 Einstein’s further conclusion that “the mass of a body is a measure of its energy content” (1905b, p. 71) does not, strictly speaking, follow from his argument. As Torretti (1996) and other philosophers and physicists have observed, Einstein’s (1905b) argument allows for the possibility that once a body’s energy store has been entirely used up (and subtracted from the mass using the mass-energy equivalence relation) the remainder is not zero. In other words, it is only an hypothesis in Einstein’s (1905b) argument, and indeed in all derivations of \(E_o = mc^2\) in special relativity, that no “exotic matter” exists that is not convertible into energy (see Ehlers, Rindler, Penrose, (1965) for a discussion of this point). However, particle-antiparticle anihilation experiments in atomic physics, which were first observed decades after 1905, strongly support “Einstein’s dauntless extrapolation” (Torretti, 1996, p. 112).