The thought experiment known as "Maxwell's demon" originally created by physicist James Clerk Maxwell deals with which scientific principle?

Maxwell's demon is a thought experiment created in 1867 by physicist James Clerk Maxwell. The idea of the thought experiment was to explore how the second law of thermodynamics might be violated. The second law states that the total entropy of an isolated system can never decrease over time. Entropy is the amount of energy in a physical system that cannot be used to do work.

In the thought experiment a demon controls a small door between two chambers filled with gas. As individual gas molecules approach the door the demon quickly opens then shuts the door so only fast moving molecules are passed into one of the chambers and slow moving molecules move into the other chamber. Since faster moving molecules are hotter the demon's actions with the door causes one chamber to warm up and the other to cool down, decreasing entropy and violating the second law of thermodynamics.

This thought experiment first appeared in a letter Maxwell wrote in December 1867 and was written about again in a letter to John William Strutt in 1871. Maxwell also included it in his 1872 book on thermodynamics entitled "Theory of Heat". Maxwell termed the agent opening the door as a "finite being". Lord Kelvin (William Thompson) was the first to use the word "demon" for Maxwell's finite being in the journal "Nature" in 1874.