From time immemorial it has been quite evident that the behaviour of conscious persons is very different from that of things which are unconscious, or of the dead bodies of persons; and from time immemorial the belief has existed that this difference is due to the presence in the body during life of something different from the body, called by the Greeks Ψυχ, or in Latin anima, and in English soul. Hence the origin of the modern words “psychology” and “animism,” while the modern word “physiology” carries us back to the Hippocratic belief that it is of the nature (ΦΣι) of life to appear to us as it does. In ancient Egyptian pictorial records the soul is represented as possessing wings, so that it can fly away from or return to the body.

The distinguishing characters of conscious behaviour are thus represented as due to the presence within the body of a soul; and I must now endeavour to follow the development of this interpretation, and to consider the difficulties to which it has led. This development has proceeded side by side with, and is evidently complementary to, the development of what may be called the Newtonian conception of the physical world. We can see the general outlines of that conception in the philosophy of Democritus and other ancient literature; but not till modern times did the Newtonian conception become quite clearly defined. Not till modern times, also, did the difficulties inherent in a conception of the relation between a soul and a body become clearly defined.

From the Newtonian standpoint the reality of a physical universe corresponding to the Newtonian conception seemed unquestionable. Yet conscious behaviour, which clearly affects and is affected by this physical reality, does not seem to correspond with that of physical phenomena, so that some influence of a supernatural or “metaphysical” character must, apparently, be interfering; and to the source of this influence the name of the soul is given. The influence seems to be definitely localized in space and time within the bodies of men and animals; and the localization has seemed to be narrowed down, with the advance of physiology, to the brain. Descartes, as we have seen, thought that the so-called pineal gland, in the centre of the brain, is the seat of the soul; but the grey matter of the cerebral hemispheres has appeared to later physiologists to be its seat, if indeed there be a soul.

What, in general terms, is the evidence for this localization? We can follow what seems to be physical influences, reinforced, it may be, on their paths, from the general surface of the body, or from the internal surface of sense-organs such as the eyes, ears, and nose, to the cerebral hemispheres along afferent nerve-fibres. An interruption at any point along these afferent nerve-fibres causes a corresponding interruption to the transmission of the influences in question, but no interference with either transmission along other nervous paths, or intelligent conscious apprehension of or response to these influences. On the other hand, damage to the cerebral hemispheres produces the most serious results as regards conscious apprehension and response—just as if the means by which the soul enters into communication with the body had been damaged. On the efferent side we can trace outwards from the cerebral hemispheres physical influences passing to muscles along definite nervous paths; and, just as in the case of the afferent influences, any particular efferent path can be interrupted at any point without interrupting conscious efferent responses of other kinds. Thus an arm or leg, or the muscles of the eye or face, may be paralyzed by interruption on an efferent path, without any paralysis of intelligent apprehension. The white matter of the brain consists of connecting nerve-fibres; only in the grey matter do we find the bodies of the nerve-cells from which the nerve-fibres emanate. Hence it is in the grey matter of the cerebral hemispheres that the seat of the soul would have to be localized, or, to use what at first sight might seem a more non-committal expression, that consciousness would have to be localized.

I should like to point out that if we accept the Newtonian conception of the physical universe there is absolutely no escape from either the assumption of a definite meeting-place and interaction between body and an invisible and impalpable soul, or a denial that there is any such thing as a soul. In the latter case, however, we must assume that a mysterious thing called consciousness is somehow produced in the cerebral hemispheres. To the majority of those who have accepted the Newtonian philosophy it has seemed that since intelligent response, and not mere sensation, is concerned, and since intelligent response seems to be something different from any physical phenomenon, there must be definite interaction between body and soul in the brain. Something, that is to say, which is not physical or chemical in its nature, and which has the character of intelligence, intervenes in the physical and chemical processes occurring in the grey matter of the brain.

