A hypothetical homunculus capable of defying the second law of thermodynamics by allowing only fast moving molecules to enter a hot area, and/or allowing only slow-moving molecules to enter a cold area.

The very implausibility of the demon may save the second law -- the demon would have to be so small, fast, smart, and knowledgeable that it may be impossible for him to exist. For instance, he would have to know both the position and momentum of some heisenbergian things.

But if the demon were stupid enough that the uncertainty principle didn't apply, it might be possible. For instance, the existence of selectively permeable membranes in biology is practically a real-world example of Maxwell's demon.