The earliest writer who deserves to be called a psychologist is Hobbes; and if we consider the time when he wrote, we cannot fail to be surprised at what I may term his prevision of the most important results which have now been established by science. He was the first clearly to sound the note which has ever since constituted the bass, or fundamental tone, of scientific thought. Let us listen to it through the clear instrumentality of his own language:—

These quotations are sufficient to show that the system of Hobbes was prophetic of a revelation afterwards declared by two centuries of scientific research. For they show how plainly he taught that all our knowledge of the external world is a knowledge of motion; and, again, that all our acquisitions of knowledge and other acts of mind themselves imply, as he elsewhere says, some kind of 'motion, agitation, or alteration, which worketh in the brain.' That he conceived such motion, agitation, or alteration to be, from its extreme minuteness, 'invisible' and 'insensible,' or, as we should now say, molecular, is likewise evident. I can therefore imagine the delight with which he would hear me speak when I say, that it is no longer a matter of keen-sighted speculation, but a matter of carefully demonstrated fact, that all our knowledge of the external world is nothing more than a knowledge of motion. For all the forms of energy have now been proved to be but modes of motion; and even matter, if not in its ultimate constitution vortical motion, at all events is known to us only as changes of motion: all that we perceive in what we call matter is change in modes of motion. We do not even know what it is that moves; we only know that when some modes of motion pass into other modes, we perceive what we understand by matter. It would take me too long to justify this general statement so that it should be intelligible to every one; but I am confident that all persons who understand such subjects will, when they think about it, accept this general statement as one which is universally true. And, if so, they will agree with Hobbes that all our knowledge of the external world is a knowledge of motion.

Now, if it would have been thus a joy to Hobbes to have heard to-day how thoroughly he has been justified in his views touching the external world, with no less joy would he have heard that he has been equally justified in his views touching the internal world. For it has now been proved, beyond the possibility of dispute, that it is only in virtue of those invisible movements which he inferred that the nervous system is enabled to perform its varied functions.

To many among the different kinds of movement going on in the external world, the animal body is adapted to respond by its own movements as best suits its own welfare; and the mechanism whereby this is effected is the neuro-muscular system. Those kinds of movement going on in the external world which are competent to evoke responsive movements in the animal body are called by physiologists stimuli. When a stimulus falls upon the appropriate sensory surface, a wave of molecular movement is sent up the attached sensory nerve to a nerve-centre, which thereupon issues another wave of molecular movement down a motor nerve to the group of muscles over whose action it presides; and when the muscles receive this wave of nervous influence they contract. This kind of response to stimuli is purely mechanical, or non-mental, and is ordinarily termed reflex action. The whole of the spinal cord and lower part of the brain are made up of nerve-centres of reflex action; and, in the result, we have a wonderfully perfect machine in the animal body considered as a whole. For while the various sensory surfaces are severally adapted to respond to different kinds of external movement—the eye to light, the ear to sound, and so on—any of these surfaces may be brought into suitable relation with any of the muscles of the body by means of the cerebro-spinal nerve-centres and their intercommunications.

So much, then, for the machinery of the body. We must now turn to consider the corporeal seat of the mind, or the only part of the nervous system wherein the agitation of nervous matter is accompanied with consciousness. This is composed of a double nerve-centre, which occurs in all vertebrated animals, and the two parts of which are called the cerebral hemispheres. In man this double nerve-centre is so large that it completely fills the arch of the skull, as far down as the level of the eyebrows. The two hemispheres of which it consists meet face to face in the middle line of the skull, from the top of the nose backwards. Each hemisphere is composed of two conspicuously distinct parts, called respectively the grey matter and the white matter. The grey matter is external, enveloping the white matter like a skull-cap, and is composed of an inconceivable number of nerve-cells connected together by nerve-fibres. It is computed that in a human brain there cannot be less than a thousand millions of cells, and five thousand millions of fibres. The white matter is composed only of nerve-fibres, which pass downwards in great strands of conducting tissue to the lower centres of the brain and spinal cord. So that the whole constitutes one system, with the grey matter of the cerebral hemispheres at the apex or crown.

That the grey matter of the cerebral hemispheres is the exclusive seat of mind is proved in two ways. In the first place, if we look to the animal kingdom as a whole, we find that, speaking generally, the intelligence of species varies with the mass of this grey matter. Or, in other words, we find that the process of mental evolution, on its physical side, has consisted in the progressive development of this grey matter superimposed upon the pre-existing nervous machinery, until it has attained its latest and maximum growth in man.

In the second place, we find that when the grey matter is experimentally removed from the brain of animals, the animals continue to live; but are completely deprived of intelligence. All the lower nerve-centres continue to perform their mechanical adjustments in response to suitable stimulation; but they are no longer under the government of the mind. Thus, for instance, when a bird is mutilated in this way, it will continue to perform all its reflex adjustments—such as sitting on a perch, using its wings when thrown into the air, and so forth; but it no longer remembers its nest or its young, and will starve to death in the midst of its food, unless it be fed artificially.