Let us now try to follow out the consequences of this conclusion. One thing that clearly follows is that our perceptions of physical things are not mere patterns of the things, but are modified by our sense-organs and nervous system, so that they appear to us distinctly different from the physical objects from which they emanate. Thus things appear to us as coloured, or possessed of odour, or as either hot or cold, dark or bright. They also appear pleasant or unpleasant, beautiful or ugly. But these can only be our subjective impressions of them. We can alter the apparent colours of things by altering the apparent colours of surrounding objects, and similarly we can make things appear bright or dark, warm or cold, by contrast; we can also produce impressions of light by mere pressure on the retina or optic nerve; and in many other ways it is evident that our perceptions can only represent greatly modified pictures of physical reality. Locke gave form to this distinction when he distinguished between the primary and secondary qualities of things, the primary being in physical objects themselves, and the secondary being qualities added in the process of perception and not inherent in the actual objects perceived.

This seems easily enough intelligible on the Newtonian conception of physical reality, and not to involve any serious modification of that conception. But, as Bishop Berkeley in particular pointed out, we cannot stop at the distinction between the primary and secondary qualities of things. All that we can directly perceive consists of the impressions produced on our souls. It can only be by interpreting these impressions that we reach the conception of a physical world around us. In an impression itself there can be nothing that tells us anything about its source in a physical world. All that we can learn from impressions is how they are grouped together; and this we can only learn from actual experience. From that experience we learn that when we experience one impression we may expect to experience also certain others; but we are not justified in inferring the existence of things outside us to correspond with the groups.

Hume carried this reasoning still further, pointing out that, just as we cannot justifiably infer the existence of a physical world, so we cannot justifiably infer the existence of a perceiving soul. All that we can really know about are the impressions or sensations, and the manner in which they are usually associated with one another and thus come to suggest one another as they actually do in the process of perception.

When we consider the reasoning of Berkeley and Hume we can easily see that it is the logical development of the Newtonian conception, when the attempt is made to apply this conception in the interpretation of the phenomena of perception. The Newtonian conception was far too valuable as a practical working hypothesis for anyone with a grain of common sense not to go on using it in practice; and as an actual fact the world so continued to use it, regardless of the sceptical conclusions of Berkeley and Hume, which had not the slightest effect on the development of physical science on Newtonian lines during the succeeding century and a half. The fact remained, however, that it could not be extended to the phenomena of perception and conscious behaviour on the ordinary assumption that it is a soul which, by perceiving the physical world around it, guides conscious behaviour. A new conception was therefore needed of the relation between perception and the physical world. This new conception was furnished by Immanuel Kant near the end of the eighteenth century. The more I have considered this new conception, the more fundamental in importance does the step which Kant took appear to me to have been.

What he saw was that mere unconnected sensations are purely imaginary entities which do not exist. In other words, each distinguishable element in perception has what we call meaning attached to it. Every element in conscious experience contains within itself the reference to other elements, and however much we may try to simplify any element of conscious experience, we find this reference still present, in however imperfectly defined a form. Thus each element refers itself to other elements as qualitatively different from them, and as arranged both after some of them in order of time, and simultaneously with others in order of space. It also refers to others as possessing qualitative identity with itself, but quantitative difference; also refers to them as being casually connected, this connexion implying substances which are acting or being acted on. Kant, in fact, saw that the Newtonian conception of the physical world, including time and space, is in reality implied in the simplest conscious perception, and so cannot be derived from simple sensations, as Berkeley and Hume had supposed. In other words, the association of sensations with one another in the form of a physical ordered experience is part of their very being.

He drew the conclusion that our minds are such that they arrange what would otherwise be formless data into the forms of what is practically the Newtonian interpretation of experience, thus giving these data definite articulation and physical order. These forms are the order of space and time, and the other general conceptions or “categories” which we make use of in interpreting the physical world. The Newtonian world is thus not a world of self-existent things, or self-existent space and time, but a subjectively interpreted world which we project outwards in the process of perception, at the same time giving it the form of the Newtonian world. Thus for Kant time, space, and the matter and energy of the physical world are not real in themselves, but only projections outwards by our minds. The same is, of course, also true of our own bodies; and the perceived world seems to be the same to all men, or to have objective existence, because the forms are the same for all men, giving the perceived world its logical structure.

