 The relationship between mass-energy equivalence and hypotheses concerning the nature of matter is twofold. First, as we have suggested, some of the interpretations of mass-energy equivalence seem to assume implicitly certain features of matter. Second, some philosophers and physicists, notably Einstein and Infeld (1938) and Zahar (1989), have argued that mass-energy equivalence has consequences concerning the nature of matter. In this section, we will discuss the first of these two relationships between \(E_o = mc^2\) and hypotheses concerning the nature of matter. We discuss the second relationship in the next section  (Section 2.5).