Again, if the grey matter of only one hemisphere be removed, the mind is taken away from the corresponding (i. e. the opposite) side of the body, while it remains intact on the other side. For example, if a dog be deprived of one hemisphere, the eye which was supplied from it with nerve-fibres continues able to see, or to transmit impressions to the lower nerve-centre called the optic ganglion; for this eye will then mechanically follow the hand waved in front of it. But if the hand should hold a piece of meat, the dog will show no mental recognition of the meat, which of course it will immediately seize if exposed to the view of its other eye. The same thing is found to happen in the case of birds: on the injured side sensation, or the power of responding to a stimulus, remains intact; while perception, or the power of mental recognition, is destroyed.

This description applies to the grey matter of the cerebral hemispheres as a whole. But of course the question next arises whether it only acts as a whole, or whether there is any localization of different intellectual faculties in different parts of it. Now, in answer to this question, it has long been known that the faculty of speech is definitely localized in a part of the grey matter lying just behind the forehead; for, when this part is injured, a man loses all power of expressing even the most simple ideas in words, while the ideas themselves remain as clear as ever. It is remarkable that in each individual only this part of one hemisphere appears to be used; and there is some evidence to show that left-handed persons use the opposite side from right-handed. Moreover, when the side which is habitually in use is destroyed, the corresponding part of the other hemisphere begins to learn its work, so that the patient may in time recover his use of language.

Within the last few years the important discovery has been made, that by stimulating with electricity the surface of the grey matter of the hemispheres, muscular movements are evoked; and that certain patches of the grey matter, when thus stimulated, always throw into action the same groups of muscles. In other words, there are definite local areas of grey matter, which, when stimulated, throw into action definite groups of muscles. The surface of the cerebral hemispheres has now been in large measure explored and mapped out with reference to these so-called motor-centres; and thus our knowledge of the neuro-muscular machinery of the higher animals (including man) has been very greatly furthered. Here I may observe parenthetically that, as the brain is insentient to injuries inflicted upon its own substance, none of the experiments to which I have alluded entail any suffering to the animals experimented upon; and it is evident that the important information which has thus been gained could not have been gained by any other method. I may also observe that as these motor-centres occur in the grey matter of the hemispheres, a strong probability arises that they are not only the motor-centres, but also the volitional centres which originate the intellectual commands for the contraction of this and that group of muscles. Unfortunately we cannot interrogate an animal whether, when we stimulate a motor-centre, we arouse in the animal's mind an act of will to throw the corresponding group of muscles into action; but that these motor-centres are really centres of volition is pointed to by the fact, that electrical stimuli have no longer any effect upon them when the mental faculties of the animal are suspended by anÆsthetics, nor in the case of young animals where the mental faculties have not yet been sufficiently developed to admit of voluntary co-ordination among the muscles which are concerned. On the whole, then, it is not improbable that on stimulating artificially these motor-centres of the brain, a physiologist is actually playing from without, and at his own pleasure, upon the volitions of the animal.

Turning, now, from this brief description of the structure and leading functions of the principal parts of the nervous system, I propose to consider what we know about the molecular movements which go on in different parts of this system, and which are concerned in all the processes of reflex adjustment, sensation, perception, emotion, instinct, thought, and volition.

First of all, the rate at which these molecular movements travel through a nerve has been measured, and found to be about 100 feet per second, or somewhat more than a mile a minute, in the nerves of a frog. In the nerves of a mammal it is just about twice as fast; so that if London were connected with New York by means of a mammalian nerve instead of an electric cable, it would require nearly a whole day for a message to pass.

Next, the time has also been measured which is required by a nerve-centre to perform its part in a reflex action, where no thought or consciousness is involved. This time, in the case of the winking reflex, and apart from the time required for the passage of the molecular waves up and down the sensory and motor nerves, is about 1/20 of a second. Such is the rate at which a nerve-centre conducts its operations when no consciousness or volition is involved. But when consciousness and volition are involved, or when the cerebral hemispheres are called into play, the time required is considerably greater. For the operations on the part of the hemispheres which are comprised in perceiving a simple sensation (such as an electrical shock) and the volitional act of signalling the perception, cannot be performed in less than 1/12 of a second, which is nearly twice as long as the time required by the lower nerve-centres for the performance of a reflex action. Other experiments prove that the more complex an act of perception, the more time is required for its performance. Thus, when the experiment is made to consist, not merely in signalling a perception, but in signalling one of two or more perceptions (such as an electrical shock on one or other of the two hands, which of five letters is suddenly exposed to view, &c.), a longer time is required for the more complex process of distinguishing which of the two or more expected stimuli is perceived, and in determining which of the appropriate signals to make in response. The time consumed by the cerebral hemispheres in meeting a 'dilemma' of this kind is from 1/5 to 1/20 of a second longer than that which they consume in the case of a simpler perception. Therefore, whenever mental operations are concerned, a relatively much greater time is required for a nerve-centre to perform its adjustments than when a merely mechanical or non-mental response is needed; and the more complex the mental operation the more time is necessary. Such may be termed the physiology of deliberation.

