In this final chapter, I propose to recapitulate the main conclusions at which we have arrived, and then to say a few words on the subject of Man’s relation to the universe in so far as philosophy has anything to teach on this subject without extraneous help.

Popular metaphysics divides the known world into mind and matter, and a human being into soul and body. Some—the materialists—have said that matter alone is real and mind is an illusion. Many—the idealists in the technical sense, or mentalists, as Dr. Broad more appropriately calls them—have taken the opposite view, that mind alone is real and matter is an illusion. The view which I have suggested is that both mind and matter are structures composed of a more primitive stuff which is neither mental nor material. This view, called “neutral monism”, is suggested in Mach’s Analysis of Sensations, developed in William James’s Essays in Radical Empiricism, and advocated by John Dewey, as well as by Professor R. B. Parry and other American realists. The use of the word “neutral” in this way is due to Dr. H. M. Sheffer,15 formerly of Harvard, who is one of the ablest logicians of our time.

Since man is the instrument of his own knowledge, it is necessary to study him as an instrument before we can appraise the value of what our senses seem to tell us concerning the world. In Part I we studied man, within the framework of common-sense beliefs, just as we might study clocks or thermometers, as an instrument sensitive to certain features of the environment, since sensitiveness to the environment is obviously an indispensable condition for knowledge about it.

In Part II we advanced to the study of the physical world. We found that matter, in modern science, has lost its solidity and substantiality; it has become a mere ghost haunting the scenes of its former splendours. In pursuit of something that could be treated as substantial, physicists analysed ordinary matter into molecules, molecules into atoms, atoms into electrons and protons. There, for a few years, analysis found a resting-place. But now electrons and protons themselves are dissolved into systems of radiations by Heisenberg, and into systems of waves by Schrödinger—the two theories amount mathematically to much the same thing. And these are not wild metaphysical speculations; they are sober mathematical calculations, accepted by the great majority of experts.

Another department of theoretical physics, the theory of relativity, has philosophical consequences which are, if possible, even more important. The substitution of space-time for space and time has made the category of substance less applicable than formerly, since the essence of substance was persistence through time, and there is now no one cosmic time. The result of this is to turn the physical world into a four-dimensional continuum of events, instead of a series of three-dimensional states of a world composed of persistent bits of matter. A second important feature of relativity-theory is the abolition of force, particularly gravitational force, and the substitution of differential causal laws having to do only with the neighbourhood of an event, not with an influence exerted from a distance, such as gravitation formerly seemed to be.

The modern study of the atom has had two consequences which have considerably changed the philosophical hearing of physics. On the one hand, it appears that there are discontinuous changes in nature, occasions when there is a sudden jump from one state to another without passing through the intermediate states. (Schrödinger, it is true, questions the need for assuming discontinuity; but so far his opinion has not prevailed.) On the other hand, the course of nature is not so definitely determined by the physical laws at present known as it was formerly thought to be. We cannot predict when a discontinuous change will take place in a given atom, though we can predict statistical averages. It can no longer be said that, given the laws of physics and the relevant facts about the environment, the future history of an atom can theoretically be calculated from its present condition. It may be that this is merely due to the insufficiency of our knowledge, but we cannot be sure that this is the case. As things stand at present, the physical world is not so rigidly deterministic as it has been believed to be during the last 250 years. And in various directions what formerly appeared as laws governing each separate atom are now found to be only averages attributable in part to the laws of chance.

From these questions concerning the physical world in itself, we were led to others concerning the causation of our perceptions, which are the data upon which our scientific knowledge of physics is based. We saw that a long causal chain always intervenes between an external event and the event in us which we regard as perception of the external event. We cannot therefore suppose that the external event is exactly what we see or hear; it can, at best, resemble the percept only in certain structural respects. This fact has caused considerable confusion in philosophy, partly because philosophers tried to think better of perception than it deserves, partly because they failed to have clear ideas on the subject of space. It is customary to treat space as a characteristic of matter as opposed to mind, but this is only true of physical space. There is also perceptual space, which is that in which what we know immediately through the senses is situated. This space cannot be identified with that of physics. From the standpoint of physical space, all our percepts are in our heads; but in perceptual space our percept of our hand is outside our percept of our head. The failure to keep physical and perceptual space distinct has been a source of great confusion in philosophy.

In Part III we resumed the study of man, but now as he appears to himself, not only as he is known to an external observer. We decided, contrary to the view of the behaviourists, that there are important facts which cannot be known except when the observer and observed are the same person. The datum in perception, we decided, is a private fact which can only be known directly to the percipient; it is a datum for physics and psychology equally, and must be regarded as both physical and mental. We decided later that there are inductive grounds, giving probability but not certainty, in favour of the view that perceptions are causally connected with events which the percipient does not experience, which may belong only to the physical world.

