Science usually advances by a succession of small steps, through a fog in which even the most keen-sighted explorer can seldom see more than a few paces ahead. Occasionally the fog lifts, an eminence is gained, and a wider stretch of territory can be surveyed—sometimes with startling results. A whole science may then seem to undergo a kaleidoscopic rearrangement, fragments of knowledge being found to fit together in a hitherto unsuspected manner. Sometimes the shock of readjustment may spread to other sciences; sometimes it may divert the whole current of human thought.

Events of this last kind are rare, but instances come readily to mind. We are likely to think first of the results of replacing the geocentric astronomy of mediaeval times by the Copernican system—man saw that his home was not the majestic fixed centre of the universe round which all else had to revolve, but one of many fragments of matter which were themselves revolving round a very ordinary one of the myriads of stars in the sky. Or we may think of the implications of the Darwinian biology—man saw that his body had not been specially designed for himself, the lord of creation, but was an adaptation and development of the bodies of animals which had preceded him on earth, and were in fact his own ancestry; all terrestrial creatures, even the meanest, proved to be his bloodrelations, and if he had dominion over them it was only because he happened to have been born into the clever branch of the big family.

We have now concluded our summary of the findings of modern physics, and may turn to consider how these findings affect the practical problems of philosophy and of everyday life. But let us first recapitulate the conclusions we have reached in our scientific discussion.

Recapitulation

Because we are human beings and not mere animals, we try to discover as much as we can about the world in which our lives are cast. We have seen that there is only one method of gaining such knowledge—the method of science, which consists in a direct questioning of nature by observation and experiment.

The first thing we learn from such questioning is that the world is rational; its happenings are not determined by caprice but by law. There exists what we have called a ‘pattern of events’, and the primary aim of physical science is the discovery of this pattern. This, as we have seen, will be capable of description only in mathematical terms.

The new quantum theory explained in the preceding chapter has provided a mathematical description of the pattern of events which is believed to be complete and perfect. For it enables us—in principle at least—to predict every possible phenomenon of physics, and not one of its predictions has so far proved to be wrong. In a sense, then, we might say that theoretical physics has achieved the main purpose of its being, and that nothing remains but to work out the details.

The new physics just described was still based largely on Newtonian ideas. Indeed, in its theoretical aspects, it might not unfairly be described as a final attempt to explain the world in materialistic terms—as particles being pushed and pulled about in space and time. Nevertheless, the new physics had found it necessary to abolish most of the forces of pushing and pulling, replacing the gradual changes of motion of the particles under these forces by sudden and unpredictable jumps. These appeared to involve violations of the law of causality, both in the disintegration of radioactive atoms and also in the internal changes of ordinary atoms. We seemed to see Fate defying this law as she picked out certain atoms for disintegration or collapse and, by her apparently capricious acts, sent the universe along one path or another according to her whim.

On such lines the new physics had explained many phenomena which had hitherto seemed inexplicable, but it had by no means met with complete success. For instance, while it gave a perfect interpretation of the simplest spectrum of all, namely that of the hydrogen atom, it failed with more complex spectra. This was not necessarily a fatal objection; a few emendations and possibly a few new ad hoc assumptions might have effected a complete reconciliation, although this seems improbable.

The aim of the present book is very simply stated; it is to discuss—and to some extent to explore—that borderland territory between physics and philosophy which used to seem so dull, but suddenly became so interesting and important through recent developments of theoretical physics.

The new interest extends far beyond the technical problems of physics and philosophy to questions which touch human life very closely, such as materialism and free-will. Thus I hope the book may interest many who are neither physicists nor philosophers by profession, and to this end I have made the discussion as simple as possible, avoiding technicalities when I could, and, when I could not, explaining them. I have also tried to arrange the book so that a reading of the first two chapters and the last shall give an intelligible view of the main argument and conclusions of the whole; many readers may prefer to read these three chapters first.

I need hardly add that my acquaintance with philosophy is simply that of an intruder, and nothing could be further from my intentions than to pose as an authority on questions of pure philosophy. If I had to choose a sub-title for my book, it might well be ‘The reflections of a physicist on some of the problems of philosophy”.

