The philosophy of materialism hols that the only thing that can be truly proven to exist is matter, and is considered a form of physicalism. Fundamentally, all things are composed of material and all phenomena are the result of material interactions; therefore, matter is the only substance. As a theory, materialism belongs to the class of monist ontology. As such, it is different from ontological theories based on dualism or pluralism. For singular explanations of the phenomenal reality, materialism would be in contrast to idealism.