An imaginary being endowed with sufficiently keen perceptions to be able to reverse entropic disorder in an isolated system by controlling in a selective way the movements of its elements.
Such a being would be capable by its action to violate the second law of thermodynamics.
However, as stated by A. RAPOPORT: "MAXWELL's argument… contains a basic fallacy. If the demon is placed inside the system, the processes going on within him must also be taken in account in computing the total change in entropy. It was subsequently shown by L. SZILARD (1929) and later by L BRILLOUIN that the processes within the demon (whether he is a mechanism or an organism) must be such that the decrease in entropy effected by his intervention is at least compensated (in general, over-compensated) by an increase of entropy in the demon. If, on the other hand, the demon intervenes from outside the system, then the system can no longer be considered to be isolated, and the Second Law does not apply" (1966, p.7).
The point can also be made by observing that the demon must acquire information through an observational process that is not gratuitous in energy terms and, moreover, needs energy in order to move the elements in an ordering way.
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Bertalanffy Center for the Study of Systems Science (2020). Title of the entry. In Charles François (Ed.), International Encyclopedia of Systems and Cybernetics (2). Retrieved from www.systemspedia.org/[full/url]
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