" Szilard began explaining. “Five or ten minutes” later, he says, Einstein understood. After only a year of university physics, Szilard had worked out a rigorous mathematical proof that the random motion of thermal equilibrium could be fitted within the framework of the phenomenological theory in its original, classical form, without reference to a limiting atomic model—“and [Einstein] liked this very much. "

― Richard Rhodes , The Making of the Atomic Bomb