  Brillouin, in particular, attempted to develop this idea into a general theory of the relationship between information and entropy. Brillouin now identified entropy with the Gibbs entropy of a system. He distinguished two kinds of information: ‘bound’ information and ‘free’ information. Bound information refers strictly to information which is embodied in the states of physical systems. Information which is, for example, only contained in someone's mind, according to Brillouin, is free, not bound. Performing a measurement could reduce the thermodynamic entropy of a system, but to do so requires the creation of an equivalent quantity of bound information in the device which holds the outcome of the measurement. However, it is not entirely clear if Brillouin's argument was that the creation of bound information is necessarily associated with compensating entropy production, as the arguments based upon scattered photons suggest, or if the bound information in itself is an additional term that must be added to normal entropy to produce a generalised second law.