A System of Psychology

The normal stimulus is some activity of what we call "matter," usually outside of, but acting on, the body of the individual. Thus, the oscillation of the air particles which act on the inner organs of the ear and produce sound; the vibration of the ether which stimulates the retina of the eye and occasions the experience of light; the chemical activity of substances, which produces odor; these are instances of stimuli. There is little difficulty in distinguishing stimuli from sensations, even for the beginner, for a little reflection convinces us that these physical facts are not experienced, but only inferred. When, for instance, I hear a tone, I am not conscious of the back-and-forth movement of the air particles; and when I see acolor I am not experiencing the undulations of the ether. In the latter case the stimulus is so far from being experienced that it has required great labor to discover its real nature (assuming that we do know it now) by inferences from observations, and it took years for those who held the present theory to convince of error those who drew different conclusions from the observations. Even to-day physicists do not profess to have a complete understanding of the actual behavior of the ether and some of them doubt its existence. Yet an ignorant man, who has never heard of ether, and whose views on the transmission of light are amusing, may experience light and color sensations which are as highly developed as those of any one. Th...