The common problem of all scientific research is the investigation and formulation of natural laws. The assumption of a unity in the happenings and of existence in the world, in accordance with definite laws, forms the indispensable foundation of all scientific study and is fully justified by experience. Experience has taught us, as a result of innumerable individual observations, the existence of such an accordance, whereas in not a single instance has it been shown that this is not the case. We are thus justified in assuming without further discussion that every scientific research, every new problem which we approach, is likewise founded on this unity of occurrences in accordance with natural laws. Only on the firm basis of this assumption has scientific investigation a purpose, and every success is a new proof of this. There is an unanimity of opinion concerning this among scientific investigators in all fields.

Not such complete agreement, however, exists in regard to the question by what symbols of human thought and speech these laws can be described in part as well as in toto, so that existing laws can not only be fully and conclusively defined, but at the same time without the use of superfluous terms. According to Ernst Mach, thought is an adaptation to facts. Our speech is simply a method of expression of our thoughts and indeed the most satisfactory form we have. We must, therefore, use those symbols which are most closely adapted to facts as the most precise expression of these existing laws. What forms of expression have we?

It might appear that a discussion of this fundamental question has not a close connection with our special subject of physiology of stimulation. This, however, is not the case. Indeed, it is an irremissibly previous requirement not only for the elucidation, but also for the understanding itself in this particular field. We could not come to a clear understanding in this field without such analysis. The interpretation of the unity of being and happenings in accordance with natural laws, which today is widely accepted in the scientific world as the only exact one, implies the assumption of a “causation” according to which things are explained by the law of “cause” and “effect.” I14 have already on various occasions taken the opportunity to criticise this view and to show the error and confusion to which it leads. I should like here to enter somewhat more in detail into the reason for this criticism. It is particularly directed against the scientific use of the term “cause” on the basis of our best-known theoretical principles. It is clear that all scientific observations and explanations are founded on experience. Can it be said that the conception of “cause” originates from experience?

We can say with absolute certainty that the conception of cause dates from prehistoric times. Its beginning reaches back to the stone age, at least to neolithic, possibly to palæolithic culture. This is demonstrated by the careful reconstruction of these prehistoric races based on a critical comparison of the remains of their culture with that of primitive races living today. The ideas of these primitive races show an inclination to an extraordinary degree to explain all happenings in the world anthropomorphously. All happenings in surrounding nature are given the same origin as the activities of man himself. To man, on this plane of phantastic religious speculation, all events in nature appear as acts of the will of invisible powers, which, having originally proceeded from the souls of dead human beings, think, feel and act exactly as he does. This anthropomorphic conception of the occurrences in the surrounding world is one of the many conclusions which ensue from the supposition of an invisible soul, which can be separated from the body. It was this conception which gave the impetus for the transition of human thought from the era of the naïvely practical to the era of the theoretical spirit in that far removed age. In this anthropomorphic transference of personal subjective impulses of will to the objectively observed events of the surrounding world, lies the origin of causal conception, which since then has been generally used as the explanation of the happenings in the world. One cannot assert that the formation of the conception of cause is purely a product of experience, but rather a result of naïve speculation. Even if a later evolution of human thought shows a continued endeavor to dismantle the conception of cause of its primitive trappings and to modernize, as it were, its outer appearance, we still find today many inner components clinging to it, which do not agree with the strict demands of critical scientific exactness, demands which must particularly be made concerning a conception which has been given such fundamental importance in theoretical knowledge.

I wish to observe here, however, that the conception of cause, even though more or less unconsciously so, is still the remains of a part of the old anthropomorphic mysticism carried over into our own times. This shows itself especially in the conception of force, which is nothing more than a form of the conception of cause. Force is the cause of movement. One has here in anthropomorphic manner transferred the action of the will of man, which produces movement of the muscles, into lifeless nature. The force of the sun attracts the earth, that of the magnet attracts iron, etc. In short, one has introduced a mysterious unknown factor instead of being content with the simple description of facts, such as Kirchhoff15 has advanced in the field of mechanics. Although of late natural science has also dispensed more and more with conception of force as a means of explanation, it is still today not wholly done away with. That which applies to the conception of force is likewise true of the conception of cause.

