In philosophy, materialism is that form of physicalism which holds that the only thing that can truly be said to exist is matter; that fundamentally, all things are composed of material and all phenomena are the result of material interactions; that 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. In terms of singular explanations of the phenomenal reality, materialism stands in sharp contrast to idealism.