first appeared in a letter Maxwell wrote to Peter Guthrie Tait on 11 December 1867. It appeared again in a letter to John William Strutt in 1871, before it was presented to the public in Maxwell's 1872 book on thermodynamics titled Theory of Heat.

In his letters and books, Maxwell described the agent opening the door between the chambers as a "finite being". Being a deeply religious man, he never used the