An imaginary being whose action appears to contradict the second law of thermodynamics. The properties of bulk systems are explained in terms of contributions from their constituent molecules. The same, presumably, should be true of certain grand properties of nature, and in particular the second law of thermodynamics, which identifies the natural direction of change with the direction of increasing entropy. The statistical basis of this law, in which entropy is identified with molecular chaos, was established largely through the work of L. Boltzmann. However, there has always been a certain degree of discomfort associated with the acceptance of the law, particularly in relation to the time reversibility of physical laws and the role of molecular fluctuations. In 1867, J. C. Maxwell considered, in this connection, the action of “a finite being who knows the paths and velocities of all the molecules by inspection.” This being was later referred to as a demon by Lord Kelvin, and the usage has been generally adopted.