 In Einstein’s own purely dynamical derivation (1935), more than half of the paper is devoted to finding the mathematical expressions that define \(\mathbf{p}\) and \(T_{\rel}\). This much work is required to arrive at these expressions for two reasons. First, the changes to the structure of spacetime must be incorporated into the definitions of the relativistic quantities. Second, \(\mathbf{p}\) and \(T_{\rel}\) must be defined so that they reduce to their Newtonian counterparts in the appropriate limit. This last requirement ensures, in effect, that special relativity will inherit the empirical success of Newtonian physics. Once the definitions of \(\mathbf{p}\) and \(T_{\rel}\) are obtained, one derives mass-energy equivalence in a straight-forward way by analyzing a collision. (For a more detailed discussion of Einstein’s (1935), see Fernflores, 2018.)