If we scrutinize more closely the science of psychology, we find that it is essentially dynamical in character. Consciousness is the subject matter of psychology; but consciousness is dynamic, it is first of all an activity, a process. Now all sciences that deal with processes cannot possibly help forming some working hypothesis that should unify the facts dealt with, and should above all be a guide for further research. Mechanics has its hypothesis of masses, forces, energy, inertia, conservation of matter and energy; thermotics its molecular energy; electricity its ether vibrations and currents; chemistry the affinity of atoms; dynamic physiology has its reflex processes; what is the fundamental hypothesis of psychology? We find the following hypotheses:
1 The Spiritualistic, or soul hypothesis,
2 The Materialistic hypothesis,
3 The Faculty hypothesis,
4 The Transmission hypothesis,
5 The Psycho-physiological hypothesis.The Metaphysical,The Positive.
We give here a brief review beginning with the spiritualistic hypothesis. At the very outset I must caution the reader against the grave error of confounding spiritualism with spiritism. The latter is a religious doctrine of life after death, and of the influences of natural or resurrected spirits; the former is a philosophical theory, hoary with age, that attempts to explain the phenomena of consciousness. Such men as Lotze and Ladd are ardent advocates of spiritualism. According to this hypothesis there exists a spiritual substance, a soul, that acts in all the processes of consciousness. The soul is the immutable principle that unifies all the phenomena of consciousness; in other words, all mental processes are but manifestations of the soul's activity. The medical man trained in the school of concrete physical sciences may smile, if not sneer, at the mention of the "soul." Such a hypothesis is in his opinion nothing but an anachronism. He may consider it as a theory long exploded by science and now only lingering among the lower ignorant classes, a theory which an intelligent scientist should be ashamed to introduce into his work even for the sake of discussion, and elucidation of his subject, the "soul" is nothing but superstition. To call a theory superstition does not refute it. The significant fact that Prof. Ladd in his volume on physiological psychology defends it valiantly, that Sigwart in his "Logic" takes up arms for it, and also that such a great thinker as Lotze, himself a medical man, takes it under his protection and finds it perfectly rational, and in fact the only tenable hypothesis, seems to show that there must be something in the "soul," and if superstition it be, it is one that has to be reckoned with, and not dismissed with contempt. We must, therefore, examine the reasons and facts that urge some thinkers and scientists to accept the soul as a working hypothesis for the phenomena of consciousness. There are two weighty considerations that are strongly in favor of spiritualism.
We have already pointed out in a previous discussion that mental phenomena are different in kind from those of the material world. A feeling, an idea, an image, a thought have neither length, nor breadth, nor height, nor weight; no psychic phenomenon can be expressed in terms of material magnitude. Hence, conclude the spiritualists, consciousness is different in kind from matter, it is a different substance, a soul.
Another great point upon which spiritualism rests is mental synthesis. We find that in consciousness, sensations, ideas, thoughts, feelings, are not juxtaposed as are the particles of some material body, but are in unity, in synthesis. The chair seen yonder consists of numerous impressions, sensations and ideas, but all these do not appear in consciousness in their bare separateness, but are synthetized in one percept, a chair. The various experiences that reach the mind, in spite of all their multitudinousness are still brought into relations and are unified, synthetized into the unity of consciousness, they are all referred to the same personality. Now reason the spiritualists, many different phenomena will remain in all their manifoldness and will not give rise to a unity, unless there is a medium through which they are unified. If a resultant is to be formed there must be something on which the forces that are to form the resultant, impinge. If then we do not assume the hypothesis of a spiritual substance, mental synthesis is incomprehensible, if not impossible.
We must now point out the weakness of the soul hypothesis. The argument of spiritualism, that because mental facts differ in kind from material facts, a spiritual substance must be assumed to exist is certainly fallacious. Phenomena may differ fundamentally and still we have no right whatever to conclude that they require two different substances. Time is different from space, but are they two different substances? Consciousness may differ widely from matter and still require no one simple substance for its existence and activity.
The only solid argument that remains for the soul hypothesis is that of mental synthesis. The very consideration, however, on which the spiritualist lays so much stress serves as his best refutation. That phenomena of consciousness differ radically from material ones is a fundamental proposition with the psychologist in general, and with the spiritualist in particular, but this is far from supporting spiritualism. On the contrary, it overthrows his last stronghold. For if mental facts differ in kind from physical material facts, it is poor reasoning to raise difficulties pertaining to one religion, and carry them over into a totally different one. It would be senseless to raise aesthetic difficulties in chemistry or mechanics, but it is no better to reason that because a medium is required for physical objects, movements, forces to combine their effects in one resultant, therefore, a medium, a substance, a soul, is also required for a synthesis of a totally different order of phenomena, those of consciousness. The two orders differ in kind, and what is found necessary in one, is not for that reason also proven to be indispensable to the other. It must first be proven that the conditions of unification are the same in both before the argument from mental synthesis may be accepted as valid. States of consciousness may become synthesized without any medium, without any tertium quid, without any soul.
The spiritualist by his "soul" hypothesis really undermines his own position. For if it be granted that the conditions of unification are the same in mental as in physical activity; that a medium is required in both in order to get a unity, a resultant, then the whole "soul" structure tumbles to the ground. Material and mental phenomena cannot possibly belong to two radically different substances, if the conditions of their activity are exactly of the same nature. It would have been perfectly logical had the difference between consciousness and the physical world been asserted and emphasized, and had the medium, the soul, been totally left out.
The greatest difficulty, however, which the spiritualist encounters is the interaction of the two substances. If matter and soul are different in nature how can they interact, how can they come into any relation? Hours in so far as they are different from pounds, or miles, have nothing in common, and as such do not interact; an hour cannot modify a pound, nor can pounds change hours, and if this holds true of phenomena of the external world where the difference after all is not so very great, it must with special force recoil on the spiritualist where the soul and body are so totally different in all respects. The only way out of the difficulty, if one is consistent and is not afraid to take the consequences, is to introduce the miraculous and say that the interaction is due to the intervention of the deity. This view was in fact taken by the followers of Descartes. The spiritualist, however, with a philosophical and scientific training will rather be inconsistent and support his view by all kinds of props than to accept such a conclusion, because he knows that it practically means defeat, it means that the hypothesis is not working, and that the soul must take shelter under the wing of the deity, the refuge of ignorance. From a purely scientific standpoint we must reject this soul-hypothesis. The first requirement of a scientific hypothesis is that its hypothetical cause should be of such a nature as to be verifiable by experiment and observation. Now in the case of the soul, this condition is not fulfilled. The soul is something that lies outside the range of experience, and could never be brought within the limits of empiricism, the basis of science. The spiritualist, in fact, has not even a positive notion of his "soul," he either frames it in wholly negative terms, that it is not changeable, that it is not material; or, if pressed hard, he falls back on the phenomena of consciousness, the very phenomena the soul is called for to explain.
Furthermore,