The behaviour of human beings is distinguished from that of inanimate matter by what are called “mnemic” phenomena, i.e. by a certain kind of effect of past occurrences. This kind of effects is exemplified in memory, in learning, in the intelligent use of words, and in every kind of knowledge. But we cannot, on this ground, erect an absolute barrier between mind and matter. In the first place, inanimate matter, to some slight extent, shows analogous behaviour—e.g. if you unroll a roll of paper, it will roll itself up again. In the second place, we find that living bodies display mnemic phenomena to exactly the same extent to which minds display them. In the third place, if we are to avoid what I have called “mnemic” causation, which involves action at a distance in time, we must say that mnemic phenomena in mental events are due to the modification of the body by past events. That is to say, the set of events which constitutes one man’s experience is not causally self-sufficient, but is dependent upon causal laws involving events which he cannot experience.

On the other hand, our knowledge of the physical world is purely abstract: we know certain logical characteristics of its structure, but nothing of its intrinsic character. There is nothing in physics to prove that the intrinsic character of the physical world differs, in this or that respect, from that of the mental world. Thus from both ends, both by the analysis of physics and by the analysis of psychology, we find that mental and physical events form one causal whole, which is not known to consist of two different sorts. At present, we know the laws of the physical world better than those of the mental world, but that may change. We know the intrinsic character of the mental world to some extent, but we know absolutely nothing of the intrinsic character of the physical world. And in view of the nature of the inferences upon which our knowledge of physics rests, it seems scarcely possible that we should ever know more than abstract laws about matter.

In Part IV we considered what philosophy has to say about the universe. The function of philosophy, according to the view advocated in this volume, is somewhat different from that which has been assigned to it by a large and influential school. Take, e.g. Kant’s antinomies. He argues (1) that space must be infinite, (2) that space cannot be infinite; and he deduces that space is subjective. The non-Euclideans refuted the argument that it must be infinite, and Georg Cantor refuted the argument that it cannot be. Formerly, a priori logic was used to prove that various hypotheses which looked possible were impossible, leaving only one possibility, which philosophy therefore pronounced true. Now a priori logic is used to prove the exact contrary, namely, that hypotheses which looked impossible are possible. Whereas logic was formerly counsel for the prosecution, it is now counsel for the defence. The result is that many more hypotheses are at large than was formerly the case. Formerly, to revert to the instance of space, it appeared that experience left only one kind of space to logic, and logic showed this one kind to be impossible. Now, logic presents many kinds of space as possible apart from experience, and experience only partially decides between them. Thus, while our knowledge of what is has become less than it was formerly supposed to be, our knowledge of what may be is enormously increased. Instead of being shut in within narrow walls, of which every nook and cranny could be explored, we find ourselves in an open world of free possibilities, where much remains unknown because there is so much to know. The attempt to prescribe to the universe by means of a priori principles has broken down; logic, instead of being, as formerly, a bar to possibilities, has become the great liberator of the imagination, presenting innumerable alternatives which are closed to unreflective common sense, and leaving to experience the task of deciding, where decision is possible, between the many worlds which logic offers for our choice.

Philosophical knowledge, if what we have been saying is correct, does not differ essentially from scientific knowledge; there is no special source of wisdom which is open to philosophy but not to science, and the results obtained by philosophy are not radically different from those reached in science. Philosophy is distinguished from science only by being more critical and more general. But when I say that philosophy is critical, I do not mean that it attempts to criticise knowledge from outside, for that would be impossible: I mean only that it examines the various parts of our supposed knowledge to see whether they are mutually consistent and whether the inferences employed are such as seem valid to a careful scrutiny. The criticism aimed at is not that which, without reason, determines to reject, but that which considers each piece of apparent knowledge on its merits and retains whatever still appears to be knowledge when this consideration is completed. That some risk of error remains must be admitted, since human beings are fallible. Philosophy may claim justly that it diminishes the risk of error, and that in some cases it renders the risk so small as to be practically negligible. To do more than this is not possible in a world where mistakes must occur; and more than this no prudent advocate of philosophy would claim to have performed.

I want to end with a few words about man’s place in the universe. It has been customary to demand of a philosopher that he should show that the world is good in certain respects. I cannot admit any duty of this sort. One might as well demand of an accountant that he should show a satisfactory balance sheet. It is just as bad to be fraudulently optimistic in philosophy as in money matters. If the world is good, by all means let us know it; but if not, let us know that. In any case, the question of the goodness or badness of the world is one for science rather than for philosophy. We shall call the world good if it has certain characteristics that we desire. In the past philosophy professed to be able to prove that the world had such characteristics, but it is now fairly evident that the proofs were invalid. It does not, of course, follow that the world does not have the characteristics in question; it follows only that philosophy cannot decide the problem. Take for example the problem of personal immortality. You may believe this on the ground of revealed religion, but that is a ground which lies outside philosophy. You may believe it on the ground of the phenomena investigated by psychical research, but that is science, not philosophy. In former days, you could believe it on a philosophical ground, namely, that the soul is a substance and all substances are indestructible. You will find this argument, sometimes more or less disguised, in many philosophers. But the notion of substance, in the sense of a permanent entity with changing states, is no longer applicable to the world. It may happen, as with the electron, that a string of events are so interconnected causally that it is practically convenient to regard them as forming one entity, but where this happens it is a scientific fact, not a metaphysical necessity. The whole question of personal immortality, therefore lies outside philosophy, and it is to be decided, if at all, either by science or by revealed religion.