So much, then, for the rate at which molecular movements travel through nerves, and the times which nerve-centres consume in performing their molecular adjustments. We may next consider the researches which have been made within the last few months upon the rates of these movements themselves, or the number of vibrations per second with which the particles of nervous matter oscillate.

If, by means of a suitable apparatus, a muscle is made to record its own contraction, we find that during all the time it is in contraction, it is under-going a vibratory movement at the rate of about nine pulsations per second. What is the meaning of this movement? The meaning is that the act of will in the brain, which serves as a stimulus to the contraction of the muscle, is accompanied by a vibratory movement in the grey matter of the brain; that this movement is going on at the rate of nine pulsations per second; and that the muscle is giving a separate or distinct contraction in response to every one of these nervous pulsations. That such is the true explanation of the rhythm in the muscle is proved by the fact that if, instead of contracting a muscle by an act of the will, it be contracted by means of a rapid series of electrical shocks playing upon its attached nerve, the record then furnished shows a similar trembling going on in the muscle as in the previous case; but the tremors of contraction are now no longer at the rate of nine per second: they correspond beat for beat with the interruptions of the electrical current. That is to say, the muscle is responding separately to every separate stimulus which it receives through the nerve; and further experiment shows that it is able thus to keep time with the separate shocks, even though these be made to follow one another so rapidly as 1,000 per second. Therefore we can have no doubt that the slow rhythm of nine per second under the influence of volitional stimulation, represents the rate at which the muscle is receiving so many separate impulses from the brain: the muscle is keeping time with the molecular vibrations going on in the cerebral hemispheres at the rate of nine beats per second. Careful tracings show that this rate cannot be increased by increasing the strength of the volitional stimulus; but some individuals—and those usually who are of quickest intelligence—display a somewhat quicker rate of rhythm, which may be as high as eleven per second. Moreover, it is found that by stimulating with strychnine any of the centres of reflex action, pretty nearly the same rate of rhythm is exhibited by the muscles thus thrown into contraction; so that all the nerve-cells in the body are thus shown to have in their vibrations pretty nearly the same period, and not to be able to vibrate with any other. For no matter how rapidly the electrical shocks are allowed to play upon the grey matter of the cerebral hemispheres, as distinguished from the nerve-trunks proceeding from them to the muscles, the muscles always show the same rhythm of about nine beats per second: the nerve-cells, unlike the nerve-fibres, refuse to keep time with the electric shocks, and will only respond to them by vibrating at their own intrinsic rate of nine beats per second.

Thus much, then, for the rate of molecular vibration which goes on in nerve-centres. But the rate of such vibration which goes on in sensory and motor nerves may be very much more rapid. For while a nerve-centre is only able to originate a vibration at the rate of about nine beats per second, a motor-nerve, as we have already seen, is able to transmit a vibration of at least 1,000 beats per second; and a sensory nerve which at the surface of its expansion is able to respond differently to differences of musical pitch, of temperature, and even of colour, is probably able to vibrate very much more rapidly even than this. We are not, indeed, entitled to conclude that the nerves of special sense vibrate in actual unison, or synchronize, with these external sources of stimulation; but we are, I think, bound to conclude that they must vibrate in some numerical proportion to them (else we should not perceive objective differences in sound, temperature, or colour); and even this implies that they are probably able to vibrate at some enormous rate.

With further reference to these molecular movements in sensory nerves, the following important observation has been made—viz. that there is a constant ratio between the amount of agitation produced in a sensory nerve, and the intensity of the corresponding sensation. This ratio is not a direct one. As Fechner states it, 'Sensation varies, not as the stimulus, but as the logarithm of the stimulus.' Thus, for instance, if 1,000 candles are all throwing their light upon the same screen, we should require ten more candles to be added before our eyes could perceive any difference in the amount of illumination. But if we begin with only 100 candles shining upon the screen, we should perceive an increase in the illumination by adding a single candle. And what is true of sight is equally true of all the other senses: if any stimulus is increased, the smallest increase of sensation first occurs when the stimulus rises one per cent, above its original intensity. Such being the law on the side of sensation, suppose that we place upon the optic nerve of an animal the wires proceeding from a delicate galvanometer, we find that every time we stimulate the eye with light, the needle of the galvanometer moves, showing electrical changes going on in the nerve, caused by the molecular agitations. Now these electrical changes are found to vary in intensity with the intensity of the light used as a stimulus, and they do so very nearly in accordance with the law of sensation just mentioned. So we say that in sensation the cerebral hemispheres are, as it were, acting the part of galvanometers in appreciating the amount of molecular change which is going on in sensory nerves; and that they record their readings in the mind as faithfully as a galvanometer records its readings on the dial.

