  His thought experiment was intended to demonstrate the possibility of a gas evolving from a higher to a lower entropy state. The later time reversal and recurrence arguments of Loschmidt and Poincare similarly challenged the H-theorem of Boltzmann (see Uffink (2006), Section 4, for a discussion of these arguments), but time reversal requires great sensitivity and recurrence takes a very long time. Although they show the law of entropy increase is not absolute, we might still be surprised to actually witness a large decrease in entropy happen through these means. With the demon we to appear to need another explanation of why such systems are not seen to occur spontaneously in nature, or why we cannot arrange for such a decrease in entropy to occur.