His next discovery, the one Einstein later considered the greatest accomplishment of the special theory, was the equivalence of mass and energy. That revelation—more an epiphany than the result of rigorous analysis—came on the heels of the June paper. It was then that Einstein became aware of the potential significance of the quantity mc2. It is argued elsewhere5 that his realization that mc2 was some sort of rest-energy (call it E0, though as yet he had no symbol for it) arose from a close reading of his formula, KE =