Let us once more take a brief survey of the reasons on the other side and arrange them in order.
That nature is everything and spirit very little seems to follow from a very simple circumstance. There are whole worlds of purely natural and corporeal existence without mind, sensation, or consciousness, which, quite untroubled by their absence, simply exist according to the everlasting laws of matter and energy. But nowhere do we find spirit or mind without a material basis. All that is psychical occurs in connection with a physical being, and with relatively few physical beings. Spirit seems wholly bound up with and dependent upon the states, development, and conditions of material being. With the body of living beings there arises what we call “soul”; with the body it grows, gains content, changes, matures, ages, and disappears. According as the body is constituted and composed, as it is influenced by heredity, race, and selection, by nutrition, mode of life, climate, and other circumstances, there are developed in a hundred different ways what we call the natural disposition or character, inclinations, virtues or vices, passions or temperaments. Even the names given to the different temperaments emphasise this dependence of what is innermost in us, the deepest tendencies of our being, on the bodily organisation and the nature of its physiological constitution. The man whose blood flows easily and freely is called sanguine, and the melancholic is the victim of his liver. According as our organs are good or bad, function freely or sluggishly, our mood rises or sinks, we are bold or cowardly, languid or impetuous, and enthusiasm is often enough only a peculiar name for a state which, physiologically expressed, might be called alcoholic poisoning. There is one soul in the sound body, another in the sickly. Fever, and the impotence of the soul against it, made Holbach a materialist. If the brain be diseased, that marvellous order of psychical processes which we call reasoning is broken; the “soul” is wholly or partly eliminated; it fades away, or becomes nothing more than a confused disconnected medley of images and desires. Even artificial interference with, and changes in the bodily organisation react upon the mind. The removal of the thyroid gland may result in idiocy. Castration not only prevents the “breaking” of the voice in the Sistine choristers, it damps the fires of life to dulness, and makes of the impetuous Abelard a comfortable discursive father-confessor. The mind is bound up almost piece by piece with its material basis. Through the “localisation” of psychic processes in the particular parts of the brain, naturalism has enormously strengthened the impression that existed even among the ancients, that sensation and imagination are nothing more than, let us say, what the note is to a tightly stretched string. Cerebrum and cerebellum are regarded as the seats of different psychic processes. The secret of the higher processes is believed to be hidden in the grey matter of the cortex of the cerebrum. We seek and find in the various lobes and convolutions of the brain the “centres” for the different capacities, the power of sight, of smell, of moving the arms, of moving the legs, of associating ideas, of co-ordinated speech, and so on. When brain and spinal cord are injured or removed piece by piece from a pigeon or a frog, it seems as if the “soul” were eliminated piece by piece,—the capacity for spontaneous free co-ordination, for voluntary action, for the various sense-impressions, and so on from the higher to the lower. It has even been maintained that the different feelings and perceptions which are gradually acquired can be apportioned among the individual cells of the brain in which they are localised, and the thought-processes, the associations of percepts, the origin of consecutive ideas, the rapid and easy recalling of memory-images, and the process of voluntary control, of instincts, can be explained as due to the “gradual laying down of nerve-paths” between the different centres and areas of localisation in the brain. All this seems to refute utterly the old belief in the unity and personality of the soul. It is different in youth and in age, and indeed varies continually. It is the ever-varied harmony of the notes of all the strings which are represented by the fibres and ganglion-cells of the nerve-substance. It apparently can not only be completely confused and brought to disharmony, but it may be halved and divided. An almost terrifying impression was produced when Trembley in 1740 made the experiment of cutting a “hydra” in two, and showed that each of the halves became a complete animal, so that obviously each of the two halves of the soul grew into a new hydra-soul.