Materialism is a form of philosophical monism that holds that matter is the fundamental substance in nature, and that all things, including mental states and consciousness, are results of material interactions. According to philosophical materialism, mind and consciousness are by-products or epiphenomena of material processes (such as the biochemistry of the human brain and nervous system), without which they cannot exist. This concept directly contrasts with idealism, where mind and consciousness are first-order realities to which matter is subject and material interactions are secondary.
Materialism is closely related to physicalism—the view that all that exists is ultimately physical. Philosophical physicalism has evolved from materialism with the theories of the physical sciences to incorporate more sophisticated notions of physicality than mere ordinary matter (e.g. spacetime, physical energies and forces, and dark matter). Thus, the term physicalism is preferred over materialism by some, while others use the terms as if they were synonymous.