Another point concerning the application of the conception of cause seems to me, however, to be of much more importance, namely that a single cause is held responsible for the taking place of a process. One endeavors to explain a process in general by seeking for its “cause.” The cause being found, the process is considered as fully accounted for. This idea is not only widely spread in everyday life, but is even found frequently in natural science, especially in biology, although here, it should be known, the processes are decidedly more complicated. The search for the “cause” of development, for the “cause” of heredity, for the “cause” of death, for the “cause” of the respiration, for the “cause” of the heart beat, for the “cause” of sleep, for the “cause” of disease, etc., was for a long time and frequently even today a characteristic of biological investigation. As if such a complicated process as development, death or disease could be explained by a single factor! In reality, one has obtained very little as a result of the analysis of a process by discovering its cause; and in addition the false impression arises that through the finding of this one factor the process has been definitely explained. It has been generally recognized in the natural sciences in recent times that no process in the world is dependent upon one single factor and attempts have been made to give this fact more consideration.

It is the custom at the present time to hold the view that every process or state is brought about by its cause, but that a series of conditions are also necessary to the production of the process. Such a view, however, which considers that two different factors existing at the same time are necessary to the accomplishment of every happening or state, namely, the cause and the conditions, leads to new difficulties, for then, upon a more exact analysis arises the question: Which is the cause and what are the conditions? It is very soon found, however, that this does not permit of any strict differentiation, as the two conceptions can not be sharply separated. This difficulty was brought to my notice with particular force during an animated discussion with a friend and colleague about twenty years ago, which I have always remembered. I had observed at that time the dependence of pseudopod formation of amœboid cells on the oxygen of the medium, and had found that the expansion phase of protoplasmic movement, that is, the extension of pseudopods, the centrifugal flowing of the protoplasm into the surrounding medium and with this the enlargement of the surface of the cell body, only takes place when oxygen is contained in the surrounding medium and never occurs in its absence. Being at that time wholly under the influence of the conception of cause, I believed that oxygen was the cause of the formation of the pseudopods. To this my friend made the objection: “Yes, I quite acknowledge the fact of the dependence of the formation of pseudopods on oxygen, but what informs me that the oxygen is really the cause? It might be simply a necessary condition.” This objection led to a long debate, which ended, however, without our being able to agree. We were not in a position to distinguish between the conception of cause and that of condition, and at that time the idea did not occur to us to emancipate ourselves from the conception of cause deeply implanted in us as a result of our training. In fact, one is greatly embarrassed if one attempts to sharply distinguish by a definition the conception of cause and that of condition. A condition is a factor on which a state or a process is dependent for its existence or its taking place. To the conception of condition belongs, besides the factor of relation, that of necessity. Every condition is necessary to the existence or taking place of this state or process. Without the condition in question the state or process does not occur. The same must be demanded for the conception of cause. No state exists, no process takes place, without its cause. The cause then has itself the specific character of a condition, it is itself a condition. Has it perhaps then some specific peculiarity in contrast to the other conditions, which would give it a prominent place? Experience teaches us that nothing, that is to say, no state or process in the world, is dependent upon a single factor alone. There are always numerous factors which bring about the state or process. Would it be possible to distinguish which of these particular conditions is of the greatest importance?

First of all, it must here be taken into consideration that the importance of a condition is not one which is capable of increase or decrease, for the simple reason that necessity, which forms an essential component of the conception of cause cannot be varied. A factor cannot be more than necessary for the existence of a state or the taking place of a process. If, however, it is less than necessary, then it is not necessary at all, and the state or process exists also without it, that is to say, the factor is not a condition. In other words: all conditions for a state or process are of equal value for its existence, as they are all necessary.

