That one's new to me ...I had to look it up .... I think Maxwell must have been drunk when he came up with that one !! ... I quote the first paragraph from Wikipedia ........ "Maxwell's Demon" paradox.
The energy required by the demon to process information (of which particles are hot and which are cold) solves the paradox..
"Maxwell's demon is a thought experiment created by the physicist James Clerk Maxwell in 1867 in which he suggested how the second law of thermodynamics might hypothetically be violated. In the thought experiment, a demon controls a small door between two chambers of gas. As individual gas molecules approach the door, the demon quickly opens and shuts the door so that only fast molecules are passed into one of the chambers, while only slow molecules are passed into the other. ...." ....can you believe that !!!
I've got a way we can have perpetual motion machines ... Imagine fairies help push the wheels round ... Brilliant .