Ontology refers to theories of reality and existence. There are several categories of ontological theories: monism believes there is only one type of reality; dualism believes there are two (e.g. mind and body); pluralism believes reality is composed of many kinds of things. Descartes' dualism defined mind and body as completely distinct substances that interact mysteriously. Other theories like behaviorism, identity theory, and functionalism are monist attempts to explain mind and body as one substance. Pluralism argues reality is too complex to fit into just one or two categories.