James Jeans: Physics and Philosophy I

James Jeans wrote Physics and Philosophy in 1942. The book, published by Cambridge University Press, was intended (in Jeans' own words from the Preface):-

... to discuss - and to some extent explore - that borderline territory between physics and philosophy which used to seem so dull, but suddenly became so interesting and important through recent developments of theoretical physics.

We give below short introduction to the book, followed by the introductory material on "What is Physics?" in which he discusses Positivism and Physical Knowledge:-


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 blood-relations, 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.

A third such rearrangement of ideas occurred when Newton's system of mechanics and law of gravitation gained general acceptance - men saw that the heavenly bodies were no longer to be feared or even consulted as influences in human affairs; they were only chunks of inert matter moving as they were driven by universal laws. The Newtonian scheme of things seemed further to suggest -although it was never quite able to prove - that all bodies, even the smallest, were subject to the same scheme of universal law, so that all change and motion were mechanical in their nature, the future following from the past with the inevitability of the motions of a machine. If this were so, man's imagined freedom to choose between good and evil or select his own path through life was a pitiable illusion; the ball could only go where the player sent it.

A fourth such revolution has occurred in physics in recent years. Its consequences extend far beyond physics, and in particular they affect our general view of the world in which our lives are cast - in a word, they affect philosophy. The philosophy of any period is always largely interwoven with the science of the period, so that any fundamental change in science must produce reactions in philosophy. This is especially so in the present case, where the changes in physics itself are of a distinctly philosophical hue; a direct questioning of nature by experiment has shown the philosophical background hitherto assumed by physics to have been faulty. The necessary emendations have naturally affected the scientific basis of philosophy and, through it, our approach to the philosophical problems of everyday life. Are we, for instance, automata or are we free agents capable of influencing the course of events by our volitions? Is the world material or mental in its ultimate nature? Or is it both? If so, is matter or mind the more fundamental - is mind a creation of matter or matter a creation of mind? Is the world we perceive in space and time the world of ultimate reality, or is it only a curtain veiling a deeper reality beyond?

The primary aim of the present book is to discuss the interrelation between physics and philosophy. While the discussion is in general terms, it naturally has very special reference to the changes of recent years, and their bearing on philosophical questions such as those just mentioned. But as a preliminary let us consider the general questions: What is physics and what is philosophy?


Both physics and philosophy had their beginnings in those dim ages in which man was first differentiating himself from his brute ancestry, acquiring new emotional and mental characteristics which were henceforth to be his distinguishing marks. Foremost among these were an intellectual curiosity out of which philosophy has grown, and a practical curiosity which was ultimately to develop into science.

For primitive man, thrown into a world which he did not understand, soon found that his comfort, his well-being, and even his life were jeopardized by this want of understanding. Inanimate nature seemed helpful and friendly to him at times, but could become hostile when the life-giving sunshine and gentle rain gave place to the thunderbolt and whirlwind; these inspired in him the same feelings of awe and fear as the wild beasts and human foes which threatened his life. His first reaction was to project his own human motives and passions on to the inanimate objects around him; he peopled his world with spirits and demons, with gods and goddesses great and small until, as Andrew Lang has said, 'all nature was a congeries of animated personalities'. Such imaginings were not confined to cave-men and savages; even Thales of Miletus (640-546 B.C.), astronomer, geometer and philosopher, maintained that all things were 'full of gods'.

Primitive man endowed these personalities with characteristics and qualities almost as definite as those of his real friends and foes. In so doing he was not altogether wrong, for they seemed to be creatures of habit; what they had done once they were likely to do again. Even the animals understand this; they avoid a place where they have suffered pain in the past, suspecting that what hurt them once may hurt them again, and they return to a place where they have once found food, considering it a hopeful place in which to look for more. What were mere associations of ideas in the brains of animals readily became translated into natural laws in the minds of thinking men, and led to the discovery of the principle of the uniformity of nature - what has happened once will, in similar circumstances, happen again; the events of nature do not occur at random but after an unvarying pattern. Once this discovery had been made, physical science became possible. Its primary aim is to discover this pattern of events, in so far as it governs the happenings of the inanimate world.


