An imaginary creature who, by opening and shutting a tiny door between two volumes of gases, could, in principle, concentrate slower molecules in one (making it colder) and faster molecules in the other (making it hotter), thus breaking the second law of thermodynamics. Essentially this law states that heat does not naturally flow from a colder body to a hotter body; work must be expended to make it do so. This hypothesis was formulated in 1871 by James C. Maxwell (1831–1879), who is considered by many to be the greatest theoretical physicist of the nineteenth century. The demon would bring about an effective flow of molecular kinetic energy. This excess energy would be useful to perform work and the system would be a perpetual motion machine. About 1950, the French physicist Léon Brillouin (1889–1969) disproved Maxwell’s hypothesis by demonstrating that the decrease in entropy resulting from the demon’s actions would be exceeded by the increase in entropy in choosing between the fast and slow molecules.