 Some philosophers and historians of science claim that Einstein’s first derivation is fallacious. For example, in The Concept of Mass, Jammer says: “It is a curious incident in the history of scientific thought that Einstein’s own derivation of the formula \(E = mc^2\), as published in his article in Annalen der Physik, was basically fallacious. . .   the result of a petitio principii, the conclusion begging the question” (Jammer, 1961, p. 177). According to Jammer, Einstein implicitly assumes what he is trying to prove, viz., that if a body emits an amount of energy \(L\), its inertial mass will decrease by an amount \(\Delta m = L/c^2\). Jammer also accuses Einstein of assuming the expression for the relativistic kinetic energy of a body. If Einstein made these assumptions, he would be guilty of begging the question. However, Stachel and Torretti (1982) have shown convincingly that Einstein’s (1905b) argument is sound. They note that Einstein indeed derives the expression for the kinetic energy of an “electron” (i.e., a structureless particle with a net charge) in his earlier (1905a) paper. However, Einstein nowhere uses this expression in the (1905b) derivation of mass-energy equivalence. Stachel and Torretti also show that Einstein’s critics overlook two key moves that are sufficient to make Einstein’s derivation sound, since one need not assume that \(\Delta m = L/c^2\).