The primitive stage of human development which we have just depicted is that which Auguste Comte (1798-1857) described as the stage of fetichism, although we now usually call it animism. In this stage man believed he could modify the course of events by his own volition and to his own advantage, by influencing the gods and spirits with which he had filled his world - sometimes through a policy of appeasement, as by worship and sacrifice, and sometimes through prayers, spells and incantations.

Comte says that in time this stage of animism gave place to a second stage of metaphysics, in which the spirits and gods of the animistic stage become depersonified, and are replaced by vaguely conceived forces, activities or essences. In this stage the world is depicted as being controlled by 'vital forces', 'chemical activities', a 'principle of gravity', and the like. These finally amalgamate into a single activity which is usually referred to as 'nature', although we still occasionally personify it and spell it with a capital N. The sequence of events has now passed beyond human control.

Comte considers that this second or metaphysical stage must in due course give place to yet a third stage - the positive stage. The 'forces' which expelled the spirits and gods now suffer expulsion in their turn. Nothing is left in the world but happenings for which no explanation or interpretation is offered or even attempted, and science has now for its single aim the discovery of the laws to which these happenings conform - the pattern of events.

Thus to primitive man the sun was a life-giving god - to the Greeks the horse-drawn chariot of a god - while a later and less pagan age supposed that angels had been entrusted with the task of pushing along the sun, moon and planets, and of maintaining the motion of the celestial spheres to which the more distant stars were supposed to be affixed. This animistic stage ended when the god, his horses and his chariot, the angels and their celestial spheres, were eliminated by the progress of science. To be more explicit, it ended when Copernicus, in accordance with the earlier teaching of Pythagoras, Aristarchus and others, showed how the apparent motion of the sun, moon and stars across the sky resulted from a daily rotation of the earth, while the motions of the planets through the stars could be explained by their revolutions round a fixed sun. Even when Kepler discovered the true shapes of these planetary orbits sixty years later, he still postulated a 'power' or influence to keep the planets moving; he thought they would all stop dead if a material emanation from the sun did not continually urge them on. The science of planetary movements had attained to its second stage.

Newton retained a 'force' of gravitation, but was fully conscious of the philosophical difficulties involved. When Leibniz attacked him for introducing occult qualities and miracles into his philosophy, he replied that 'to understand the motions of the planets under the influence of gravity, without knowing the cause of gravity, is as good a progress in philosophy as to understand the frame of a clock, and the dependence of the wheels upon one another, without knowing the cause of the gravity of the weight which moves the machine, is in the philosophy of clockwork'. Astronomy was beginning to move into the third stage, to which it has only recently fully attained. The astronomer of today makes no claim to understand why the planets move as they do; he is content to know that the pattern of events can be described very neatly and concisely by picturing planetary motions as taking place in a curved space.

Comte believed that every science must inevitably go through these three stages in turn - this is his famous 'law of the three stages'. He further claimed that the abstract sciences could be arranged in a hierarchy, in the order

mathematics, astronomy, physics, chemistry, biology, sociology,

in which each science is

(a) historically older,

(b) logically simpler,

(c) more widely applicable,

than any of the sciences which come after it on the list. Certain sciences which loom large in present-day knowledge, as for instance geology and psychology, are absent from the list and do not fit at all naturally into the hierarchy. If, however, we merge the minor sciences into the greater, the hierarchy assumes the simpler form

mathematics, physics, biology, sociology,

and now possesses all the virtues claimed for it by its author.

Comte further claimed that each science in the hierarchy is independent of all that follow it, and also must reach the final or positive stage before them. Since mathematics must have been in the positive stage from its first beginnings, the claim for physics is that it depends only on mathematics, and must be the first experimental science to attain to the positive stage. We shall investigate these claims in due course, but first let us examine the true nature of physical knowledge.

Physical Knowledge

We each live our mental life in a prison-house from which there is no escape. It is our body; and its only communication with the outer world is through our sense-organs - eyes, cars, etc. These form windows through which we can look out on to the outer world and acquire knowledge of it. A man lacking all five senses could know nothing of this outer world, because he would have no means of contact with it; the whole content of his mind would be an expansion of what had been in it at birth.

