  Nevertheless Bennett also argued that the Szilard engine was a process where the logically irreversible erasure step could not be avoided to complete the cycle. The information acquired during the non-dissipative measurement had to be stored in the state of the demon's memory. While this information allowed the demon to extract work from the engine, this memory could not be reset without incurring an erasure cost at least equal to the work extracted. Unlike the analysis from Szilard to Brillouin, the consistency of statistical mechanics now requires us to say at least something about the demon itself as a physical being, constrained by a particular kind of physical law. The demon may be able to perform measurements non-dissipatively, but its memory must be representable by a physical device, and this device must be included in the statistical mechanical description. This contrasts sharply with Brillouin. Brillouin characterised information as “bound” if it was embodied in the states of a physical device, but he explicitly stated that information contained only in the mind was “free”, not “bound”.