If one attempts to prove by means of concrete examples this statement obtained by purely logical deduction—a control which, considering the experimental nature of modern thought, never should be neglected even in the simplest of reasoning—it might appear that an objection could still be made against its general validity. From various instances it might be concluded that there are conditions, which as such are not absolutely necessary for a state or process, but can be replaced by other factors. An example may serve to make this clear. I pour diluted hydrochloric acid on powdered carbonate of sodium, and carbon dioxide is set free. The addition of hydrochloric acid is here a condition for the liberation of the carbon dioxide. Without the presence of the hydrochloric acid the process does not occur. Nevertheless I can substitute diluted sulphuric acid for the hydrochloric acid. Here it would appear that one condition can be replaced by another. But one must not be deceived. A closer observation soon shows that the process has not been sufficiently analyzed if we look upon the addition of hydrochloric acid as a condition for the liberation of carbon dioxide. It is not the presence of hydrochloric acid or sulphuric acid, as such, which is a condition for the process, but rather the separation of the sodium atoms from their combinations with the oxygen in the molecule of the carbonate. This reaction can occur as a partial component in very different complexes of processes. Or to quote another example, taken from the subject with which we are especially here concerned. I allow an induction shock to act on the nerve of a nerve muscle preparation of the frog. The muscle contracts. The electric stimulus is the condition for the muscle contraction. But I can substitute for the induction shock a mechanical stimulus by sudden pressure of the nerve. The muscle again contracts. The analysis again shows that the induction shock as such was not the condition for the muscle contraction, but the excitation of the nerve which it produced and which is conducted as a specific impulse to the muscle. This excitation of the nerve can, however, be induced by very different kinds of processes, namely, by all processes which possess in common the condition that they suddenly increase certain disintegration processes in the living nerve substance. Indeed, the further analysis of the whole process shows in addition that the nerve impulse as such likewise does not form a condition for the contraction of the muscle, but it first of all produces the necessary condition for the muscle contraction by suddenly greatly increasing certain chemical processes, which take place in the living substance of the resting muscle. The nerve impulse can, therefore, also be replaced by other processes, if only these contain the condition for an increase of disintegration of the muscle substance, as in the case of the direct stimulation of the curarized muscle, where the influence of nervous impulses is totally eliminated. In a further analysis of this process we should penetrate even more deeply into the differentiation of the individual constituent processes and the isolating of the special conditions on which each link in the chain is dependent.

Such an analysis then shows us the following: Every thing, every state or process, is a complex of numerous components, of which one always conditions the other in the manner that the individual conditioning components are themselves in their turn contained as constituents of other complexes and are conditioned here again by other factors. These factors in themselves as such are not directly necessary to the taking place or existing of the special component and can, therefore, be replaced by others. Closer observation shows that there is a constant interdependence between all things in the world. Every thing in the world is indirectly dependent upon every other, although often so remotely that we are not able to trace the connection. Absolute things, completely isolated and independent of others, do not exist in the world. In observing and studying complexes individually, we must not forget that we only think of them as isolated from the great eternal coherence, from which they are in reality not separated. The conception of condition, however, only then has meaning, if we refer to it in connection with the direct dependence of one factor upon another. Nevertheless if we understand by conditions those which are connected by multitudinous intermediate components, then we would render the conception of conditions useless. For if every thing in the world were the condition for every other, the conception of relation would lose its value in special states or processes. Should the conception of condition have a meaning in regard to a certain state or process, then we should only look upon that part of a complex upon which the other is directly dependent as a condition. When, however, we meet with a factor for a process or state, which can apparently be replaced by another factor, we have not carried the analysis far enough. Upon deeper penetration into the subject, it is found that the essential condition for the process, which exists, is a component common to both factors, one of which in consequence can replace the other.

It is the task of all scientific research to penetrate deeper and deeper into these relations, these connections and the order of succession of states and processes and to separate them into their individual components, and in this way gain a more thorough knowledge of the constancy of existence and happenings in the world.

This analytical process, it is true, only advances very gradually, and we must accept for the present, especially in the complex biological processes, that a whole complexity of members appear conditioned, and that a complex aggregate is a condition of the whole process. We are not yet in the position to define the special components of the constituent processes. It is only step by step that we are able to differentiate the necessary from the accessory parts in these complexes. However, we are here only concerned for the present with a purely theoretical question and we may be permitted to say: If we maintain that the conception of condition has as an integral part the element of necessity and of relation to a special thing, then there are no substituting conditions. For then every condition for a state or process is of equal value. There is no justification to give more prominence to one condition and place it in the position of being the “cause.”

If the cause is elevated, then it is done from some superficial motive. This is confirmed by a glance at the practical use of the term cause. The cases in which the cause is always at once clearly recognized and named without doubt or hesitation are those where a new factor is added to an already existing system of conditions, which bring about a process. When such a process is produced, the last added condition is considered as “cause.” A shock acts on an explosive body, the body explodes: the shock is considered the cause. An induction shock acts on a muscle, the muscle contracts; the induction shock is looked upon as the cause of the muscle contraction. To regard only the last added condition as being of especial importance to the taking place and the explanation for a process is, however, a standpoint which could satisfy only the most superficial of observers.

