 At a very general level, purely dynamical derivations of Einstein’s equation and derivations that appeal to Maxwell’s theory really follow the same approach. In both styles of derivations, although it may not seem like it at first glance, we are dealing with one of the most basic dynamical interactions: a collision. So, for example, we can construe the physical configuration of Einstein’s original 1905 derivation (Einstein 1905b) as a collision in which the total number of objects is not conserved. This is even easier to do if one adopts a “particle” description of light. In both the purely dynamical derivations and the derivations that appeal to an interaction with electromagnetic radiation, one then examines the collision in question and shows that in order for dynamical principles to be satisfied, the relationship among the masses and energies of the objects involved in the collisions must satsify Einstein’s equation. 