Ivan Pavlov was born in a small village in central Russia. His family hoped that he would become a priest, and he went to a theological seminary. After reading Charles Darwin, he found that he cared more for scientific pursuits and left the seminary for the University of St. Petersburg. There he studied chemistry and physiology, and he received his doctorate in 1879. He continued his studies and began doing his own research in topics that interested him most: digestion and blood circulation. His work became well known, and he was appointed professor of physiology at the Imperial Medical Academy.The work that made Pavlov a household name in psychology actually began as a study in digestion. He was looking at the digestive process in dogs, especially the interaction between salivation and the action of the stomach. He realized they were closely linked by reflexes in the autonomic nervous system. Without salivation, the stomach didn't get the message to start digesting. Pavlov wanted to see if external stimuli could affect this process, so he rang a metronome at the same time he gave the experimental dogs food. After a while, the dogs -- which before only salivated when they saw and ate their food -- would begin to salivate when the metronome sounded, even if no food were present. In 1903 Pavlov published his results calling this a "conditioned reflex," different from an innate reflex, such as yanking a hand back from a flame, in that it had to be learned. Pavlov called this learning process (in which the dog's nervous system comes to associate the sound of the metronome with the food, for example) "conditioning." He also found that the conditioned reflex will be repressed if the stimulus proves "wrong" too often. If the metronome sounds repeatedly and no food appears, eventually the dog stops salivating at the sound.

One cannot but be struck by a comparison of the following facts. First, the cerebral hemispheres, the higher part of the central nervous system, is a rather impressive organ. In structure it is exceedingly complex, comprising millions and millions (in man - even billions) of cells, i. e., centres or foci of nervous activity. These cells vary in size, shape and arrangement and are connected with each other by countless branches. Such structural complexity naturally suggests a very high degree of functional complexity. Consequently, it would seem that a boundless field of investigation is offered here for the physiologist. Secondly, take the dog, man's companion and friend since prehistoric times, in its various roles as hunter, sentinel, etc. We know that this complex behaviour of the dog, its higher nervous activity (since no one will dispute that this is higher nervous activity), is chiefly associated with the cerebral hemispheres. If we remove the cerebral hemispheres in the dog (Goltz and others), it becomes incapable of performing not only the roles mentioned above, but even of looking after itself. It becomes profoundly disabled and will die unless well cared for. This implies that both in respect of structure and function, the cerebral hemispheres perform considerable physiological work.

Let us turn now to man. His entire higher nervous activity is also dependent on the normal structure and functioning of the cerebral hemispheres. The moment the complex structure of his hemispheres is damaged or disturbed in one way or another, he also becomes an invalid; he can no longer freely associate with his fellows as an equal and must be isolated.

In amazing contrast to this boundless activity of the cerebral hemispheres is the scant content of the present-day physiology of these hemispheres. Up to 1870 there was no physiology of the cerebral hemispheres at all; they seemed inaccessible to the physiologist. It was in that year that Fritsch and Hitzig first successfully applied the ordinary physiological methods of stimulation and destruction to their study. Stimulation of certain parts of the cerebral cortex regularly evoked contractions in definite groups of the skeletal muscles (the cortical motor region). Extirpation of these parts led to certain disturbances in the normal activity of the corresponding groups of muscles.

Shortly afterwards H. Munk, Ferrier and others demonstrated that other regions of the cortex, seemingly not susceptible to artificial stimulation, are also functionally differentiated. Removal of these parts leads to defects in the activity of certain receptor organs - the eye, the ear and the skin.

Many researchers have been thoroughly investigating these phenomena. More precision and more details have been obtained, especially as regards the motor region, and this knowledge has even found practical application in medicine; however, investigation as vet has not gone far beyond the initial point. The essential fact is that the entire higher and complex behaviour of the animal, which is dependent on the cerebral hemispheres, as shown by the previously mentioned experiment by Goltz with the extirpation of the hemispheres in a dog, has hardly been touched upon in these investigations and is not included even in the programme of current physiological research, what do the facts relating to the cerebral hemispheres, which are now at the disposal of the physiologist, explain with regard to the behaviour of the higher animals? Is there a general scheme of the higher nervous activity? What kind of general rules govern this activity? The contemporary physiologist finds himself truly empty-handed when he has to answer these lawful questions. While the object of investigation is highly complex in relation to structure, and extremely rich in function, research in this sphere remains, as it were, in a blind alley, unable to open up before the physiologist the boundless vistas which might have been expected.

