SUMMARY The advocates of the mechanistic conception of the universe have met with several obstacles in their attempts to reconcile mechanism with the facts of experience. In the mechanistic hypothesis, all phenomena must be reversible, while experience shows that many phenomena are irreversible. It has been suggested that the apparent irreversibility of natural phenomena is due merely to the fact that molecules are too small and too numerous for our gross senses to deal with them, although a "Maxwell demon" could do so and would thereby be able to prevent irreversibility.

The kinetic theory of gases is up to now the most serious attempt to reconcile mechanism and experience, but it is still faced with the difficulty that a mechanical system cannot tend toward a permanent final state but must always return eventually to a state very close to its initial state. This difficulty can be overcome only if one is willing to assume that the universe does not tend irreversibly to a final state, as seems to be indicated by experience, but will eventually regenerate itself and reverse the second law of thermodynamics.

Everyone knows the mechanistic conception of the universe which has seduced so many good men, and the different forms in which it has been dressed. Some represent the material world as being composed of atoms which move in straight lines because of their inertia; the velocity and direction of this motion cannot change except when two atoms collide.

Others allow action at a distance, and suppose that the atoms exert on each other an attraction (or a repulsion) which depends on their distance according to some law. The first viewpoint is clearly only a particular case of the second; what I am going to say will be as true of one as of the other. The most important conclusions apply also to Cartesian mechanism, in which one assumes a continuous matter.

It would perhaps be appropriate to discuss here the metaphysical difficulties that underlie these conceptions; but I do not have the necessary authority for that. Rather than discussing with the readers of this review that which they know better than I do, I prefer to speak of subjects with which they are less familiar, but which may interest them indirectly.

I am going to concern myself with the obstacles which the mechanists have encountered when they wished to reconcile their system with experimental facts, and the efforts which they have made to overcome or circumvent them.

In the mechanistic hypothesis, all phenomena must be reversible; for example, the stars might traverse their orbits in the retrograde sense without violating Newton's law; this would be true for any law of attraction whatever. This is therefore not a fact peculiar to astronomy; reversibility is a necessary consequence of all mechanistic hypotheses.

Experience provides on the contrary a number of irreversible phenomena. For example, if one puts together a warm and a cold body, the former will give up its heat to the latter; the opposite phenomenon never occurs. Not only will the cold body not return to the warm one the heat which it has taken away when it is in direct contact with it; no matter what artifice one may employ, using other intervening bodies, this restitution will be impossible, at least unless the gain thereby realized is compensated by an equivalent or large loss. In other words, if a system of bodies can pass from state A to state B by a certain path, it cannot return from B to A, either by the same path or by a different one. It is this circumstance that one describes by saying that not only is there not direct reversibility, but also there is not even indirect reversibility.

There have been many attempts to escape this contradiction; first there was Helmholtz's hypothesis of "hidden movements". Recall the experiment made by Foucault and Pantheon with a very long pendulum. This apparatus seems to turn slowly, indicating the rotation of the earth. An observer who does not know about the movement of the earth would certainly conclude that mechanical phenomena are irreversible. The pendulum always turns in the same sense, and there is no way to make it turn in the opposite sense; to do that it would be necessary to change the sense of rotation of the earth. Such a change is of course impractical, but for us it is conceivable; it would not be so for a man who believed our planet to be immobile.

Can one not imagine that there exist similar motions in the molecular world, which are hidden from us, which we have not taken account of, and of which we cannot change the sense? This explanation is seductive, but it is insufficient; it shows why there is not direct reversibility; but one can show that it still requires indirect reversibility.

The English have proposed a completely different hypothesis. To explain it, I will make use of a comparison: if one had a hectolitre of wheat and a grain of barley, it would be easy to hide this grain in the middle of the wheat; but it would be almost impossible to find it again, so that the phenomenon appears to be in a sense irreversible. This is because the grains are small and numerous; the apparent irreversibility of natural phenomena is likewise due to the fact that the molecules are too small and too numerous for our gross senses to deal with them.

To clarify this explanation, Maxwell introduced the fiction of a "demon" whose eyes are sharp enough to distinguish the molecules, and whose hands are small and fast enough to grab them. For such a demon, if one believes the mechanists, there would be no difficulty in making heat pass from a cold to a warm body.

The development of this idea has given rise to the kinetic theory of gases, which is up to now the most serious attempt to reconcile mechanism and experience, But all the difficulties have not been overcome.

I do not know if it has been remarked that the English kinetic theories can extricate themselves from this contradiction. The world, according to them, tends at first toward a state where it remains for a long time without apparent change; and this is consistent with experience; but it does not remain that way forever, if the theorem cited above is not violated; it merely stays there for an enormously long time, a time which is longer the more numerous are the molecules. This state will not be the final death of the universe, but a sort of slumber, from which it will awake after millions of millions of centuries. According to this theory, to see heat pass from a cold body to a warm one, it will not be necessary to have the acute vision, the intelligence, and the dexterity of Maxwell's demon; it will suffice to have a little patience.

One would like to be able to stop at this point and hope that some day the telescope will show us a world in the process of waking up, where the laws of thermodynamics are reversed.

Unfortunately, other contradictions arise; Maxwell made ingenious efforts to conquer them. But I am not sure that he succeeded. The problem is so complicated that it is impossible to treat it with complete rigour. One is then forced to make certain simplifying hypotheses; are they legitimate, are they self-consistent? I do not believe they are. I do not wish to discuss them here; but there is no need for a long discussion in order to challenge an argument of which the premises are apparently in contradiction with the conclusion, where one finds in effect reversibility in the premises and irreversibility in the conclusion.

Thus the difficulties that concern us have not been overcome, and it is possible that they never will be. This would amount to a definite condemnation of mechanism, if the experimental laws should prove to be distinctly different from the theoretical ones.

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