This derivation came very early in the development of relativity, and the formal concept of “rest-energy” had not yet evolved, nor had E0 been introduced to symbolize it. Unless Einstein was willing to assume that whenever E0 = 0, m = 0 that all of the mass of a material entity was equivalent to energy he could not take the analysis any farther than he had in September 1905. Unfortunately, he did not overtly share his concerns about this issue with his readers.