temperature between + and - would continually be increased. Heat would thus flow from a region of low to a region of high temperature without an equivalent amount of work being expended. [20] Impossible, in the sense that while we are unable to “abrogate” a physical law, Maxwell’s finite demon could, although his faculties were similar in nature to ours. Now we must not introduce demonology into science, so, lest this fiction of Maxwell’s should savour of mysticism, or something equally repugnant, we shall state the idea involved in it in quite unexceptionable terms. The conclusions of physics are founded on the assumption that we cannot control the motions of individual molecules. In a mass of gas, or liquid, or in a solid, the molecules are free to move and do move. Their individual velocities and free paths vary considerably from each other. These motions and paths are un-co-ordinated--“helter-skelter”--if we like so to term them. Physics considers only the statistical _mean_ velocities and free paths. The irreversibility of physical phenomena, the fact that energy tends to dissipate itself, the second law of thermodynamics, depend on the assumption that Maxwell’s demons exist only in imagination. We must appeal to experience now. There is no _a priori_ reason why the phenomena of physics should be directed one way and not the other, for it is possible to conceive a condition of our Universe in which, for instance, solid iron would fuse when exposed to the atmosphere. In such conditions organisms would grow backwards from old age to birth, with conscious knowledge of the future but no recollections of the past. Experience shows, however, that phenomena do tend in one way--_but this experience is that of experimental physics_, so that for the latter science Maxwell’s demons do not exist. Now physiology has borrowed from physics, not only the experimental methods, but also the fundamental concepts of thermodynamics. The organism, therefore (so physiology must conclude), cannot control the motions of individual molecules, and so vital processes are irreversible. But we have seen that the processes of terrestrial life as a whole are reversible, or tend to reversibility. We must therefore seek for evidence that the organism can control the, otherwise, un-co-ordinated motions of the individual molecules.