Why is this so? The reason is clear, the work of the cerebral hemispheres has never been regarded from the same point of view as that of other organs of the body, or even other parts of the central nervous system. It has been described as special psychical activity - which we feel and apprehend in ourselves and which we suppose exists in animals by analogy with human beings. Hence the highly peculiar and difficult position of the physiologist. On the one hand, the study of the cerebral hemispheres, as of all other parts of the organism, seems to come within the scope of physiology, but on the other hand, it is an object of study by a special branch of science - psychology. What, then, should be the attitude of the physiologist? Should he first acquire psychological methods and knowledge and only then begin to study the activity of the cerebral hemispheres? But there is a real complication here. It is quite natural that physiology, in analysing living matter, should always base itself on the more exact and advanced sciences - mechanics, physics and chemistry. But here we are dealing with an altogether different matter, since in this particular case we should have to rely on a science which has no claim to exactness as compared with physiology. Until recently discussion revolved even around the question whether psychology should be considered a natural science or a science at all. Without going deeply into this question, I should like to cite some facts which, although crude and superficial, seem to me very convincing. Even the psychologists themselves do not regard their science as being exact. Not so long ago James, an outstanding American psychologist, called psychology not a science, but a "hope for science." Another striking illustration has been provided by Wundt, formerly a physiologist, who became a celebrated psychologist and philosopher and even the founder of the so-called experimental psychology. Prior to the war, in 1913, a discussion took place in Germany as to the advisability of separating the psychological branch of science from the philosophical in the universities, i. e., of having two separate chairs instead of one. Wundt opposed separation, one of his arguments being the impossibility of establishing a common and obligatory examination programme' in psychology, since each professor had his own ideas of the essence of psychology. Is it not clear, then, that psychology has not yet reached the stage of an exact science?

This being the case, there is no need for the physiologist to have recourse to psychology. In view of the steadily developing natural science it would be more logical to expect that not psychology should render assistance to the physiology of the cerebral hemispheres, but, on the contrary, physiological investigation of the activity of this organ in animals should lay the foundation for the exact scientific analysis of the human subjective world. Consequently, physiology must follow its own path - the path blazed for it long ago. Taking as his starting-point the assumption that the functioning of the animal's organism, unlike that of the human being, is similar to the work of a machine, Descartes' three hundred years ago evolved the idea of the reflex as the basic activity of the nervous system. Descartes regarded every activity of the organism as a natural response to certain external agents and believed that the connection between the active organ and the given agent, that is, between cause and effect, is achieved through a definite nervous path. In this way the study of the activity of the animal nervous system was placed on the firm basis of natural science. In the eighteenth, nineteenth and twentieth centuries the idea of the reflex had been extensively used by physiologists, but only in their work on the lower parts of the central nervous system; gradually, however, they began to study its higher parts, until finally, after Sherrington's classical works on spinal reflexes, Magnus, his successor, established the reflex nature of all the basic locomotor activities of the organism. And so experiment fully justified the idea of the reflex which , thereafter, was used in the study of the central nervous system almost up to the cerebral hemispheres. It is to be hoped that the more complex activities of the organism, including the basic locomotor reflexes - states so far referred to in psychology as anger, fear, playfulness, etc. - will soon be related to the simple reflex activity of the subcortical parts of the brain.