Hitherto we have been considering certain features in the physiology of nervous action, so far as this can be appreciated by means of physiological instruments. But we have just seen that the cerebral hemispheres may themselves be regarded as such instruments, which record in our minds their readings of changes going on in our nerves. Hence, when other physiological instruments fail us, we may gain much additional insight touching the movements of nervous matter by attending to the thoughts and feelings of our own minds; for these are so many indices of what is going on in the cerebral hemispheres. I therefore propose next to contemplate the mind, considered thus as a physiological instrument.

The same scientific instinct which led Hobbes so truly to anticipate the progress of physiology, led him not less truly to anticipate the progress of psychology. For just as he was the first to enunciate the fundamental principle of nerve-action in the vibration of molecules, so was he likewise the first to enunciate the fundamental principle of psychology in the association of ideas. And the great advance of knowledge which has been made since his day with respect to both these principles, entitles us to be much more confident than even he was that they are in some way intimately united. Moreover, the manner in which they are so united we have begun clearly to understand. For we know from our study of nerve-action in general, that when once a wave of invisible or molecular movement passes through any line of nerve-structure, it leaves behind it a change in the structure such that it is afterwards more easy for a similar wave, when started from the same point, to pursue the same course. Or, to adopt a simile from Hobbes, just as water upon a table flows most readily in the lines which have been wetted by a previous flow, so the invisible waves of nerve-action pass most readily in the lines of a previous passage. This is the reason why in any exercise requiring muscular co-ordination, or dexterity, 'practice makes perfect:' the nerve-centres concerned learn to perform their work by frequently repeating it, because in this way the needful lines of wave-movement in the structure of the nerve-centre are rendered more and more permeable by use. Now we have seen that in the nerve-centres called the cerebral hemispheres, wave-movement of this kind is accompanied with feeling. Changes of consciousness follow step by step these waves of movement in the brain, and therefore when on two successive occasions the waves of movement pursue the same pathway in the brain, they are attended with a succession of the same ideas in the mind. Thus we see that the tendency of ideas to recur in the same order as that in which they have previously occurred, is merely an obverse expression of the fact that lines of wave-movement in the brain become more and more permeable by use. So it comes that a child can learn its lessons by frequently repeating them; so it is that all our knowledge is accumulated; and so it is that all our thinking is conducted.

A wholly new field of inquiry is thus opened up. By using our own consciousness as a physiological instrument of the greatest delicacy, we are able to learn a great deal about the dynamics of brain-action concerning which we should otherwise remain in total ignorance. But the field of inquiry thus opened up is too large for me to enter upon to-day. I will therefore merely observe, in general terms, that although we are still very far from understanding the operations of the brain in thought, there can be no longer any question that in these operations of the brain we have what I may term the objective machinery of thought. 'Not every thought to every thought succeeds indifferently,' said Hobbes. Starting from this fact, modern physiology has clearly shown why it is a fact; and looking to the astonishing rate at which the science of physiology is now advancing, I think we may fairly expect that within a time less remote than the two centuries which now separate us from Hobbes, the course of ideas in a given train of thought will admit of having its footsteps tracked in the corresponding pathways of the brain. Be this, however, as it may, even now we know enough to say that, whether or not these footsteps will ever admit of being thus tracked in detail, they are all certainly present in the cerebral structures of each one of us. What we know on the side of mind as logical sequence, is on the side of the nervous system nothing more than a passage of nervous energy through one series of cells and fibres rather than through another: what we recognize as truth is merely the fact of the brain vibrating in tune with Nature.

Such being the intimate relation between nerve-action and mind-action, it has become the scientifically orthodox teaching that the two stand to one another in the relation of cause to effect. One of the most distinguished of my predecessors in this place, the President of the Royal Society, has said in one of the most celebrated of his lectures:—'We have as much reason for regarding the mode of motion of the nervous system as the cause of the state of consciousness, as we have for regarding any event as the cause of another.' And, by way of perfectly logical deduction from this statement, Professor Huxley argues that thought and feeling have nothing whatever to do with determining action: they are merely the bye-products of cerebration, or, as he expresses it, the indices of changes which are going on in the brain. Under this view we are all what he terms conscious automata, or machines which happen, as it were by chance, to be conscious of some of their own movements. But the consciousness is altogether adventitious, and bears the same ineffectual relation to the activity of the brain as a steam-whistle bears to the activity of a locomotive, or the striking of a clock to the time-keeping adjustments of the clock-work. Here, again, we meet with an echo of Hobbes, who opens his work on the Commonwealth with these words:—

Now, this theory of conscious automatism is not merely a legitimate outcome of the theory that nervous changes are the causes of mental changes, but it is logically the only possible outcome. Nor do I see any way in which this theory can be fought on grounds of physiology. If we persist in regarding the association between brain and thought exclusively from a physiological point of view, we must of necessity be materialists. Further, so far as we are physiologists our materialism can do us no harm. On the contrary, it is to us of the utmost service, as at once the simplest physiological explanation of facts already known, and the best working hypothesis to guide us in our further researches. But it does not follow from this that the theory of materialism is true. The bells of St. Mary's over the way always ring for a quarter of an hour before the University sermon; yet the ringing of the bells is not the cause of the sermon, although, as long as the association remains constant, there would be no harm in assuming, for any practical purposes, that it is so. But just as we should be wrong in concluding, if we did not happen to know so much about the matter as we do, that the University sermon is produced by the vibration of bells in the tower of St. Mary's Church, so we may be similarly wrong if we were definitely to conclude that the sermon is produced by the vibration of a number of little nerve-cells in the brain of the preacher.