Pre-Newtonian Mechanics

The earliest attempts to discover the pattern of events were limited, naturally enough, to the visible movements of objects either on what we have called the man-sized scale or on the far grander scale of astronomy—these were the only movements which could be studied without instrumental aid.

The movements of the astronomical bodies were treated only in their geometrical aspect. The ‘fixed stars’ hardly came under discussion at all, since they appeared to have no motion beyond their diurnal rotation round the pole. This was of course a consequence of their great distance from the earth, but it was explained by supposing them to be immovably attached to a sphere which rotated round the earth as centre.

There remained the sun, moon, and planets. A whole succession of astronomers—from Aristarchus through Ptolemy to Copernicus and Kepler—had investigated the paths in which these bodies moved, but had shown very little concern as to why they moved in these particular paths rather than in others. Aristotle's pronouncement that a circular motion was natural to all bodies, because the circle was the perfect geometrical figure, seems to have stifled curiosity fairly thoroughly for nearly two thousand years; it was uncritically accepted by Copernicus, and even at one time by Galileo.

It was different with terrestrial bodies; there had been many attempts to explain their movements in what we should now describe as dynamical terms.

Preliminary Survey

With the coming of the twentieth century, there came into being a new physics which was especially concerned with phenomena on the atomic and sub-atomic scale. It brought with it a new way of interpreting the phenomena of inanimate nature, which was destined in time to sweep away all the difficulties besetting the old classical mechanics. A preliminary glance over the vast territory of this new physics reveals three outstanding landmarks.

First we notice an investigation which Prof. Planck of Berlin published in 1899. His aim was so to amend the classical mechanics that it should fit the observed facts of radiation, and show why the energy of bodies was not wholly transformed into radiation. We have already seen that this was likely to involve giving up either continuity or causality or the representation of phenomena as changes taking place in space and time. Actually his investigation seemed to show that continuity had to be given up, suggesting that in the last resort changes in the universe do not consist of continuous motions in space and time, but are in some way discontinuous.

The classical mechanics had envisaged a world constructed of matter and radiation, the matter consisting of atoms and the radiation of waves. Planck's theory called for an atomicity of radiation similar to that which was so well established for matter.

THE SOURCES OF KNOWLEDGE

We have already noticed how knowledge is gained by establishing relations between an inner process of understanding in our private minds and the facts of that public outer world which is common to us all. As Plato pointed out, the use of a common language is based on the supposition that such relations can be established by all of us.

In the period we have been considering, science claimed only one source of knowledge of the facts and objects of the outer world, namely the impressions they make on the mind through the medium of the senses. Yet the untrustworthiness of the senses had been one of the commonplaces of philosophy from Greek times on, and if the same facts and objects of the outer world made different impressions on different minds, where did science stand? If we trusted to individual sense-impressions, we could never get beyond the position described by Protagoras (c. 481—411 B.C.): ‘What seems to me is so to me, what seems to you is so to you’; each individual would become his own final arbiter of truth, and there could be no body of objective knowledge. Six centuries before Christ, in the earliest days of Greek philosophy, Thales of Miletus had urged the importance of gaining a substratum of facts, independent of the judgment of individuals, on which a body of objective knowledge could be built.

We have seen that knowledge of the external world can come only through observation and experiment. These tell us that the world is rational—its events follow one another according to definite laws, and so form a regular pattern. The primary aim of physics is the discovery of this pattern; we have seen that it can be described only in mathematical language.

We have seen that physics cannot clothe the mathematical symbols of this description with their true physical meaning, but physics and philosophy may properly engage in joint discussion as to their possible meanings, and the most probable interpretation of the pattern of events. Yet there are many hindrances to such discussion. In the present chapter we shall try to unearth some of these and eliminate them with a view to clearing the ground for the discussions which are to follow.

DIFFERENCES OF LANGUAGE

Foremost among these hindrances are differences of language and of terminology; when science and philosophy are not speaking entirely different languages, they, often seem at least to employ different idioms.

More than three hundred years have elapsed since Francis Bacon wrote of the ‘Idols’ which beset men's minds when they try to discover truth. The most troublesome of these, he said, are the idols of the market-place, the place where men meet to talk with one another. For words are unsuited to the expression of accurate or scientific thought, and apparent differences of opinion often result from inadequate definition of the terms employed in the discussion.