[From Bruce Abbott (2017.11.13.1210 EST)]
Continuing my series of posts on Kenneth J. W. Craik’s unfinished book, The Mechanism of Human Action, Craik has this to say about equilibrium and stability:
Living organisms, as has been remarked by Herbert Spencer and later by Pavlov and many others, are in a state of dynamic equilibrium with their environments. If they do not maintain this equilibrium they die; if they do maintain it they show a degree of spontaneity, variability, and purposiveness of response unknown in the non-living world. This is what is meant by ‘adaptation to environment’. Before we review the main result of analytical study of the structure and behaviour of living organisms, let us consider from first principles—synthetically as it were—the essential features which any device must possess in order to show this type of dynamic equilibrium.
The essential feature of equilibrium is stability—that is, the ability to withstand disturbances. Some objects, such as stones, have great apparent stability, because they are so refractory that very few of the forces which they encounter are able to make any impression on them; but if they do encounter a sufficiently great disturbance they break up passively. This is a static type of equilibrium.
Other objects, such as the sea, or a steel spring, yield under an applied disturbance, but tend to return to their previous state after the disturbance has passed, by virtue of the potential energy which they have stored up while the disturbance lasted.
The third, and dynamic, type of equilibrium is that where energy derived from some outside source is utilized by the object to counter the disturbances which it encounters. For example, if you push someone, they put out one of their feet and push against the floor, so using energy derived from the food they have eaten to counter the disturbance which you caused. This device of using energy derived from outside to restore equilibrium is, I think, one of the first and most important features of living organisms to appear, and is more characteristic of dynamic equilibria than the mere fact that the rate of anabolism and catabolism are equal. Thus, the often quoted example of dynamic equilibrium—the level of water in a river—fails to emphasize this important feature: the use of external energy to restore equilibrium, even while the disturbing cause is still acting. One of the reasons why this device is so important is that it opens, to living organisms, an immense realm of possible reactions which, if it were not for the external supply of energy, would be thermodynamically ‘uphill’ and therefore unable to occur. To take an extreme case, no bird or machine could fly but for the supply of energy from outside. If it were not for living organisms the course of the world would be limited almost entirely to the steady downhill degradation of energy from its potential state in the sun and stars to its kinetic state in the thermal agitation of matter. Living organisms were, with very few exceptions, the first devices to use a downhill reaction—such as the combustion of carbohydrates—to provide them with a store of energy by which to drive a few uphill reactions for their own benefit. This does not, of course, imply that living organisms run contrary to the second law of thermodynamics and can prevent the gradual degradation of energy; it merely means that they have means of storing external energy in potential form for driving some local uphill reaction. If we consider the whole picture, the reaction is, as far as we know, always downhill on the average; but some parts of it may go uphill by virtue of the energy derived from the other parts.
Sherwood, Stephen L. (Ed.). (1966). The nature of psychology: A selection of papers, essays, and other writings by the late Kenneth J. W. Craik. London: Cambridge. Pp 13-14.