I will take up another matter in regard to which what I have said may have been disappointing to some readers. It is sometimes thought that philosophy ought to aim at encouraging a good life. Now, of course, I admit that it should have this effect, but I do not admit that it should have this as a conscious purpose. To begin with, when we embark upon the study of philosophy we ought not to assume that we already know for certain what the good life is; philosophy may conceivably modify our views as to what is good, in which case it will seem to the non-philosophical to have had a bad moral effect. That, however, is a secondary point. The essential thing is that philosophy is part of the pursuit of knowledge, and that we cannot limit this pursuit by insisting that the knowledge obtained shall be such as we should have thought edifying before we obtained it. I think it could be maintained with truth that all knowledge is edifying, provided we have a right conception of edification. When this appears to be not the case it is because we have moral standards based upon ignorance. It may happen by good fortune that a moral standard based upon ignorance is right, but if so knowledge will not destroy it; if knowledge can destroy it, it must be wrong. The conscious purpose of philosophy, therefore, ought to be solely to understand the world as well as possible, not to establish this or that proposition which is thought morally desirable. Those who embark upon philosophy must be prepared to question all their preconceptions, ethical as well as scientific; if they have a determination never to surrender certain philosophic beliefs, they are not in the frame of mind in which philosophy can be profitably pursued.

But although philosophy ought not to have a moral purpose, it ought to have certain good moral effects. Any disinterested pursuit of knowledge teaches us the limits of our power, which is salutary; at the same time, in proportion as we succeed in achieving knowledge, it teaches the limits of our impotence, which is equally desirable. And philosophical knowledge, or rather philosophical thought, has certain special merits not belonging in an equal degree to other intellectual pursuits. By its generality it enables us to see human passions in their just proportions, and to realise the absurdity of many quarrels between individuals, classes, and nations. Philosophy comes as near as possible for human beings to that large, impartial contemplation of the universe as a whole which raises us for the moment above our purely personal destiny. There is a certain asceticism of the intellect which is good as a part of life, though it cannot be the whole so long as we have to remain animals engaged in the struggle for existence. The asceticism of the intellect requires that, while we are engaged in the pursuit of knowledge, we shall repress all other desires for the sake of the desire to know. While we are philosophising, the wish to prove that the world is good, or that the dogmas of this or that sect are true, must count as weaknesses of the flesh—they are temptations to be thrust on one side. But we obtain in return something of the joy which the mystic experiences in harmony with the will of God. This joy philosophy can give, but only to those who are willing to follow it to the end, through all its arduous uncertainties.

The world presented for our belief by a philosophy based upon modern science is in many ways less alien to ourselves than the world of matter as conceived in former centuries. The events that happen in our minds are part of the course of nature, and we do not know that the events which happen elsewhere are of a totally different kind. The physical world, so far as science can show at present, is perhaps less rigidly determined by causal laws than it was thought to be; one might, more or less fancifully, attribute even to the atom a kind of limited free will. There is no need to think of ourselves as powerless and small in the grip of vast cosmic forces. All measurement is conventional, and it would be possible to devise a perfectly serviceable system of measurement according to which a man would be larger than the sun. No doubt there are limits to our power, and it is good that we should recognise the fact. But we cannot say what the limits are, except in a quite abstract way, such as that we cannot create energy. From the point of view of human life, it is not important to be able to create energy; what is important is to be able to direct energy into this or that channel, and this can do more and more as our knowledge of science increases. Since men first began to think, the forces of nature have oppressed them; earthquakes, floods, pestilences, and famines have filled them with terror. Now at last, thanks to science, mankind is discovering how to avoid much of the suffering that such events have hitherto entailed. The mood in which, as it seems to me the modern man should face the universe is one of quiet self-respect. The universe as known to science is not in itself either friendly or hostile to man, but it can be made to act as a friend if approached with patient knowledge. Where the universe is concerned, knowledge is the one thing needful. Man, alone of living things, has shown himself capable of the knowledge required to give him a certain mastery over his environment. The dangers to man in the future, or at least in any measurable future, come, not from nature, but from man himself. Will he use his power wisely? Or will he turn the energy liberated from the struggle with nature into struggles with his fellow-men? History, science, and philosophy all make us aware of the great collective achievements of mankind. It would be well if every civilised human being had a sense of these achievements and a realisation of the possibility of greater things to come, with the indifference which must result as regards the petty squabbles upon which the passions of individuals and nations are wastefully squandered.

Philosophy should make us know the ends of life, and the elements in life that have value on their own account. However our freedom may be limited in the causal sphere, we need admit no limitations to our freedom in the sphere of values: what we judge good on its own account we may continue to judge good, without regard to anything but our own feeling. Philosophy cannot itself determine the ends of life, but it can free us from the tyranny of prejudice and from distortions due to a narrow view. Love, beauty, knowledge, and joy of life: these things retain their lustre however wide our purview. And if philosophy can help us to feel the value of these things, it will have played its parts in man’s collective work of bringing light into a world of darkness.