With this the history of the doctrine of irritability enters into its present phase of development. To future research remains then the problem of further analyzing irritability, this common property of living substance, and finally rendering it into its simplest chemical and physical components. This last goal can only be approached very gradually, step by step. With the analysis of irritability we shall investigate life itself. In the following lectures it will be my endeavor to show how far, with our present knowledge, we can penetrate by this path into the great secret.
CHAPTER II
THE NATURE OF STIMULATION
Contents: Principles of scientific knowledge and research. Origin and meaning of the conception of cause. Cause and condition. Criticism of the conception of cause. The conditional point of view. Conception of cause. The conditional point of view applied to the investigation of life. Conception of vital conditions. Definition of the conception of stimulation.
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.” I[14] 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 Kirchhoff[15] 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?