Now, if time permitted, and if I supposed that you would all care to go with me into matters of some abstruseness, I could certainly prove that whatever the connexion between body and mind may be, we have the best possible reasons for concluding that it is not a causal connexion. These reasons are, of course, extra-physiological; but they are not on this account less conclusive. Within the limits of a lecture, however, I can only undertake to give an outline sketch of what I take to be the overwhelming argument against materialism.

We have first the general fact that all our knowledge of motion, and so of matter, is merely a knowledge of the modifications of mind. That is to say, all our knowledge of the external world—including the knowledge of our own brains—is merely a knowledge of our own mental states. Let it be observed that we do not even require to go so far as the irrefutable position of Berkeley, that the existence of an external world without the medium of mind, or of being without knowing, is inconceivable. It is enough to take our stand on a lower level of abstraction, and to say that whether or not an external world can exist apart from mind in any absolute or inconceivable sense, at any rate it cannot do so for us. We cannot think any of the facts of external nature without presupposing the existence of a mind which thinks them; and therefore, so far at least as we are concerned, mind is necessarily prior to everything else. It is for us the only mode of existence which is real in its own right; and to it, as to a standard, all other modes of existence which may be inferred must be referred. Therefore, if we say that mind is a function of motion, we are only saying, in somewhat confused terminology, that mind is a function of itself.

Such, then, I take to be a general refutation of materialism. To use but a mild epithet, we must conclude that the theory is unphilosophical, seeing that it assumes one thing to be produced by another thing, in spite of an obvious demonstration that the alleged effect is necessarily prior to its cause. Such, I say, is a general refutation of materialism. But this is far from being all. 'Motion,' says Hobbes, 'produceth nothing but motion;' and yet he immediately proceeds to assume that in the case of the brain it produces, not only motion, but mind. He was perfectly right in saying that with respect to its movements the animal body resembles an engine or a watch; and if he had been acquainted with the products of higher evolution in watch-making, he might with full propriety have argued, for instance, that in the compensating balance, whereby a watch adjusts its own movements in adaptation to external changes of temperature, a watch is exhibiting the mechanical aspect of volition. And, similarly, it is perhaps possible to conceive that the principles of mechanism might be more and more extended in their effects, until, in so marvellously perfected a structure as the human brain, all the voluntary movements of the body might be originated in the same mechanical manner as are the compensating movements of a watch; for this, indeed, as we have seen, is no more than happens in the case of all the nerve-centres other than the cerebral hemispheres. If this were so, motion would be producing nothing but motion, and upon the subject of brain-action there would be nothing further to say. Without consciousness I should be delivering this lecture; without consciousness you would be hearing it; and all the busy brains in this University would be conducting their researches, or preparing for their examinations, mindlessly. Strange as such a state of things might be, still motion would be producing nothing but motion; and, therefore, if there were any mind to contemplate the facts, it would encounter no philosophical paradox: it would merely have to conclude that such were the astonishing possibilities of mechanism. But, as the facts actually stand, we find that this is not the case. We find, indeed, that up to a certain level of complexity mechanism alone is able to perform all the compensations or adjustments which are performed by the animal body; but we also find that beyond this level such compensations or adjustments are never performed without the intervention of consciousness. Therefore, the theory of automatism has to meet the unanswerable question—How is it that in the machinery of the brain motion produces this something which is not motion? Science has now definitely proved the correlation of all the forces; and this means that if any kind of motion could produce anything else that is not motion, it would be producing that which science would be bound to regard as in the strictest sense of the word a miracle. Therefore, if we are to take our stand upon science—and this is what materialism professes to do—we are logically bound to conclude, not merely that the evidence of causation from body to mind is not so cogent as that of causation in any other case, but that in this particular case causation may be proved, again in the strictest sense of the term, a physical impossibility.

To adduce only one other consideration. Apart from all that I have said, is it not in itself a strikingly suggestive fact that consciousness only, yet always, appears upon the scene when the adjustive actions of any animal body rise above the certain level of intricacy to which I have alluded? Surely this large and general fact points with irresistible force to the conclusion, that in the performance of these more complex adjustments, consciousness—or the power of feeling and the power of willing—is of some use. Assuredly on the principles of evolution, which materialists at all events cannot afford to disregard, it would be a wholly anomalous fact that so wide and important a class of faculties as those of mind should have become developed in constantly ascending degrees throughout the animal kingdom, if they were entirely without use to animals. And, be it observed, this consideration holds good whatever views we may happen to entertain upon the special theory of natural selection. For the consideration stands upon the general fact that all the organs and functions of animals are of use to animals: we never meet, on any large or general scale, with organs and functions which are wholly adventitious. Is it to be supposed that this general principle fails just where its presence is most required, and that the highest functions of the highest organs of the highest animals stand out of analogy with all other functions in being themselves functionless? To this question I, for one, can only answer, and answer unequivocally, No. As a rational being who waits to take a wider view of the facts than that which is open to the one line of research pursued by the physiologist, I am forced to conclude that not without a reason does mind exist in the frame of things; and that apart from the activity of mind, whereby motion is related to that which is not motion, this planet could never have held the wonderful being, who in multiplying has replenished the earth and subdued it—holding dominion over the fish of the sea, and over the fowl of the air, and over every living thing that moveth.

