James Clark Maxwell and his demon, artwork. Maxwell (1831-1879) is best known for his laws of electromagnetism, which laid the foundations for modern physics. His demon" was a thought experiment to question the Second Law of Thermodynamics: the entropy of a system tends to increase. This means that a hot region cannot draw heat from a colder region. Maxwell's demon is a being that can operate a door between a hot and cold gas, opening it to allow a fast-moving (hot) molecule to pass to the hot side, or a slow- moving one to pass to the cold side. This would violate the second law. However, as the demon requires energy to distinguish between fast and slow approaching molecules, and to operate the door, the entropy of the system increases anyway."