From the Preface. This monograph is the outgrowth of an experimental investigation begun in the psychological laboratory of the University of Pennsylvania in 1897 and continued at irregular intervals during the two following years. In 1900 it was presented substantially in its present form to the Faculty of the Department of Philosophy in the University of Pennsylvania and accepted as a thesis in partial fulfillment of the requirements for the degree of Doctor of Philosophy. The experiments were directed, at first, exclusively to the investigation of local differences in the sensitivity of cutaneous areas; it was assumed that such differences, if found, would furnish conclusive evidence of the existence of a distinct terminal apparatus for pain. As the investigation progressed, however, it became evident that it was necessary to subject this point of view itself to a critical and experimental examination, if the results were to be given an adequate interpretation. The first step in this examination led to the question, "How do the several parts in a nexus of processes - connected with stimulus, sense organ or brain, and psychosis - present themselves as material for investigation?" It is commonly assumed that physical, physiological, and psychical processes are discretely and directly open to investigation. A little reflection will show the error of such an assumption. A pain stimulus that can be studied without reference to the pain process it excites in a sense organ, a pain organ to be investigated without reference to the pain sensation, a pain psychosis pure and simple to be studied apart from the subject's judgment or bodily reaction, - these are accessible to no experimental method. In even the simplest experiment it is impossible to deal separately with any one of the three parts of the total psychophysical process.