The concept of the stimulus in psychology.

r seems to me that there is a weak link in the :hain of reasoning by which we explain experimce and behavior, namely, our concept of the stimulus. The aim of this paper is to find out what psychologists mean by the term stimulus, with the hope of deciding what they ought to mean by it. After a short look at the history of the term, I will try to uncover the sources of confusion in modern usage. In the end, perhaps, the concept will be clarified. If not, certain contradictions will have been brought to light. The experimental study of the stimulus began in the eighteenth century, so far as I can tell, with an investigation of the curious things that could be done to make a frog's leg twitch. The experimenters discovered what is now called the nerve-muscle preparation. Galvani and later Volta gave their names to electricity as well as to physiology by their experiments. In the early nineteenth century Johannes Miiller applied these discoveries to the philosophers' problem of the human senses, the gates of knowledge. The nerves of sense, he pointed out, can be excited by a variety of unnatural agencies such as electrical current. Since the mind is acquainted only with the qualities specific to the sensory nerves, not with the stimuli, how it gets knowledge of the material world became more puzzling than ever. Later in the century, Sherrington was to emphasize the extent to which receptors are naturally protected against such irrelevant stimuli by the structural specialization of sense organs. But meanwhile it had been discovered that the skin would yield sensations only at certain discrete points. Here was a fresh puzzle. The separate receptor cells of all the sense organs came to be seen under the microscope, and the punctate character of the sensory process seemed to be established. During all this time, the physical scientists were discovering the laws of energy and triumphantly measuring it in its various forms, electricity, momentum, light, heat, sound, and the results of chemical reaction. It became possible to measure certain