A bold attempt to apply the idea of the reflex to the cerebral hemispheres not only of animals but also of man, was made by I. M. Sechenov, the Russian physiologist, on the basis of the contemporary physiology of the nervous system. In a paper published in Russian in 1863 and entitled Reflexes of the Brain Sechenov characterised the activity of the cerebral hemispheres as reflex, i. e., determined activity. He regarded thoughts as reflexes in which the effector end is inhibited, and affects as exaggerated reflexes with a wide irradiation of excitation. A like attempt has been made in our time by Ch. Richet who introduced the concept of the psychical reflex in which the reaction to a given stimulus is determined by its union with the traces left in the cerebral hemispheres by previous stimuli. Generally, the recent physiology of the higher nervous activity related to the cerebral hemispheres tends to associate acting stimulation with traces left by previous ones (associative memory - according to J. Loeb; training, education by experience - according to other physiologists). But this was mere theorising. The time had come for a transition to the experimental analysis of the subject, and from the objective external aspect, as is the case with any other branch of natural science. This transition was determined by comparative physiology which had just made its appearance as a result of the influence of the theory of evolution. Now that it had turned its attention to the entire animal kingdom, physiology, in dealing with its lower representatives, was forced, of necessity, to abandon the anthropomorphic concept and concentrate on the scientific elucidation of the relations between the external agents influencing the animal and the responsive external activity, the locomotor reaction of the latter. This gave birth to J. Loeb's doctrine of animal tropisms; to the suggestion by Beer, Bethe and Uexküll of an objective terminology for designating the animal reactions; and finally, to the investigation by zoologists of the behaviour of the lower representatives of the animal world, by means of purely objective methods, by comparing the effect of external influences on the animal with its responsive external activity - as for example in the classical work of Jennings, etc.

Influenced by this new tendency in biology and having a practical cast of mind, American psychologists who also became interested in comparative psychology displayed a tendency to subject the external activity of animals to experimental analysis under deliberately induced conditions. Thorndike's Animal Intelligence (1898) must be regarded as the starting-point for investigations of this kind. In these investigations the animal was kept in a box and food placed outside, within sight. The animal, naturally, tried to reach the food, but to do so it had to open the door which in the different experiments was fastened in a different way. Tables and charts registered the speed and the manner in which the animal solved this problem. The entire process was interpreted as the formation of an association, connection between the visual and the tactile stimulation and the locomotor activity. Afterwards by means of this method, and by modifications of it, researchers studied numerous questions relating to the associative ability of various animals. Almost simultaneously with the above-mentioned work by Thorndike, of which I was not then aware, I too had arrived at the idea of the need for a similar attitude to the subject. The following episode, which occurred in my laboratory, gave birth to the idea.

While making a detailed investigation of the digestive glands I had to busy myself also with the so-called psychical stimulation of the glands. When, together with one of my collaborators, I attempted a deeper analysis of this fact, at first in the generally accepted way, i. e., psychologically, visualising the probable thoughts and feelings of the animal, I stumbled on a fact unusual in laboratory practice. I found myself unable to agree with my colleague; each of us stuck to his point of view, and we were unable to convince each other by certain experiments. This made me definitely reject any further psychological discussion of the subject, and I decided to investigate it in a purely objective way, externally, i. e., strictly recording all stimuli reaching the animal at the given moment and observing its corresponding responses either in the form of movements or in the form of salivation (as occurred in this particular case).

This was the beginning of the investigations that I have carried on now for the past twenty-five years with the participation of numerous colleagues who joined hand and brain with me in this work and to whom I am deeply grateful. We have, of course, passed through different stages, and the subject has been advanced only gradually. At first we had but a few separate facts at our disposal, but today so much material has been accumulated by us that we can make an attempt to present it in a more or less systematised form. I am now in a position to place before you a physiological theory of the activity of the cerebral hemispheres which at any rate conforms much more to the structural and functional complexity of this organ than the theory which until now has been based on a few fragmentary, though very important, facts of modern physiology.

Thus, research along these new lines of strictly objective investigation of the higher nervous activity has been carried out mainly in my laboratories (with the participation of a hundred colleagues); work along the same lines has been carried out also by American psychologists. As for other physiological laboratories, so far only a few have begun, starting somewhat later, to investigate this subject, but in most cases their work is still in the initial stage. So far there has been one essential point of difference in the research of the Americans and in ours. Since in the case of the Americans the objective investigation is being conducted by psychologists, this means that, although psychologists study the facts from the purely external - aspect, nevertheless, in posing the problems, in analysing and formulating the results, they tend to think more in terms of psychology. The result is that with the exception of the group of "behaviourists" their work does not bear a purely physiological character. Whereas, we, having started from physiology, invariably and strictly adhere to the physiological point of view, and we are investigating and systematising the whole subject solely in a physiological way.

I shall now pass to an exposition of our material, but before doing so I should like to touch on the concept of the reflex in general, on reflexes in physiology and the so-called instincts.

