The thermodynamics of the Einstein universe was first examined in 1925-26, and a few years later Tolman greatly developed the study of the thermodynamical properties of model universes. According to Lewis, entropy was to be understood as a measure of our ignorance of the actual distribution of a system of particles, and therefore as a purely subjective concept. Nernst first thought of a mechanism for maintaining energy equilibrium in the universe in a lecture of 1912 and nine years later he developed the idea into a cosmic world picture, now justifying it by the new quantum theory. Braun and Gutberlet were among those who stressed that the heat death of the future proved the contingent nature of the universe. Theoretical cosmology in the 1920s, which essentially meant the rival world models of Einstein and de Sitter, rarely aroused public debate and was ignored by most philosophers and theologians.