  Szilard's analysis emphasised the necessity of knowing which side the molecule is on, for work to be extracted. Without this it would not be possible to know in which direction the partition needs to move. This connects the apparent second law violation to the state of a demon's knowledge. Szilard argued that the second law would be saved if the acquisition of knowledge by the demon came with a compensating entropy cost. Much of the succeeding literature is concerned with whether, and how, this knowledge comes with the cost that Szilard believed, or with whether such knowledge is, in fact, necessary for the engine's operation.