What, then, shall we say touching this mysterious union of mind and motion? Having found it physically impossible that there should be a causal connexion proceeding from motion to mind, shall we try to reverse the terms, and suppose a causal connexion proceeding from mind to motion? This is the oldest and still the most popular theory—the theory of spiritualism. And, no doubt, in one important respect it is less unphilosophical than the opposite theory of materialism. For spiritualism supposes the causation to proceed from that which is the source of our idea of causality—the mind: not from that into which this idea has been read—the brain. Therefore, if causation were to be accepted as a possibility either way, it would be less unreasonable to suppose mental changes the causes of material changes than vice versÂ; for we should then at least be starting from the basis of immediate knowledge, instead of from the reflection of that knowledge in what we call the external world. Seeing that the external world is known to us only as motion, it is logically impossible for the mind to infer its own causation from the external world; for this would be to infer that it is an effect of motion, which would be the same as saying that it is an effect of its own knowledge; and this would be absurd. But, on the other hand, it is not thus logically impossible for the mind to infer that it may be the cause of some of its own knowledge, or, in other words, that it may have in some measure the power of producing what it knows as motion. And when the mind does infer this, no logic on earth is able to touch the inference; the position of pure idealism is beyond the reach of argument. Nevertheless, it is opposed to the whole momentum of science. For if mind is supposed, on no matter how small a scale, to be a cause of motion, the fundamental axiom of science is impugned. This fundamental axiom is that energy can neither be created nor destroyed—that just as motion can produce nothing but motion, so, conversely, motion can be produced by nothing but motion. Regarded, therefore, from the stand-point of physical science, the theory of spiritualism is in precisely the same case as the theory of materialism: that is to say, if the supposed causation takes place, it can only be supposed to do so by way of miracle.

And this is a conclusion which the more clear-sighted of the idealists have expressly recognized. That subtle and most entertaining thinker, for example, the late Professor Green of Oxford, has said that the self-conscious volition of man 'does not consist in a series of natural events, ... is not natural in the ordinary sense of that term; not natural at any rate in any sense in which naturalness would imply its determination by antecedent events, or by conditions of which it is not itself the source.'

Thus the theory of spiritualism, although not directly refutable by any process of logic, is certainly enfeebled by its collision with the instincts of physical science. In necessarily holding the facts of consciousness and volition super-natural, extra-natural, or non-natural, the theory is opposed to the principle of continuity.

Spiritualism being thus unsatisfactory, and materialism impossible, is there yet any third hypothesis in which we may hope to find intellectual rest? In my opinion there is. If we unite in a higher synthesis the elements both of spiritualism and of materialism, we obtain a product which satisfies every fact of feeling on the one hand, and of observation on the other. The manner in which this synthesis may be effected is perfectly simple. We have only to suppose that the antithesis between mind and motion—subject and object—is itself phenomenal or apparent: not absolute or real. We have only to suppose that the seeming duality is relative to our modes of apprehension; and, therefore, that any change taking place in the mind, and any corresponding change taking place in the brain, are really not two changes, but one change. When a violin is played upon we hear a musical sound, and at the same time we see a vibration of the strings. Relatively to our consciousness, therefore, we have here two sets of changes, which appear to be very different in kind; yet we know that in an absolute sense they are one and the same: we know that the diversity in consciousness is created only by the difference in our modes of perceiving the same event—whether we see or whether we hear the vibration of the strings. Similarly, we may suppose that a vibration of nerve-strings and a process of thought are really one and the same event, which is dual or diverse only in relation to our modes of perceiving it.

The great advantage of this theory is that it supposes only one stream of causation, in which both mind and motion are simultaneously concerned. The theory, therefore, escapes all the difficulties and contradictions with which both spiritualism and materialism are beset. Thus, motion is supposed to be producing nothing but motion; mind-changes nothing but mind-changes: both producing both simultaneously, neither could be what it is without the other, because without the other neither could be the cause which in fact it is. Impossible, therefore, is the supposition of the materialist that consciousness is adventitious, or that in the absence of mind changes of brain could be what they are; for it belongs to the very causation of these changes that they should have a mental side. The use of mind to animals is thus rendered apparent; for intelligent volition is thus shown to be a true cause of adjustive movement, in that the cerebration which it involves could not otherwise be possible: the causation would not otherwise be complete.