In a scientific investigation such methods should play no rôle. For to every careful observer it must appear quite clear from the beginning, that the previously existing conditions have as great a value for the taking place of the process and its explanation as that last added.

The induction shock would not have produced the characteristic effect had not the other conditions been already previously combined, had not certain special atoms in the molecule of the explosive combination in consequence of former processes assumed quite a peculiar labile position, had not in the evolution of the muscle in the growth and metabolism certain combinations been formed, and certain chemical processes taken place.

Therefore if I do not analyze these previously existing processes and the conditions brought about by them in the system of the explosive substances or the muscle, and simply know the condition added last, then I have learned nothing of the process itself, have explained nothing. The time of application of a new condition does not justify in any degree the assignment of a dominant position to a factor. But more: in many cases there is not a question at all of the addition of a process to an existing state, but rather of the simultaneous interference of two or more processes. Several conditions can appear at the same time. In other cases the sequence of the combination can be reversed. Which then is the cause? Has the process several causes, or has it no cause? Here one sees plainly to what absurd results it leads if time alone is used as a basis of the conception of cause. To illustrate this I return to the case of the liberation of carbon dioxide from carbonate of sodium. I place anhydrous carbonate of sodium in a beaker and add hydrochloric acid. The carbon dioxide escapes. Here the addition of hydrochloric acid would be assumed to be the cause of the freeing of the gas. Then I put hydrochloric acid in a beaker and add carbonate of sodium. The same process takes place, but now the addition of carbonate of sodium would be considered the cause for the formation of gas. Now I put both simultaneously into a beaker. Again the same process. Which was now the cause? Has the process now two or has it no cause at all? Finally I put anhydrous carbonate of sodium and hydrochloric acid in ether solution into the beaker. The formation of gas does not take place, and yet both causes for this formation of gas are present, the carbonate of sodium and the hydrochloric acid. Only when I add water to the mixture does the formation of carbon dioxide take place. Here water would be considered the cause. Hence every condition would be in succession the cause for one and the same process. Under some circumstances the same process would have several and in others no cause at all. It is scarcely necessary for further comments upon the value of the conception of cause for the scientific explanation of a state or process. If we do not seek to introduce into exact science the antiquated symbols which have become useless and belong to a primitive phase of development of human thought, there cannot be a moment’s doubt that a strict scientific analysis in whatever field of investigation it may be carried on can consist only in the study of all the conditions concerned in a state or process. If this is done, then the work of exact research is accomplished. Further problems do not exist. The use of superfluous terms or symbols for the definition of things would be in opposition to the fundamental principle, already brought forward by Kirchhoff, especially for mechanics, namely, that of formulating comprehensively and in the simplest manner the processes which take place in nature.

At first glance one might be tempted to find an incompleteness in the observation and description, when a conditional standpoint is adopted. It might be thought that conditionalism were a purely formal method of observation, and only considered the interdependence of things, but not the properties, the nature of the objects themselves. Regarded more closely, however, it is seen that this objection does not hold good. For what is a condition?

A condition is in itself a thing of quite distinct properties. The properties of a thing are, however, determined by the specific combination of conditions which characterize the thing. The conditions by which a thing, that is to say, a state or process, is determined, are identical with its being and nature; in other words, they are the thing itself. Purely formal relations without essence would be altogether an absurd fiction not in accord with reality, and which even the science of mathematics does not acknowledge, for we cannot have a conception without concrete content, just as in nature we do not find a form existing independently of a thing. Every thing is equal to the sum of all its conditions and depending upon the uniform constancy in accordance with natural laws is solely determined by its conditions. The problem of all scientific research consists wholly in the ascertaining of the conditional interdependency.

A state or process is solely determined by the sum total of its conditions. A state or process is identical with all of its conditions in totality. From this it follows that equal states or processes are always the expression of equal conditions and wherever unequal conditions exist, unequal states or processes will result; and further, a state or process is completely investigated when the entire number of its conditions is ascertained.

This fundamental statement of conditionism should be engraved over the portals to the entrance of every scientific investigation.

