  Maxwell's demon was first mentioned in a letter written to Tait in 1867. Maxwell was one amongst a number of researchers in the developing field of thermodynamics who was interested in seeking an understanding of thermal phenomena in terms of an underlying atomic physics. However, unlike Boltzmann and Clausius, who were attempting to prove the law of entropy increase from such atomic physics, Maxwell had realised that if thermodynamics was ultimately grounded in atomic theory, then the second law of thermodynamics could have only a statistical validity.