Materialism is the theory that the only thing that exists is matter or energy; that all things are composed of material and all phenomena (including consciousness) are the result of material interactions. In other words, matter is the only substance, and reality is identical with the actually occurring states of energy and matter. To many philosophers, materialism is synonymous with physicalism. However, materialists have historically held that everything is made of matter, but physics has shown that gravity, for example, is not made of matter in the traditional sense so physicalism is used to emphasize a connection to physics and the physical sciences.