Behind this world of perception there was, for Kant, a real world of things in themselves. Apart from this assumption, his philosophy would seem to imply that we made the world of perception by the mere act of perceiving it. This is certainly not consistent with our experience. We cannot predict or deduce in any way the details of the physical universe. All we might be able to say of it would be that its appearance as perceived will be consistent with the Newtonian conception. On Kant's interpretation of perception it must be.

Kant's immediate philosophical successors, and particularly Hegel, who was the greatest of them, pointed out that the “things-in-themselves” of Kant's conception are nothing but the ghosts of the Newtonian physical reality. We have no evidence of their existence as “things-in-themselves,” any more than of the Newtonian things-in-themselves. Hegel also pointed out that the Kantian general conceptions or categories are incomplete, since we perceive not merely physically interpreted existence, but also existence, such as that of life or of conscious activity, in which the whole is essentially present in each of the parts and actions, so that mere physical interpretation of this existence is impossible. For Hegel the world of Nature seemed to present an embodiment of different categories, from the most empty, abstract, and unarticulated, like mere existence, to the fullest and most articulated, like human behaviour as manifested in history and social life. He also insisted that these categories are essentially related to, or implied in, one another. To the question, however, why the embodiments of these categories should be, as it were, scattered abroad throughout our experience, there is no really satisfactory answer in the Hegelian philosophy; and I do not think that Kant himself would ever have felt satisfied with this philosophy.

To the Hegelian philosophy as a whole I shall return in the second course of the lectures; but meanwhile we must follow out the development of conceptions as to a relation between soul and body. For Kant the experimental investigation of any such relation was impossible, since all that we can actually perceive is necessarily interpreted mechanically as part of a Newtonian world, while the soul belongs to a real world of things-in-themselves, which real world cannot be perceived. Conscious behaviour, as well as organic behaviour, is thus necessarily interpreted by us mechanically, in accordance with Newtonian principles. So far, therefore, as scientific investigation is concerned, the Kantian philosophy is as mechanistic as that of Democritus, and much more mechanistic than that of Newton, since Newton never questioned the intervention of supernatural influences in the physical world, or the possibility of actually observing the results of this intervention, as for instance in voluntary human behaviour, or in the occurrence of miracles. Kant's philosophy is also inconsistent with the scientific attitude of Hippocrates, a development of which is the attitude which I have taken up towards biology in the first five lectures. The Kantian attitude towards the scientific interpretation of all that we can perceive of conscious behaviour seems, therefore, to be the mechanistic attitude; and as such it will be discussed in the next lecture, though Kant had re-established the mechanistic conception in an entirely renovated form.

On the other hand, the attitude of Hegel and Kant's other immediate successors was essentially animistic and vitalistic. I think, however, that it may be truly said that their philosophy had, outside Germany, very little influence in shaping ideas as to either the relation of soul to body or the relation of the “vital principle” to the bodies of animals and plants. In Germany itself the influence of what was called the “Philosophy of Nature” was only a passing one. This philosophy seemed to add nothing substantial to ideas already current, and led to various very empty theories; but at least it helped to support or re-establish animism and vitalism, which the Kantian philosophy seemed to be inconsistent with. Whether the world as perceived is an ideal construction or not, and even if it is the case that in the perception of conscious behaviour there is a different ideal construction from that in the perception of physical and merely organic phenomena, the question as to the relation between psychical and physical phenomena still remained.