A simple illustration may serve at once to render this doctrine more easily intelligible, and to show that, if accepted, the doctrine, as it appears to me, terminates the otherwise interminable controversy on the freedom of the will.

In an Edison lamp the light which is emitted from the burner may be said indifferently to be caused by the number of vibrations per second going on in the carbon, or by the temperature of the carbon; for this rate of vibration could not take place in the carbon without constituting that degree of temperature which affects our eyes as luminous. Similarly, a train of thought may be said indifferently to be caused by brain-action or by mind-action; for, ex hypothesi, the one could not take place without the other. Now, when we contemplate the phenomena of volition by themselves, it is as though we were contemplating the phenomena of light by themselves: volition is produced by mind in brain, just as light is produced by temperature in carbon. And just as we may correctly speak of light as the cause, say, of a photograph, so we may correctly speak of volition as the cause of bodily movement. That particular kind of physical activity which takes place in the carbon could not take place without the light which causes a photograph; and, similarly, that particular kind of physical activity which takes place in the brain could not take place without the volition which causes a bodily movement. So that volition is as truly a cause of bodily movement as is the physical activity of the brain; seeing that, in an absolute sense, the cause is one and the same. But if we once clearly perceive that what in a relative sense we know as volition is, in a similar sense, the cause of bodily movement, we terminate the question touching the freedom of the will. For this question in its last resort—and apart from the ambiguity which has been thrown around it by some of our metaphysicians—is merely the question whether the will is to be regarded as a cause of Nature. And the theory which we have now before us sanctions the doctrine that it may be so regarded, if only we remember that its causal activity depends upon its identity with the obverse aspect known as cerebration, without which identity in apparent duality neither volition nor cerebration could be the cause which in fact they are. It thus becomes a mere matter of phraseology whether we speak of the will determining, or being determined by, changes going on in the external world; just as it is but a matter of phraseology whether we speak of temperature determining, or being determined by, molecular vibration. All the requirements alike of the free-will and of the bond-will hypotheses are thus satisfied by a synthesis which comprises them both. On the one hand, it would be as impossible for an unconscious automaton to do the work or to perform the adjustments of a conscious agent, as it would be for an Edison lamp to give out light and cause a photograph when not heated by an electric current. On the other hand, it would be as impossible for the will to originate bodily movement without the occurrence of a strictly physical process of cerebration, as it would be for light to shine in an Edison lamp which had been deprived of its carbon-burner.

It may be said of this theory that it is highly speculative, not verifiable by any possible experiment, and therefore at best is but a mere guess. All which is, no doubt, perfectly true; but, on the other hand, we must remember that this theory comes to us as the only one which is logically possible, and at the same time competent to satisfy the facts alike of the outer and of the inner world. It is a speculation in the sense of not being verifiable by experiment; but it has much more value than ordinarily attaches to an unverifiable speculation, in that there is really no alternative hypothesis to be considered: if we choose to call it a guess, we must at the same time remember it is a guess where it does not appear that any other is open. Once more to quote Hobbes, who, as we have seen, was himself a remarkable instance of what he here says: 'The best prophet naturally is the best guesser; and the best guesser, he that is most versed and studied in the matters he guesses at.' In this case, therefore, the best prophet is not the physiologist, whose guess ends in materialism; nor the purely mental philosopher, whose guess ends in spiritualism; but rather the man who, being 'versed and studied' in all the facts appertaining to both sides of the matter, ends in the only alternative guess which remains open. And if that most troublesome individual, the 'plain man' of Locke, should say it seems at least opposed to common sense to suppose that there is anything in a burning candle or a rolling billiard-ball substantially the same as mind, the answer is that if he could look into my brain at this moment he would see nothing there but motion of molecules, or motion of masses; and apart from the accident of my being able to tell him so, his 'common sense' could never have divined that these motions in my brain are concerned in the genesis of my spoken thoughts.

It is obvious that from this hypothesis as to the substantial identity of mind and motion, two important questions arise; and I feel that some reference to these questions is in present circumstances forced upon me, because they have both been considered in precisely the same connexion by one of the most powerful intellects that was ever sent out into the world by this University. I mean the late Professor Clifford. As my intimate and valued friend, I desire to mention his name in this place with all the affection, as well as with all the admiration, to which I well know it is so fully entitled; and if I appear to mention him only in order to disagree with him, this is only because I know equally well that in his large and magnanimous thought differences of philosophical opinion were never felt to weaken the bonds of friendship.

