Materialism is the idea that nothing exists independently of the material or physical world.
Many ancient thinkers appeal to supernatural or extranatural entities in order to account for certain features of the natural world. Materialists, however, deny the existence of any non-natural events, entities, or forces.
Early materialists include the Greek atomists, Democritus (c. 460-c. 370 BCE) and Leucippus (fl. early fifth century BCE), who argued that the world consists of nothing but atoms in empty space (even the soul was thought to be composed of atoms), and Epicurus (341-270 BCE), who postulated that the atoms move only in an up-down direction.
The significance of materialism is typically found in discussions of philosophical questions, such as how to account for the properties of objects and how to explain consciousness. For example, while Plato (c. 424-c. 348 BCE) sought to explain why, say, two blue objects look exactly the same by arguing that they participate in pre-existing (ante rem) universals, Aristotle (384-322 BCE) argued that all universals are present in existing objects (in re), and was thus a materialist about properties. However, both men seem to appeal to an immaterial divine being to explain the origin of physical reality, and to an immaterial soul to explain consciousness. Thus, it was deemed possible to be a materialist about some things and not others.
The comprehensive materialism of the sort defended by the atomists gained popularity in the late nineteenth and early twentieth centuries as advancements in science reduced the apparent need for extra-natural explanations, and pluralism in mathematics challenged the idea of a unique, Platonic reality of mathematical forms. More recently, advancements in our understanding of the brain have undermined older appeals to immaterial substances or properties to explain consciousness, but they have also served to highlight the limitations of materialism.