That there is not the least difficulty in presenting scientific observations strictly according to these principles of conditionism, and that one can perfectly well do without the causal conception in a scientific description, I have shown by a concrete example, namely, in the fifth edition of my “General Physiology.” In the whole volume the conception of cause is only mentioned in one place, where its theoretical value is criticised, elsewhere not at all, and yet I do not think that any one will miss this conception, and indeed, if their attention is not especially called to the fact, even notice the omission.

These principles of an exact conditional investigation must also guide us in the analysis of the processes of stimulation. The process of stimulation is especially apt to tempt one to employ the old conception of cause, for it belongs to that group of processes which originate from an already existing system by the addition of a new factor. An electric stimulus acts on the muscle. The muscle contracts. The stimulus is considered the cause of the contraction. But what would I explain if I were to prove that the stimulation is the cause of the contraction?

The history of physiology shows us that this subject has advanced long since far beyond the stage of being satisfied with such an explanation. Today the process would only then be fully investigated if we knew the entire number of its conditions and had traced the dependency of the individual partial constituents of the whole complex process upon one another. For this, however, it is essential that we study the conditions already existent in the entire system previous to the action of the stimulus.

That which we describe with the word life is an exceedingly complex process. If we analyze life, it is found to be composed of an immense number of separate constituent processes, each one being conditioned by the others. These constituent processes are the vital conditions. A vital process occurs, and must occur, where and when the whole sum of vital conditions is realized. It is identical with the sum total of the vital conditions. If only one condition is absent, then life does not exist. It is then expedient to reserve the expression “life” for the entire sum of the vital conditions. When we speak of the individual constituent processes as “vital processes” in the plural, we must bear in mind that in reality each is not in itself life. Only the whole complex “lives,” not an individual constituent of the same. Living substance is rather the whole system, and not a constituent part of the same, not a piece of protoplasm, not a nucleus and not a specific protein combination in the cell.

A property of this system should receive our consideration at this point. It is a characteristic of every system in the world, namely, the fact that a system is not isolated from its surroundings. It is a deception resulting from the selective action of our sensory organs, if we consider the bodies as separated and isolated from their environment. This deception disappears upon further analysis and when we assist our organs of sense, which only respond to certain parts of the whole process, by experimental methods of investigation. Our experience then shows us that an isolated system does not exist, but that there are instead everywhere connections which extend further and further into the infinity of the world. An organism is consequently no delimitated system and the vital process cannot, therefore, be sharply separated from the processes in the medium. We cannot draw a sharp line between vital processes and say: on the right we have factors which are necessary for the maintenance of life, and on the left factors which are not necessary. The conditional connection between individual processes extends to the entire world, and likewise a great series of constituents, each influencing the others, extend from the medium into the organism. The nature of our sense perception, and consequently the knowledge derived therefrom, is such that we are obliged to arbitrarily take into consideration merely fragments from the endless interdependence of all things in the world, and so we separate the vital conditions of the organisms from their surrounding factors, as though they were independent. A conscientious theoretical analysis requires that we should never forget that in reality such an isolation does not exist. Only with the recognition of this can we distinguish for practical purposes between internal and external vital conditions. In such a differentiation the internal vital conditions which compose the living system conceived to be isolated, are the organs, the tissues, the cells, the protoplasm and the cell nucleus, and within the protoplasm and the nucleus the arrangement and quantitative relations of certain substances, such as proteins, salts, water and the thousands of special components with their interactions and continued alterations. On the other hand, the external vital conditions, which act on the periphery, are the conditions of the surrounding medium, as foodstuffs, water, oxygen, static and osmotic pressure, temperature, light, etc. But this distinction has only a practical value for the study of the organism as an independent system. Theoretically it is as impossible to make a sharp distinction between internal and external vital conditions, as to distinguish between the vital conditions generally and the more remote conditions of the environment. All these conditions form a widely branching system of factors of which one is conditioned by the other reaching continually from the interior of the vital system into the surrounding medium, so that on the periphery of the system it cannot always be said whether or not a component still belongs to life. Considering these circumstances we can roughly for the present define the conception of stimulus as follows:

A stimulus is every change in the vital conditions.

