Branch of philosophy that studies the ultimate structure and constitution of reality—i.e., of that which is real, insofar as it is real. The term, which means literally “what comes after physics,” was used to refer to the treatise by Aristotle on what he himself called “first philosophy.” In the history of Western philosophy, metaphysics has been understood in various ways: as an inquiry into what basic categories of things there are (e.g., the mental and the physical); as the study of reality, as opposed to appearance; as the study of the world as a whole; and as a theory of first principles. Some basic problems in the history of metaphysics are the problem of universals—i.e., the problem of the nature of universals and their relation to so-called particulars; the existence of God; the mind-body problem; and the problem of the nature of material, or external, objects. Major types of metaphysical theory include Platonism, Aristotelianism, Thomism, Cartesianism (see also dualism), idealism, realism, and materialism.