I think the real trick with the thought experiment is: which law of physics is the Demon ignoring? The answer (spoilers) is that the demon is ignoring the entr

I think the real trick with the thought experiment is: which law of physics is the Demon ignoring? The answer (spoilers) is that the demon is ignoring the entropy of the information it must learns about the particles that pass through the gate - either the Demon stores info about which particles are let through, which has entropy of its own, or the Demon erases that data, which comes with an energy cost that must subtly heat something up and again increases entropy elsewhere. Either way, the entropy of the total system still increases, even though the entropy of the gas decreases.