developed in Ulrici’s Leib und Seele in 1866—to the body of modern science. As it is likely to be some time yet before scientific terminology shall have attained such clearness and perfection in a sphere by no means easily accessible, that we may count on banishing all obscurity, I must content myself with the term “natural or inherited impulse”[6] as the basis of my classification. In far the greater number of cases it is equivalent to simple instinct. But in the imitative impulse we have something which is _analogous_ only to instinct, and in reference to the higher mental dispositions to activity, the term “impulse” must be expanded beyond its usual significance. I am well aware that my classification lacks precision, but I venture to think that it affords deeper insight into the problem than may be had by other means and that some aspects of the subject, not evident from other standpoints, may be brought out by this method of treatment. The first important distinction made is that between the impulses by which the individual wins supremacy over his own psycho-physical organism without regard to other individuals prominent in his environment, and such other impulses as are directly concerned with his relations to others. To the first group belong all the manifold impulses which issue in human activity, those controlling his sensory and motor apparatus[7] as well as the higher mental dispositions which impel him to corresponding acts. To the second group we assign the fighting and sexual impulses, imitation, and the social dispositions closely connected with these. Each of these manifests its own peculiar play activity. Unfortunately, an adequate terminology here, too, is wanting, and as the opposites “egotism and altruism,” “individualism and socialism,” are not admissible in our classification, it is difficult to designate the two groups with propriety. While awaiting better names for them, I am forced to the very unsatisfactory expedient of calling them impulses of the first order and impulses of the second order.[8] To denote the playful exercise of the first order of impulses, I shall use the expression “playful experimentation,” which is already adopted in child-psychology, and also, by myself at least, in animal psychology. As all further subdivisions will be effected without difficulty in the course of our investigation, I add here only a brief note on the general characteristics of the playful exercise of these impulses. The biological criterion of play is that it shall deal not with the serious exercise of the special instinct, but with practice preparatory to it. Such practice always responds to definite needs, and is accompanied by pleasurable feelings. The psychological criterion corresponds with it; thus, when an act is performed solely because of the pleasure it affords, there is play. Yet, the consciousness of engaging in sham occupation is not a universal criterion of play. PART I _PLAYFUL EXPERIMENTATION_