The arrow and the wheel have both been used to represent time, which has been measured by the flowing of water or sand, unilateral and definitive, or by the movement of a needle on a dial, an ever recurring cycle. These two aspects of time—the unlimited advance without stop or return and the cycle which passes again and again through exactly the same states— have for various reasons preoccupied both philosophers and astronomers. A universe of infinite duration which never returns to any of its preceding states, the sort suggested by application of the second principle of thermodynamics, has little appeal for certain thinkers and leads them to a sort of cosmic pessimism. A cyclical evolution, on the other hand, with an eternal return to states, if not identical, at least very similar, to the one in which we live, gains more sympathy by providing the mind with a sort of repose, a vague consolation for death. This preference can be observed in action whenever the progress of astrophysics presents a new hypothesis on cosmic evolution. The expansion of the universe, an irremediable evolution, met with a hostility which, under cover of scientific arguments, concealed an affective malaise induced by metaphysical anguish in the presence of this final fading-away, far as it might be from our day.