 As we will see, if Baierlein’s assessment of his own derivation is correct, the price to pay for this particular simplification is that the derivation ceases to be relativistic, in the sense that none of the core principles at the heart of special relativity seem to be required to derive Einstein’s equation. Nevertheless, it may help readers get a “feel” for why \(E_o = mc^2\) and it does familiarize those not already conversant in the methods of relativistic physics with a style of reasoning that is common in that field. Those interested in a presentation of Einstein’s own 1946 derivation, which explicitly shows all of the relativistic assumptions and key approximation steps Einstein takes, may be interested in consulting the exposition by Fernflores (Fernflores 2018, vol. II, §3.3). 