The sense-organs of a normal man receive stimuli - rays of light, waves of sound, etc. - from the outer world, and these produce electric changes which are propagated over his nerves to his brain. Here they produce further changes, as the result of which - after a series of processes we do not in the least understand - his mind acquires perceptions - to use Hume's terminology - of the outer world. These give rise to impressions and ideas in turn, an impression denoting a sensation, emotion or feeling at the moment when a perception first makes its appearance in the mind, and an idea denoting what is left of an impression when its first vigour is spent, including for instance the memory of an impression or the repetition of it in a dream.

Thus the whole content of a man's mind. can consist of three parts at most - a part that was in his mind at birth, a part that has entered through his sense-organs, and a part which has been developed out of these two parts by processes of reflection and ratiocination. Some have denied that the first part exists at all, holding with Hobbes (1588-1679) that 'there is no conception in a man's mind which hath not at first been begotten upon the organs of sense' - nihil est in intellectu quod non fuerit in sensu. Others have thought with Leibniz (1646-1716) that this should be amended by the addition of the words nisi intellectus ipse - there is nothing in the understanding that has not come through the senses, except the understanding itself. We shall discuss these questions more fully as the need arises.

Whenever a man increases the content of his mind he gains new knowledge, and this occurs each time a new relation is established between the worlds on the two sides of the sense-organs - the world of ideas in an individual mind, and the world of objects existing outside individual minds which is common to us all.

The study of science provides us with such new knowledge. Physics gives us exact knowledge because it is based on exact measurements. A physicist may announce, for instance, that the density of gold is 19.32, by which he means that the ratio of the weight of any piece of gold to that of a volume of water of equal size is 19.32; or that the wave-length of the line Ha in the spectrum of atomic hydrogen is 0.000065628 centimetre, by which he means that the ratio of the length of a wave of Ha light to that of a centimetre is 0.000065628, a centimetre being defined as a certain fraction of the diameter of the earth, or of the length of a specified bar of platinum, or as a certain multiple of the wave-length of a line in the spectrum of cadmium.

These statements import real knowledge into our minds, since each identifies a specific number, the idea of which is already in our minds, with the value of a ratio which has an existence in the world outside; this idea of a ratio is again something with which our minds are familiar. Thus the statements tell us something new in a language we can understand.

Each ratio expresses a relation between two things neither of which we understand separately, such as gold and water. Our minds can never step out of their prison-houses to investigate the real nature of the things - gold, water, atomic hydrogen, centimetres or wave-lengths - which inhabit that mysterious world out beyond our sense-organs. We are acquainted with such things only through the messages we receive from them through the windows of our senses, and these tell us nothing as to the essential nature of their origins. But our minds can understand and know ratios - which are pure numbers - even of quantities which are themselves incomprehensible. We can, then, acquire real knowledge of the external world of physics, but this must always consist of ratios, or in other words, of numbers.

The raw material of every science must always be an accumulation of facts; the values of ratios of which we have just been speaking constitute the raw material of physics. But, as Poincaré remarked, an accumulation of facts is no more a science than a heap of stones is a house. When we set to work to build our house - i.e. to create a science - we must first coordinate and synthesize the accumulated piles of facts. It is then usually found that a great number of separate facts can be summed up in a much smaller number of general laws. This indeed is the most fundamental and also the most general fact disclosed by the experimental study of science the stones fit together and combine, out of their intrinsic nature, to make a house. In brief, nature is rational. The house, being a rational structure and not a shapeless pile of stones, will show certain marked features. These express the pattern of events for which we are searching.

In physics the separate stones are numbers - the ratios just described - and the features of the house are relations between large groups of numbers. Clearly these relations will be most easily recorded and explained by embodying them in mathematical formulae, so that our scientific house will consist of a collection of mathematical formulae; in this way, and this alone, can we express the pattern of events. To take a simple illustration, the physicist finds that the spectrum of atomic hydrogen contains the line Ha which we have already mentioned, and also a very great number of other lines which are usually designated as Hb, Hg, Hd, etc. The wave-lengths of these lines can be measured, and are found to be related with one another in a very simple way which can be expressed by a quite simple mathematical formula. This is typical of the way in which the particular scientific house of physics is built up; a great number of separate facts of observation are all subsumed in a single mathematical formula, and our knowledge of the physical world is expressed by a number of such formulae.

The second part of Jeans' What is Physics? is given at this link

JOC/EFR March 2006

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