Now, this mystery is not of the nature of those which disappear under the light of knowledge. On the contrary, science only brings it out in sharper relief, and emphasizes its absolute unsolvableness. Suppose an absolutely perfect knowledge, perfect in degree, but human in kind. Suppose an ideally perfect science—a science which has so completely subdued its domain, and reduced it to such perfect simplicity, that the whole cosmos may be expressed in a single mathematical formula—a formula which, worked out with plus signs, would give every phenomenon and event which shall ever occur in the future, and with minus signs every phenomenon and event which has ever occurred in the past. Surely, this is an ideally perfect science. Yet, even to such a science, the relation of brain-changes to mental states would be as great a mystery as now. It would even come out in stronger relief, because so many other apparent mysteries would disappear. Like the essential nature of matter or the ultimate cause of force, this relation lies evidently beyond the domain of science. It requires some other kind of knowledge than human to understand it.
But materialists insist so much on the identity of brain-physiology with psychology, that even at the risk of tediousness we will multiply illustrations in order, if possible, to make this point still clearer. Suppose, then, we exposed the brain of a living man in a state of intense activity. Suppose, further, that our senses were absolutely perfect, so that we could see every change, of whatever sort, taking place in the brain-substance. What would we see? Obviously nothing but molecular changes, physical and chemical; for to the outside observer there is absolutely nothing else there to see. But the subject of this experiment sees nothing of all this. His experiences are of a different order, viz., consciousness, thought, emotions, etc. Viewed from the outside, there is—there can be—nothing but motions; viewed from the inside, nothing but thought, etc.—from the one side, only physical phenomena; from the other side, only psychical phenomena. Is it not plain that, from the very nature of the case, it must ever be so? Certain vibrations of brain molecules, certain oxidations with the formation of carbonic acid, water, and urea on the one side, and there appear on the other sensations, consciousness, thoughts, desires, volitions. There are, as it were, two sheets of blotting-paper pasted together. The one is the brain, the other the mind. Certain ink-scratches or blotches, utterly meaningless on the one, soak through and appear on the other as intelligible writing, but how we know not, and can never hope to guess. But when the paste dissolves, shall the writing remain? We shall see.
But some will object. There is nothing specially strange and unique in all this, for the same mystery underlies the essential nature of all kinds of force and matter, and therefore all phenomena. True enough, but with this difference. Physical and chemical forces and phenomena are indeed incomprehensible in their essential nature; but once accept their existence, and all their different forms are mutually convertible, construable in terms of each other and all in terms of motion. But it is impossible by any stretch of the imagination to thus construe mental forces and mental phenomena. It may, indeed, be impossible to conceive how came the plane of material existence, but, standing on that plane, all phenomena fall into intelligible order. But there is another plane above this one, having no intelligible relation with it. We must climb up and stand on this before its phenomena fall into intelligible order. In a word, material forces and phenomena are, indeed, a mystery, but only of the first order. But mental and moral forces and phenomena are a mystery even from the standpoint of the other, and are therefore a mystery of the second order—a mystery within a mystery.
We repeat, then, with additional emphasis after this examination, that we can not imagine between physical and psychical phenomena a relation of cause and effect in the same sense in which we use these terms in physical science, although in some sense there is doubtless such a relation. If man were the only animal we had to deal with, there would be no standing ground left for materialism. But there is still another difficulty which sticks deeper. It is that suggested by the law of evolution and enforced by the comparative method.
Relation of Man to Animals.—Man, we say, is endowed with, is, in fact, an immortal spirit. What is spirit? We know things only by their phenomena; what are the phenomena of spirit? Consciousness, will, intelligence, memory, love, hate, fear, desire—surely these are some of them. But has not a dog or a monkey all these? Pressed with this difficulty, some have indeed felt compelled to accord immortal spirit to higher animals. But we can not stop here. If to these, then also to all animals; for we have here only a sliding scale without break. Can we stop now and make it coextensive with sentiency? No; for the lowest animals and lowest plants merge into each other so completely that no one can draw the line between them with certainty. We must extend it to plants also. Shall we stop here and make immortal spirit coextensive with life? We can not; for life-force is certainly correlated with, transmutable into, and derivable from, physical and chemical forces. We must extend it into dead nature also. Therefore, everything is immortal or none. Our boasted immortality by continued extension becomes thinner and thinner until it evaporates into thin air. It becomes naught else than conservation of energy, and not, as we had hoped, conservation of self-conscious personality. This may be interesting as a scientific fact; but of what value to us personally is a continued existence of our spiritual forces as heat, light, electricity, or any other form of unconscious force? Thus, then, if once we pass the gap between man and the higher animals, there is no possibility of a stopping-place anywhere.
