An analogous position is tenable in the treatment of human play, although the word instinct, while generally applicable, is not universally so—a difficulty which is much more conspicuous here than in the classification of animal play. We lack a comprehensive and yet specific term for those unacquired tendencies which are grounded in our psycho-physical organism as such. The word instinct does not cover the ground with its commonly accepted definition as inherited association between stimuli and particular bodily reactions. Even the imitative impulse, which is responsible for the important group of imitative plays, is not easily included in this idea, because no specific reaction characterizes it.[3] It is safer, therefore, to speak of such play as the product of “natural or hereditary impulse,” although even that is not entirely satisfactory, since many psychologists connect the idea of impulse with a tendency to physical movement. There are undoubtedly deep-rooted requirements of our nature which this definition does not include, and which must be given due weight in our study of play. Thus, as Jodl, in agreement with Beaunis and others, maintains, every sensory tract has not only the ability to receive and act upon certain stimuli, but betrays itself originally through desire for their realization.[4] And if we keep in mind the tension toward special sensation, always present even in a state of comparative rest and distraction of the sense organ, as well as those external movements which are no longer the particular object of desire, we find ourselves still further from the narrow idea of instinct in relation to psycho-physical processes. In this dilemma we can only hold fast to the fact of the primal need for activity, which, while it can not, any more than the other, be included in the narrower use of the terms, has nevertheless an unmistakable relation to the life of impulse and instinct. And while it is true that mere intellectual fiat is not adequate to the establishment of such causal connections, one might be tempted, under the stress of dire need, to coin some such term as “central instinct,” did not any added burden threaten to plunge the already over-weighted term into a very chaos of obscurity. The case is much the same, too, with other mental attributes. Who is to decide whether it is lawful to assume a universal “impulse to activity” (Ribot approaches such an assumption)[5] which may, according to circumstances, become now effort after emotional excitement, now desire for logical expression and the like? Or who shall pass on the legitimacy of a revival of the hereditary central-impulse theory which directs attention not to external physical movement, but exclusively to such internal dispositions as are dependent on the psycho-physical organization? Should this latter view prevail, biological psychology will have before it the task of linking an ancient idea—it was