With the advance of physiological and medical knowledge it seemed to become more and more evident that all the phenomena of conscious behaviour are dependent on the influence of what was admitted to be, or was interpreted as, the physical environment or mere physiological conditions in the conscious organism. In the early part of last century this was not by any means as evident as it has now become. One discovery after another has revealed what are naturally interpreted as abnormal physical or physiological conditions on which both mental and moral failure depend; and with this revelation has come the corresponding revelation that normal conscious behaviour depends, down to the minutest detail, on what are interpreted as the physical and physiological conditions of hereditary transmission. These are hard facts of observation, and cannot be ignored now, though at a time when few of them were known their full significance was not realized, and it still seemed possible to adhere to the conception of a physical body, with an essentially independent soul to guide it. In the next lecture I shall discuss these facts more fully.

The supposed independent soul has turned out to be something which is dependent in every respect on the supposed physical body and environment. We cannot possibly separate their influences. If we start with the provisional assumption that there is a physical or biological living body, with an independent soul to guide it, the facts lead us inevitably to a correction of this assumption. Body and soul cannot be separated from one another as independent entities. The conception of soul must either include within itself the body and the physical environment, or the conception of body and physical environment must include within itself the phenomena provisionally attributed to interference by the soul.

It is to the former solution that the reasoning sketched out in the previous lecture points: but we must explore the latter solution, which will be done in the next lecture. Meanwhile I wish to leave no doubt or ambiguity about the conclusion reached in this lecture. The conclusion is that we can no longer uphold the animistic conception of a physical body guided—in other words, interfered with—by an independently existing soul. Such a conception would amount to a denial of the full validity of the law of conservation of energy; but this in itself seems to me a trifling matter, since a physical “law” is only a formula in which we summarize our observations. If the observations pointed to an exception to the law, we should just have to accept it as an exception. The objections to the animistic conception go far deeper than our natural reluctance to admit any exception to a well-established law. These objections depend on the fact that the observed phenomena are not consistent with the conception of a soul independent of bodily existence. I am perfectly at one with the most thorough-going materialists on this point, though not at all at one with them as to further points.

It seems to me that animism is a counterpart to Newtonian realism and the vaguer representations of it in the days before Galileo and Newton. Conscious behaviour is apparently so different in kind from the behaviour of physical bodies that it seemed necessary to postulate the existence in the body of a soul which interferes with physical processes. Newton was far too good a physicist not to see the need of this counterpart to his physical philosophy; and the common sense of the majority of mankind has been with him and other physicists in this respect. It is only on a wide philosophical view of our experience that it turns out that the Newtonian conception, whether in its original form or in the renovated form given to it by Kant, is inconsistent with the conception of a soul which can be regarded as independent of the body. If we accept the Newtonian conception as part of a philosophical conception of our universe, the observed facts are bound to lead us on to the conclusion that all psychical phenomena are mechanically determined, so that there is no such thing as an independent soul. And if it is nothing but common sense to accept as philosophical the Newtonian conception, then after we have examined the facts revealed by physiological and medical investigation it seems to be nothing but common sense to accept what is commonly called materialism, and to reject entirely the hypothesis of an independent soul.

This is why so many earnestly minded persons have become, and are becoming, materialists. No amount of mere personal authority, whether theological, ecclesiastical, scientific, or State authority, will in the end avail against this trend. It is a trend among persons who are honestly and sincerely striving after the truth, and to whom that striving is part of their religion—part, as I shall endeavour to point out later, of the most powerful influence which determines human behaviour. It may be that, on the whole, materialism makes for personal unhappiness and social disorder; but this also, if it be a fact, will not avail. The only thing that could avail is the result of a perfectly free and open discussion of all the evidence bearing on the subject.

In the present lecture I have discussed the animistic conception of conscious behaviour, and tried to point out its essential defects. The next lecture will include a discussion of the mechanistic, or what is usually called the materialistic conception. Meanwhile, however, I may perhaps again point out that animism—the conception of a soul essentially independent of the body—is only a counterpart to the Newtonian conception of what is visible and tangible. It was not to animism, but to a complete denial of the possibility of interpreting by Newtonian conceptions the visible and tangible phenomena of conscious behaviour, including the whole of our experience in perception, that the discussion in the last lecture pointed.