The most essential point in this definition is the relation of the conception of stimulus to that of vital conditions. These relations, however, call for a brief explanation. Here again the conditional method of observation saves us from error, for it would be wrong to place the conception of stimulus and vital conditions in contrast to one another, one excluding the other. On the other hand, this method of observation shows that the stimuli are likewise only conditions, but conditions producing certain changes in the vital system. If a stimulus acts, that is, if there is any change whatever in the vital conditions, the whole complex of life in consequence of the dependency of the constituent parts upon each other is also changed, and a new state of living substance occurs. Stimuli are, therefore, also only vital conditions, but vital conditions for new vital manifestations. The relation of one given state to another, forms an indispensable point in the understanding of vital conditions as well as that of the stimulus. The stimulus becomes a vital condition for the new state which it produces. It is only a stimulus relatively to the original state, which previously existed. The essential point, therefore, in the conception of the stimulus is that of alteration. An example will serve to make this clearer. If Amœba limax are bred in a hay infusion they appear in countless masses. Observed in water in a watch glass they show at first the well-known form of Amœba proteus with short, broad, lobate pseudopods. (Figure 1, A.) After a period of rest, however, they gradually assume the characteristic elongated limax form. (Figure 1, B.) In this shape they constantly move about. But if I add to the water only a faint trace of diluted solution of caustic potash, the amœbæ first assume the shape of a ball (Figure 1, C), and then after a time, stretch out long, pointed pseudopods, which give them the characteristic form of Amœba radiosa. I have observed them for several hours at a time. (Figure 1, D and E.) They remain permanently16 in this form. They move in the same manner as Amœba radiosa. They draw in one pseudopod, stretch out another and float freely in the water in contrast to their limax state, in which they are always attached to some support. The long, pointed, often threadlike pseudopods, yield to every movement of the water, bending in consequence like whipcords. In this example the amœbæ under the vital conditions existing in tap water have limax form. The vital conditions undergo a change by the addition of a solution of caustic potash, which acts as a stimulus. The consequence is a reaction, in which the animal assumes radiosa form. By the action of the stimulus a new state of the living substance is produced, and remains as long as the solution of caustic potash is contained in the medium. The solution of caustic potash is, therefore, a stimulus for the state of the vital system, which is manifested in the limax form, whilst for the state of the system which shows itself in the radiosa form, it is a vital condition. If I place the amœbæ of the radiosa form once again in tap water, they assume the proteus and then the limax form. The withdrawal of the solution of caustic potash, the presence of which is a vital condition for the radiosa state, acts as a stimulus, which results in a transition of the vital system to another state. By altering the medium I can at will bring about this change of form in the same individuals. In this way one and the same factor can figure as stimulus and vital condition, according to the state of the vital system on which it acts. Whilst its addition acts as stimulus in the one state, its withdrawal acts as a stimulus in the other state, which it has produced. The same fact is shown by the well-known example of Artemia salina, which on being placed in fresh water changes into Branchipus stagnalis and, when again introduced into sea water, becomes once more Artemia salina.

These facts show clearly that some stimuli can also be considered as vital conditions. In the absence of certain stimuli, life could not exist for any length of time. In the highly differentiated cell community of the animal organism, for instance, as a result of the coexistence of the cells and the tissues, many parts have forfeited in a measure their independence. An example of this is the skeletal muscle, which, in the absence of impulses from the nervous system, reaches a low level of chemical change and energy transformation. Here the nervous impulses which act as momentary stimuli, are also in the course of time indispensable vital conditions. Without them the muscle would gradually become atrophied from inactivity. The same applies to all other tissues of our bodies. The functional stimuli are for them at the same time vital conditions. These vital conditions undergo fluctuations and interruptions but at each alteration from a given state they act as stimuli.

Stimulus is every change in the vital conditions. But is this definition complete? Are we really justified in regarding every alteration in the vital conditions as a stimulus?

In considering this question, one point must not be omitted. This is the fact that one of the chief characteristics of the vital process is, that it undergoes continuous change. A vital process involves not simply an alteration in metabolism or transformation of energy in the sense that the same chemical processes continuously reoccur in the same manner. Such a view could only be admissible for the observation of living substance during a limited period. An investigation over a long period of time shows rather that every living system alters as long as it exists, although this alteration is very gradual. The constituent processes, in short, continuously undergo metabolic change both quantitative and qualitative in nature.