In the main we base ourselves on Descartes' concept of the reflex. Of course, this is a genuinely scientific concept, since the phenomenon implied by it can be strictly determined. It means that a certain agent of the external world, or of the organism's internal medium produces a certain effect in one or other nervous receptor, which is transformed into a nervous process, into nervous excitation. The excitation is transmitted along certain nerve fibres, as if along an electric cable, to the central nervous system; thence, thanks to the established nervous connections, it passes along other nerve fibres to the working organ, where it in its turn is transformed into a special activity of the cells of this organ. Thus, the stimulating agent proves to be indispensably connected with the definite activity of the organism, as cause and effect.

It is quite obvious that the entire activity of the organism is governed by definite laws. If the animal were not (in the biological sense) strictly adapted to the surrounding world, it would, sooner or later, cease to exist. If instead of being attracted by food, the animal turned away from it, or instead of avoiding fire threw itself into it, and so on, it would perish. The animal must so react to the environment that all its responsive activity ensures its existence. The same is true if we think of life in terms of mechanics, physics and chemistry. Every material system can exist as an entity only so long as its internal forces of attraction, cohesion, etc., are equilibrated with the external forces influencing it. This applies in equal measure to such a simple object as a stone and to the most complex chemical substance, and it also holds good for the organism. As a definite material system complete in itself, the organism can exist only so long as it is in equilibrium with the environment; the moment this equilibrium is seriously disturbed, the organism ceases to exist as a particular system. Reflexes are the elements of this constant adaptation or equilibration. Physiologists have studied and are studying numerous reflexes, these indispensable, machine-like reactions of the organism, which at the same time are inborn, i. e., determined by the peculiar organisation of the given nervous system. Reflexes, like the belts of machines made by human hands, are of two kinds: the positive and the negative inhibitory, in other words, those which excite certain activities and those which inhibit them. Although investigation of these reflexes by physiologists has been under way for a long time, it is, of course, a long way from being finished. More and more new reflexes are being discovered; the properties of the receptor organs, on the surface on which it is walking. In what way does it differ, say, from inclining the head and closing the lids when something flashes near the eye? We should call the latter a defensive reflex, and the first an alimentary instinct, although in the case of the pecking, if it is caused by the sight of a stain, nothing but inclining the head and a movement of the beak occurs.

Further, it has been noted that instincts are more complex than reflexes. But there are exceedingly complex reflexes which no one designates as instincts. Take, for example, vomiting. This is a highly complex action and one that involves extraordinary co-ordination of a large number of muscles, both striated and smooth, usually employed in other functions of the organism and spread over a large area. It also involves the secretion of various glands which normally. participate in quite different activities of the organism.

The fact that instincts involve a long chain of successive actions, while reflexes are, so to speak, one-storeyed, has also been regarded as a point of distinction between them. By way of example let us take the building of a nest, or of animal dwellings in general. Here, of course, we have a long chain of actions: the animal must search for the material', bring it to the site and put it together and secure it. If we regard this as a reflex, we must assume that the ending of one reflex excites a new one, or, in other words, that these are chain-reflexes. But such chain activities are by no means peculiar to instincts alone. We are familiar with many reflexes which are also interlocked. Here is an instance. When we stimulate an afferent nerve, for example, the n. ischiadicus, there takes place a reflex rise of blood pressure. This is the first reflex. The high pressure in the left ventricle of the heart and in the first part of the aorta acts as a stimulus to another reflex: it stimulates the endings of the n. depressoris cordis which evokes a depressor reflex moderating the effect of the first reflex. Let us take the chain-reflex recently established by Magnus. A cat, even deprived of the cerebral hemispheres will in most cases fall on its feet when thrown from a height. How does this occur? The change in the spatial position of the otolithic organ of the ear causes a certain reflex contraction of the muscles in the neck, which restores the animal's head to a normal position in relation to the horizon. This is the first reflex. The end of this reflex - the contraction of the muscles in the neck and the righting of the head in general - stimulates a fresh reflex on certain muscles of the trunk and limbs which come into action and, in the end, restore the animal's proper standing posture.

