doctrine of methods, treats of thought for its own distinct purpose. The former determines what the truth is; the latter determines how we ought to think. The didactic sciences are of two kinds: the sciences of the general principles or ends to be obtained, and the sciences of the means to attain these ends. Among the former is General Pedagogy, which determines the ends to be sought for in education. Psychology furnishes the foundation of fact; the science of general pedagogy judges which of these facts are desirable, in much the same way as epistemology judges which are true. In a former article (_Mind_ No. 56) Mr. Marshall showed that Pleasure and Pain are primitive qualities which, under proper conditions, _may_ appear with any psychosis, whatever be its content. He now finds that all the most notable pleasure-pain theories may in the first instance be placed in four groups, determined by the emphasis of certain kinds of pleasure or pain. An examination of pleasure-pain theories shows, _first_ that there is a general agreement, with but few dissenting voices, that all pleasure is at bottom the same thing, and that all pain in its essence is a single psychological phenomenon, and further that pleasures and pains are unifiable; _second_, that there are certain facts so marked in experience as to have become the basis of the majority of pleasure-pain theories. Mr. Marshall proceeds to consider the theory that “the activity of the organ of any content if efficient is pleasurable, if inefficient is painful.” He concludes that pleasures and pains are involved with the nutritive conditions of the active organ, and lays down the principle that “all pleasure-pain phenomena are determined by the action in the organs concomitant of the conscious state, as related to the nutritive conditions of the organs at the time of the action.” The difference between the hypernormality of pain and of pleasure, turns upon the fact that pleasure is obtained where the organ has been _rested_. Rest in an organ which is sometimes active means storage of energy derived from blood supply; and action after rest means the use of stored energy. But as action of an organ after rest gives a psychic content which is pleasurable, we have the working hypothesis: “Pleasure is experienced wherever the physical action which determines the content involves the use of stored force—the resolution of potential into actual energy; or, in other words, whenever the energy involved in the reaction to a stimulus is greater in amount than the energy of the stimulus.” By a similar process of reasoning we obtain the hypothesis: “Pain is experienced whenever the physical action which determines the content is so related to the supply of nutriment that the energy involved in the reaction to the stimulus is less in amount than the energy of the stimulus.” We may also say in general, “Pleasure and pain are primitive qualities of psychic states which are determined by the relation