Such is the difficulty presented by comparison in the taxonomic series. Take now the embryonic series. Each one of us, individually, was formed gradually by a process of evolution, from a microscopic spherule of protoplasm undistinguishable in structure from the lowest forms of protozoal life. Now, in this gradual process of evolution, where did immortal spirit come in? Was it in the germ-cell? Then why deny it to the protozoan? Was it at the quickening, or at the birth, or at the moment of first self-consciousness, or at some later period of capacity of abstract thought? Again, when it did come in, was it something superadded or did it grow out of something already existing in the embryo or the infant?
Or take the evolution series from protozoan to man. This we have already seen is similar in outline to the other two. Now, in the gradual evolution of the animal kingdom throughout all geological time, terminating in man, when did immortal spirit come in? Did it enter with life, or with sentient life, or somewhere in the ascending scale of animals, or with the advent of man? If with man, was it some new thing added at once out of hand, or did it grow out of something already existing in animals?
This last, we are persuaded, is the only tenable view—the only view that can effect that reconciliation between the two extreme, mutually excluding views now usually held, which, as already seen, is the true test of a rational philosophy. I believe that the spirit of man was developed out of the anima or conscious principle of animals, and that this, again, was developed out of the lower forms of life-force, and this in its turn out of the chemical and physical forces of Nature; and that at a certain stage in this gradual development, viz., with man, it acquired the property of immortality precisely as it now, in the individual history of each man at a certain stage, acquires the capacity of abstract thought. This is, in brief, the view which I wish to enforce. The reader must understand, however, that this is my own view only, a view for which I have earnestly contended for twenty years. It appeals, therefore, not to authority, but only to reason. I wish now to present it as briefly as possible.
First, then, I would draw attention to the fact that there is nothing wholly exceptional in such transformation with the sudden appearance of new powers and properties; but, on the contrary, it is in accordance with many analogies in the lower forces, and therefore a priori not only credible but probable. For example, force and matter may be said to exist now on several distinct planes raised one above another. There is a sort of taxonomic scale of force and matter. These are, 1, the plane of elements; 2, the plane of chemical compounds; 3, the plane of vegetal life; 4, the plane of animal life; and 5, the plane of rational and, as we hope, immortal life. Each plane has its own appropriate force and distinctive phenomena. On the first operates physical forces, producing physical phenomena only—for the operation of chemical affinity immediately raises matter to the next plane. On the second plane operates, in addition to physical, also chemical forces, producing all those changes by action and reaction, the study of which constitutes the science of chemistry. On the third plane, in addition to the two preceding forces, with their characteristic phenomena, operates also life-force, producing the distinctive phenomena characteristic of living things. On the fourth plane, in addition to all lower forces and their phenomena, operates also a higher form of life-force characteristic of animals, producing the phenomena characteristic of sentient life, such as sensation, consciousness, and will. On the fifth plane, in addition to all the preceding forces and phenomena, we have also the forces and phenomena characteristic of rational and moral life.
Now, although there are doubtless great differences of level on each of these planes, yet there is a very distinct break between each. Although there are various degrees of the force characteristic of each, yet the difference between the characteristic forces is one of kind as well as of degree. Although energy by transmutation may take all these different forms, and thus does now circulate up and down through all these planes, yet the passage from one plane upward to another is not a gradual passage by sliding scale, but at one bound. When the necessary conditions are present, a new and higher form of force at once appears, like a birth into a higher sphere. For example, when hydrogen and oxygen are brought together under proper conditions, water is born—a new thing with new and wholly unexpected properties and powers, entirely different from those of its components. When CO2, H2O, and NH3 are brought together under suitable conditions, viz., in the green leaves of plants, in the presence of sunlight, living protoplasm is then and there born, a something having entirely new and unexpected powers and properties. It is no gradual process but sudden, like birth into a higher sphere.