Physiology adds its quota to the physical arguments. Physics deals with the phenomena which occur outside our organisms and which correspond to our percepts. Physiology seeks to investigate the processes which go on in man's own body when a certain sensation is evoked in him. It teaches us that the epidermis is wholly insensitive to the stimuli in the external world. Thus, e.g., if external stimuli are to affect the end-organs of our touch-nerves on the surface of our bodies, the oscillations which occur outside our bodies have to be transmitted through the epidermis. In the case of the senses of hearing and of sight, the external motions have, in addition, to be modified by a number of structures in the sense-organs, before they reach the nerves. The nerves have to conduct the effects produced in the end-organs up to the central organ, and only then can take place the process by means of which purely mechanical changes in the brain produce sensations. It is clear that the stimulus which acts upon the sense-organs is so completely changed by the transformations which it undergoes, that every trace of resemblance between the initial impression on the sense-organs and the final sensation in consciousness must be obliterated. Hartmann sums up the outcome of these considerations in these words: "This content of consciousness consists, originally, of sensations which are the reflex responses of the soul to the molecular motions in the highest cortical centres, but which have not the faintest resemblance to the molecular motions by which they are elicited." If we think this line of argument through to the end, we must agree that, assuming it to be correct, there survives in the content of our consciousness not the least element of what may be called "external existence."