In an enclosure at constant temperature, the radiation is that of a ``blackbody,'' and the demon cannot see the molecules. Hence, he cannot operate the trap door and is unable to violate the second principle. If we introduce a source of light, the demon can see the molecules, but the over‐all balance of entropy is positive. This leads to the consideration of a cycleNegentropy→Information→Negentropyfor Maxwell's demon as well as for the scientist in his laboratory. Boltzmann's constantkis shown to represent the smallest possible amount of negative entropy required in an observation.