If we observe the occurrences in a living system at various moments of the cycle of life, we will find that the condition differs qualitatively at each period. The progressive alteration of the system is such that every state of living substance conditions another, by which it is followed. No state can permanently exist as such. Every state is the product of the preceding, as it in turn conditions its successor. Consequently the relations of the system to the surrounding medium also undergo alteration, even when the external factors themselves in no way alter. That which today is still a vital condition, is not in consequence necessarily one tomorrow. These progressive changes exist continuously until the death of the system takes place. They characterize life. It is development, and life cannot exist without development. Death is only the last phase of development. The individual constituent processes of metabolism gradually change to such a degree that they can no longer work harmoniously together. Then the chain of processes is interrupted at one point or another. The system develops into death or, on the other hand—and this, as Weissman especially emphasizes, is realized in the case of unicellular organisms—a corrective process takes place, a process of cell division by which the original state of the cell is restored and development begins anew and in a similar manner.

Ought we to designate these constant alterations in the inner vital conditions as “stimuli”? Usage in this connection has already answered in the negative, by applying to them the word “development.” And this use is in a certain sense justified. Let us imagine an organism or any other object for the purpose of investigation as isolated from its surroundings. This conception, which we have already stated, proves untenable on closer analysis, but it, however, is based on the nature of the methods of human observation and is indispensable for practical use within certain limits. Then the inner vital conditions belong to the organism, the external to the medium. They differ in so far that the external vital conditions can exist permanently without alteration, that is, independently of the development of living systems, whilst the inner vital conditions of every living organism continuously and progressively undergo alteration. In this sense, but only in this, there is evidently a difference between the inner and outer vital conditions, which permits a separation of the two groups. But we should always bear in mind that this separation cannot be sharply defined. On the same basis we assume that the organism for purposes of study is separated from its surroundings as an independent system, which leads us in consequence to contrast the alterations in the internal with those in the external vital conditions, in which we designate the first as processes of development, the latter as stimuli. This distinction, as all differentiations and separations in nature, gives us only a practical working basis.

In this way we confine the conception of the stimulus to all alterations in the external vital conditions of a living system, considered as isolated. This view does not exclude the fact that stimuli can also occur and act within an organism. If a nervous impulse is conducted from the cerebral cortex through the pyramidal tract to a skeletal muscle, this impulse acts upon the muscle cells as a stimulus. Although the explosion of the impulse is an alteration within the body, nevertheless, as far as the muscle is concerned, it may be looked upon as an external vital condition, therefore as a stimulus. As the conception of stimulus involves the relation to a given state, it likewise involves at the same time the relation to a given living system, upon which it acts from the exterior.

What is the value then of all this theoretical discussion?

In presenting the conception of stimulation from a conditional standpoint, I desired to show what difficulties stand in the way of a theoretical isolation of a fundamental conception in the field of physiology, which indeed is used in our practical research work at every step. “Natura non facit saltus.” I wished to demonstrate that the sharp separation of the conception of stimulation, like all artificial divisions which we make in nature, must always contain an arbitrary note, as in reality isolated systems do not exist in the world. I wished to show that, for this reason, the conception of vital system, the conception of life, the conception of vital conditions are not sharply defined. I wished likewise to show that as a necessary consequence of this fact a sharp separation of the conception of stimulation, which can only be made in relation to that of vital conditions, cannot be maintained theoretically. I wished to show further that there is no sharp line of division between inner and outer vital conditions, and that we cannot, therefore, make a strictly theoretical distinction between the conception of stimulation and that of the processes of development. I wished to show that, for these reasons, we must not expect from the conception of stimulation, as we understand it, anything beyond its possibilities. But finally I wished also to show that, whilst fully conscious of and with due consideration of all these difficulties, it is possible to work out a definition of stimulation which is of great practical working value. The definition in short is: “Stimulus is every alteration in the external vital conditions.”

This definition gives to the conception of stimulation its most complete, that is to say, its generally applicable and simplest form. The great importance from a methodical standpoint of this definition of stimulation for the research of life is evident. Our whole experimental natural science always employs for investigation of any state or process the same method: the state or process to be observed is studied under systematically altered conditions. By stimulating the living substance it is brought under changed external conditions. A systematic employment of stimulus is, therefore, the experimental means for the research of life.