, but Aristotle flatly denied this view. In fact, he believed, for example, that God would have no knowledge of evil. Thus Aristotle's conception is full of paradoxes. God is the ultimate cause of everything in the world, but it also remains completely detached.