 So far, we have been focusing on what physicists such as Baierlein (2007) call the incremental version of mass-energy equivalence, because we have focused on the strict correlation in special relativity between a change in the rest energy of a body \(E_o\) and a change in its mass \(m\). However, Einstein also emphasized two subtly different “readings” of his equation. First, as early as 1906, Einstein argued that when one considers physical systems in which there are electromagnetic processes, such as a “complex” of light being emitted from the inside wall of a freely-floating box and absorbed by the opposite wall, one could avoid a fundamental conflict with the laws of mechanics “if one ascribes the inertial mass \(E/V^2\) to any energy \(E\)” (Einstein 1906, p. 206. Note that \(V\) represents the speed of light. See also Taylor and Wheeler 1992, p. 254, for a detailed discussion of this example). This insight, sometimes expressed with talk about the “inertia of energy,” was an important step in the development of general relativity, because that theory uses the principle of equivalence, which states very roughly that inertial mass and gravitational mass are directly proportional, as a foundational principle. 