Irritability: A Physiological Analysis of the General Effect of Stimuli in Living Substance

by Max Verworn

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?