The standard formulation of energy conservation involves the subsidiary ideas of kinetic energy ("KE"), work ("W"), thermal energy, internal energy, and a half-dozen different kinds of potential energy ("PE"): elastic, chemical, nuclear, gravitational, and so forth. These quantities came to be recognized during the centuries over which the principle developed. The final conservation law, although rich in specificity, is fairly involved. More significantly, it obscures a fundamental underlying simplicity, which could only be appreciated post-relativity (1905). Energy is the scalar measure of physical change. Using the special theory it will be shown that there are only two all-encompassing classifications of energy--"energy of rest" and "energy of motion"--and that we can apply the idea of conservation of energy to all physical processes using only these two energy types as quantified by "mass" and "KE."