A term that traditionally refers to the substance of which all bodies consist. In Aristotelian physics, each type (species) of material body had a distinct “essential form.” Early modern scientists, however, asserted that there is one universal type of matter. For Isaac Newton, this matter consisted of “solid, massy, hard, impenetrable, movable,” ultimate particles (“atoms”), which were discrete, localized, indivisible bodies. Modern analyses distinguish two types of mass in classical (Newtonian) matter: inertial mass, by which matter retains its state of rest or uniform rectilinear motion in the absence of external forces; and gravitational mass, by which a body exerts forces of attraction on other bodies and by which it reacts to those forces. Expressed in appropriate units, these two properties are numerically equal—a purely experimental fact, unexplained by theory. Albert Einstein made the equality of inertial and gravitational mass a fundamental principle (principle of equivalence), as one of the two postulates of the theory of general relativity. In the equation E = mc² (where c is the velocity of light), Einstein recognized the equivalence (interconvertibility) of mass (m) and energy (E), which had been distinguished in classical theories.