In philosophy, the theory of materialism holds 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 of "'an inert, senseless substance, in which extension, figure, and motion do actually subsist'… So it is tempting to use 'physicalism' to distance oneself from what seems a historically important but no longer scientifically relevant thesis of materialism, and related to this, to emphasize a connection to physics and the physical sciences." Therefore much of the generally philosophical discussion below on materialism may be relevant to physicalism.
Also related to materialism are the ideas of methodological naturalism (i.e. "let's at least do science as though physicalism is true") and metaphysical naturalism (i.e. "the physical world is all that exists").
Contrasting philosophies include idealism, other forms of monism, dualism, and pluralism.
 Is there a single kind of matter (hyle) which everything is made of, or multiple kinds? Is matter a continuous substance capable of expressing multiple forms (hylomorphism), or a number of discrete, unchanging constituents (atomism)? Does it have intrinsic properties (substance theory), or is it lacking them (prima materia)?
One challenge to the traditional concept of matter as tangible "stuff" came with the rise of field physics in the 19th century. Relativity shows that matter and energy (including the spatially distributed energy of fields) are interchangeable. This enables the ontological view that energy is prima materia and matter is one of its forms. On the other hand, the Standard Model of Particle physics uses quantum field theory to describe all interactions. On this view it could be said that fields are prima materia and the energy is a property of the field.