This is the meaning of all those psycho-physical experiments that have been carried on with so much ingenuity and persistence (usually associated with attempts to explain vital phenomena in terms of mechanism). First, they attempt to interpret the expressions of will, feeling and need, the spontaneous activities and movements of the lowest forms of life—protists—as “pure reflexes,” as processes which take place in obedience to stimuli, and thus are ultimately due to chemical and physical influences and causes without the intervention of a psychical motive; and, secondly, when this has been apparently or really achieved, the theory of irritability and reflex mechanism is pushed from below upwards, until even the most intricate and complex movements and operations of our own body, which we have wrongly distinguished as acts or behaviour from mere processes of stimulation, are finally recognised as reflexes and liberations due to stimuli. Some stimulus or other, from light or sound or something else, is, according to this theory, conducted to the nervous centre, the ganglion, the spinal cord, the cerebellum or the cerebrum. Here it produces an effect, not of a psychical nature, but some minute chemical, or physical,—or purely mechanical change, which goes through many permutations within the nervous centre itself, unites there with the stored energies, and then, thus altered, returns by the efferent nerve paths to effect a muscle-contraction in some organ, a stretching of the hand, or a movement of the whole body. The physical process is accompanied by a peculiar inward mirroring, which is the psychical penumbra or shadow of the whole business. Thus what is in reality a purely mechanical and reflex sequence appears like a psychical experience, like choice and will and psychical causality. We may be compared to Spinoza’s stone; it was thrown, and it thought it was flying. The reasons for interpreting things in this way lie in the principles of investigation. It is only in this way, we are told, that nature can be reduced to natural terms, that is, to chemistry, physics, and mechanics. Only in this way is it possible to gain a true insight into and understanding of things, and to bring them under mathematical formulæ. Thus only, too, can “the miraculous” be eliminated. For if we are obliged to admit that the will has a real influence on the corporeal, for instance upon our brain, and nerves, and arm-muscles, this would be a violation of the law of the constancy of the sum of energy. For in this case there would occur, at a certain point in the nexus of phenomena, a piece of work done, however small it might be, for which there was no equivalent of energy in the previous constitution. But this is, since the days of Helmholtz, an impossible assumption. And thus all those experiments and theories on what we have called the “second line” of mechanistic interpretation of the universe show themselves to be relevant to our present subject.