In his well-known lecture on Body and Mind, Professor Clifford adopted the hypothesis of identity which we are now considering, and from it was led to the conclusion that if in the case of cerebral processes motion is one with mind, the same must be true of motion wherever it occurs; or, as he expressed it subsequently, the whole universe must be made of mind-stuff. But in his view, although matter in motion presents what may be termed the raw material of mind, it is only in the highly elaborated constitution of the human brain that this raw material is sufficiently wrought up to yield a self-conscious personality. Hence the dissolution of a human brain implies the dissolution of a human mind; and hence also the universe, although entirely composed of mind-stuff, is itself mindless. Now, all I have to say about these two deductions is this—they do not necessarily follow from the theory which is before us. In holding that the mind of man perishes with his body, and that above the mind of man there is no other, Clifford may have been right, or may have been wrong. I am not here to discuss at length any questions of such supreme importance. But I feel that I am here to insist upon the one point which is immediately connected with my subject; and this is, that whether or not Clifford was right in his conclusions, these conclusions certainly did not follow by way of any logical sequence from his premises. Because within the limits of human experience mind is only known as associated with brain, it clearly does not follow that mind cannot exist in any other mode. It does not even follow that any probability upon this matter can be thus established. The basis of analogy on which Clifford sought to rear an inference of cosmical extent, was restricted to the one instance of mind as known upon one planet; and, therefore, it is hard to imagine a more precarious use of that precarious method which is called by logicians simple enumeration. Indeed, even for what it is worth, the inference may be pointed with quite as much effect in precisely the opposite direction. For we have seen how little it is that we understand of the one mode in which we certainly know that mind does exist; and if from this little we feel impelled to conclude that there is a mode of mind which is not restricted to brain, but co-extensive with motion, is consubstantial and co-eternal with all that was, and is, and is to come; have we not at least a suggestion, that high as the heavens are above the earth, so high above our thoughts may be the thoughts of such a mind as this? I offer no opinion upon the question whether the general order of Nature does not require some one explanatory cause; nor upon the question whether the mind of man itself does not point to something kindred in the self-existing origin of things. I am not concerned to argue any point upon which I feel that opinions may legitimately differ. I am only concerned to show that, in so far as any deductions can be drawn from the theory which is before us, they make at least as much against as in favour of the cosmical conclusions arrived at by Clifford.

On February 17, in the year 1600, when the streets of Rome were thronged with pilgrims from all the quarters of Christendom, while no less than fifty cardinals were congregated for the Jubilee; into the densely crowded Campo di Fiori a man was led to the stake, where, 'silent and self-sustained,' before the eyes of all nations, he perished in the flames. That death was the death of a martyr: it was met voluntarily in attestation of truth. But most noble of all the noble army to which he belonged, the name of that man is written large in history, as the name of one who had fortitude to die, not in the cause of religious belief, but in that of scientific conviction. For why did Bruno suffer? He suffered, as we all know, because he refused to recant his persuasion of the truth of the Copernican theory. Why, then, do I adduce the name of Bruno at the close of this lecture? I do so because, as far as I have been able to ascertain, he was the first clearly to enunciate the monistic theory of things to which the consideration of my subject has conducted us. This theory—or that as to the substantial identity of mind and motion—was afterwards espoused, in different guises, by sundry other writers; but to Bruno belongs the merit of its original publication, and it was partly for his adherence to this publication that he died. To this day Bruno is ordinarily termed a pantheist, and his theory, which in the light of much fuller knowledge I am advocating, Pantheism. I do not care to consider a difference of terms, where the only distinction resides in so unintelligible an idea as that of the creation of substance. It is more to the purpose to observe that in the mind of its first originator—and this a mind which was sufficiently clear in its thought to die for its perception of astronomical truth—the theory of Pantheism was but a sublime extension of the then contracted views of Theism. And I think that we of to-day, when we look to the teaching of this martyr of science, will find that in his theory alone do we meet with what I may term a philosophically adequate conception of Deity. If the advance of natural science is now steadily leading us to the conclusion that there is no motion without mind, must we not see how the independent conclusion of mental science is thus independently confirmed—the conclusion, I mean, that there is no being without knowing? To me, at least, it does appear that the time has come when we may begin, as it were in a dawning light, to see that the study of Nature and the study of Mind are meeting upon this greatest of possible truths. And if this is the case—if there is no motion without mind, no being without knowing—shall we infer, with Clifford, that universal being is mindless, or answer with a dogmatic negative that most stupendous of questions—Is there knowledge with the Most High? If there is no motion without mind, no being without knowing, may we not rather infer, with Bruno, that it is in the medium of mind, and in the medium of knowledge, we live, and move, and have our being?

This, I think, is the direction in which the inference points, if we are careful to set the logical conditions with complete impartiality. But the ulterior question remains, whether, so far as science is concerned, it is here possible to point any inference at all: the whole orbit of human knowledge may be too narrow to afford a parallax for measurements so vast. Yet even here, if it be true that the voice of science must thus of necessity speak the language of agnosticism, at least let us see to it that the language is pure; let us not tolerate any barbarisms introduced from the side of aggressive dogma. So shall we find that this new grammar of thought does not admit of any constructions radically opposed to more venerable ways of thinking; even if we do not find that the often-quoted words of its earliest formulator apply with special force to its latest dialects—that if a little knowledge of physiology and a little knowledge of psychology dispose men to atheism, a deeper knowledge of both, and, still more, a deeper thought upon their relations to one another, will lead men back to some form of religion, which, if it be more vague, may also be more worthy than that of earlier days.