Yet another difference between reflexes and instincts has been assumed, namely, that instincts often depend on the internal state or condition of the organism. For instance, a bird builds its nest only in the mating season. Or, to take a simpler example, when the animal is sated, it is no longer attracted by food and stops eating. The same applies to the sexual instinct, which is connected with the age of the organism, as well as with the state of the reproductive glands. In general the hormones, products of the glands of internal secretion, are of considerable importance in this respect. But this, too, is not a peculiar property of the instincts alone. The intensity of any reflex, as well as its presence or absence, directly depends on the state of excitability of the reflex centres which in turn always depends on the chemical and physical properties of the blood (automatic stimulation of the centres) and on the interaction of different reflexes.

Finally, importance is sometimes attached to the fact that reflexes are related to the activity of separate organs, whereas instincts involve the activity of the organism as a whole, i. e., actually the whole skeleto-muscular system. However, we know from the works of Magnus and de Kleyn that standing, walking, and bodily balance in general, are reflexes.

Thus, reflexes and instincts alike are natural reactions of the organism to certain stimulating agents, and consequently there is no need to designate them by different terms. The term "reflex" is preferable, since a strictly scientific sense has been imparted to it from the very outset.

The aggregate of these reflexes constitutes the foundation of the nervous activity both in men and animals. Consequently, thorough study of all these fundamental nervous reactions of the organism is, of course, a matter of great importance. Unfortunately, as already mentioned, this is a long way from having been accomplished, especially in the case of those reflexes which are called instincts. Our knowledge of these instincts is very limited and fragmentary. We have but a rough classification of them - alimentary, self-defensive, sexual, parental and social. But almost each of these groups often includes numerous separate reflexes, some of which have not been even identified by us, while some are confused with others or, at least, they are not fully appreciated by us as to their vital importance. To what extent this subject remains unelucidated and how full it is still of gaps can be demonstrated by this example from my own experience.

Once, in the course of our experimental work which I shall describe presently, we were puzzled by the peculiar behaviour of our animal. This was a tractable dog with which we were on very friendly terms. The dog was given a rather easy assignment. It was placed in the stand and had its movements restricted only by soft loops fastened round its leys (to which at first it did not react at all). Nothing else was done except to feed it repeatedly at intervals of several minutes. At first the dog was quiet and ate willingly, but as time went on it became more and more excited: it began to struggle against the surrounding objects, tried to break loose, pawing at the floor, gnawing the supports of the stand, etc. This ceaseless muscular exertion brought on dyspnoea and a continuous secretion of saliva; this persisted for weeks, becoming worse and worse, with the result that the dog was no longer fit for our experimental work. This phenomenon puzzled us for a long time. We advanced many hypotheses as to the possible reason for this unusual behaviour, and although we had by then acquired sufficient knowledge of the behaviour of dogs, our efforts were in vain until it occurred to us that it might be interpreted quite simply - as the manifestation of a freedom reflex, and that the dog would not remain quiet so long as its movements were constrained. We overcame this reflex by means of another - a food reflex, We began to feed the dog only in the stand. At first it ate sparingly and steadily lost weight, but gradually it began to eat more - until it consumed the whole of its daily ration. At the same time it became quiet during the experiments; the freedom reflex was thus inhibited. It is obvious that the freedom reflex is one of the most important reflexes, or, to use a more general term, reactions of any living being. But this reflex is seldom referred to, as if it were not finally recognised. James does not enumerate it even among the special human reflexes (instincts). Without a reflex protest against restriction of an animal's movements any insignificant obstacle in its way would interfere with the performance of certain of its important functions. As we know, in some animals the freedom reflex is so strong that when placed in captivity they reject food, pine away and die.

Let us turn to another example. There is a reflex which is still insufficiently appreciated and which can be termed the investigatory reflex. I sometimes call it the "What-is-it?" reflex. It also belongs to the fundamental reflexes and is responsible for the fact that given the slightest change in the surrounding world both man and animals immediately orientate their respective receptor organs towards the agent evoking the change. The biological significance of this reflex is enormous. If the animal were not provided with this reaction, its life, one may say, would always hang by a thread. In man this reflex is highly developed, manifesting itself in the form of an inquisitiveness which gives birth to scientific thought, ensuring for us a most reliable and unrestricted orientation in the surrounding world. Still less elucidated and differentiated is the category of negative, inhibitory reflexes (instincts) induced by any strong stimuli, or even by weak but unusual stimuli. So-called animal hypnotism belongs, of course, to this category.

