swer to this question in the course of his book on the Voluntary Act, of the relevant parts of which the following is a brief summary.
Physiologically considered, the voluntary act is the final outcome of a physico-chemical process taking place in the sensory-motor arc; that is, in the apparatus composed of sense-organs, centripetal nerves, brain, centrifugal nerves, and striate muscles. By means of this apparatus the innumerable stimuli which act upon the animal body elicit from it those external movements which are useful for its preservation. The sum of the stimuli, taken in connection with the structure and momentary condition of the nervous apparatus, must furnish a complete explanation of every resulting movement. Such a useful movement is known in physiology as a reflex action. Every act, even the free voluntary act, is, physiologically speaking, a reflex action, differing from the spinal reflex of the brainless frog only in its vastly greater complexity.
In the normal frog with nervous system intact, the after-effects of previous excitations preponderate among the conditions producing movement, and the movement seems uncaused because the visible stimuli form so small a part of the total cause. Just as the eye and ear enable the organism to be affected by objects not in immediate contact with it, so memory — the power of the brain to associate to an impression the image of another impression experienced with it before — enables the organism to adapt its movements to objects before they have as yet entered the field of sense. The power of the human brain, finally, to analyze its impressions into their elements and recombine these into new complexes, enables it to form complexes corresponding to objects never actually experienced; so that civilized man is able to act with reference to objects the most distant in space, the most remote in time, or altogether ideal.
But the usefulness which characterizes animal movements demands some explanation. How could a nervous apparatus come into existence so constructed that, despite the infinite variety of external conditions, it should respond at every moment with a useful and appropriate reaction? The answer to this question is furnished by the Darwinian theory, which tells us that the nervous apparatus came into existence and was gradually perfected by means of natural selection. If natural selection explains the origin of the digestive apparatus, it can explain that of the nervous apparatus also; for the latter is no more useful than the former. But since natural selection explains the origin of the nervous apparatus, and since, given the apparatus, a definite complex of conditions necessarily evokes a definite external movement, it follows that every voluntary act, regarded solely on its physiological side, is completely explicable in accordance with the principles of physico-chemical science.
Approaching the problem from the psychological side, we find, if we