Thus, the fundamental nervous reactions both of man and animals are inborn in the form of reflexes. And I repeat once more that it is highly important to have a complete list of these reflexes and properly to classify them, since, as we shall see later, all the remaining nervous activity of the organism is based on these reflexes.

However, although the reflexes just described constitute the fundamental condition for the safety of the organism in the surrounding nature, they in themselves are not sufficient to ensure a lasting, stable and normal existence for the organism. This is proved by the following experiment, carried out on a dog in which the cerebral hemispheres have been extirpated. Besides the internal reflexes, such a dog retains the fundamental external reflexes. It is attracted by food; it keeps away from destructive stimuli; it displays the investigatory reflex pricking up its ears and lifting its head to sound. It possesses the freedom reflex as well, and strongly resists any attempt at capture. Nevertheless, it is an invalid and would not survive without care. Evidently something vital is missing in its nervous activity. But what? It is impossible not to see that the number of stimulating agents evoking reflex reactions in this dog has decreased considerably, that the stimuli act at a very short distance and are of a very elementary and very general character, being undifferentiated. Hence, the equilibrium of this higher organism with the environment in a wide sphere of its life has also become very elementary, limited and obviously inadequate.

Let us now revert to the simple example with which we began our investigations. When food or some unpalatable substance gets into the mouth of the animal, it evokes a secretion of saliva which moistens, dissolves and chemically alters the food, or in the case of disagreeable substances removes them and cleanses the mouth. This reflex is caused by the physical and chemical properties of the above-mentioned substances when they come in contact with the mucous membrane of the oral cavity. However, a ' similar secretary reaction is produced by the same substances when placed at a distance from the dog and act on it only by appearance and smell. Moreover, even the sight of the vessel from which the dog is fed suffices to evoke salivation, and what is more, this reaction can be produced by the sight of the person who usually brings the food, even by the sound of his footsteps in the next room. All these numerous, distant, complex and delicately differentiated stimuli lose their effect irretrievably when the dog is deprived of the cerebral hemispheres; only the physical and chemical properties of substances, when they come in contact with the mucous membrane of the mouth, retain their effect. Meanwhile, the processing significance of the lost stimuli is, in normal conditions, very great. Dry food immediately encounters plenty of the required liquid; unpalatable substances, which often destroy the mucous membrane of the mouth, are removed from it by a layer of saliva rapidly diluted and so on. But their significance is still greater when they bring into action the motor component of the alimentary reflex, i. e., when the seeking of food is effected.

Here is another important example of the defensive reflex. The strong animals prey on those smaller and weaker, and the latter must inevitably perish if they begin to defend themselves only when the fangs and claws of the enemy are already in their flesh. But the situation is quite different when the defensive reaction arises at the sight and sound of the approaching foe. The weak animal has a chance of escaping by seeking cover or in flight.

What, then, would be our general summing up of this difference in attitude of the normal and of the decorticated animal to the external world? What is the general mechanism of this distinction and what is its basic principle?

It is not difficult to see that in normal conditions the reactions of the organism are evoked not only by those agents of the external world that are essential for the organism, i. e., the agents that bring direct benefit or harm to the organism, but by other countless agents which are merely signals of the first agents, as demonstrated above. It is not the sight and sound of the strong animal which destroy the smaller and weaker animal, but its fangs and its claws. However. the signalling, or to use Sherrington's term, the distant stimuli, although comparatively limited in number, play a part in the afore-mentioned reflexes. The essential feature of the higher nervous activity, with which we shall be concerned and which in the higher animal is probably inherent in the cerebral hemispheres alone, is not only the action of countless signalling stimuli, rather it is the important fact that in certain conditions their physiological action changes.

In the above-mentioned salivary reaction now one particular vessel acted as a signal, now another, now one man, now another - strictly depending on the vessel that contained the food or the unpalatable substances before they were introduced in the dog's mouth, and which person brought and gave them to the dog. This, clearly, makes the machine-like activity of the organism still more precise and perfect. The environment of the animal is so infinitely complex and is so continuously in a state of flux, that the intricate and complete system of the organism has the chance of becoming equilibrated with the environment only if it is also in a corresponding state of constant flux.

Hence, the fundamental and most general activity of the cerebral hemispheres is signalling, the number of signals being infinite and the signalisation variable.