It may of course be contended, by those who advocate the importance of the last-mentioned interpretations, that the variety of art-forms compels us to assume, not one, but several art-impulses. At this stage of our research we cannot enter upon a discussion of such views; but it will at least be admitted that explanations which can be applied in the whole field of art must be preferable to partial definitions. This merit of universality, at least, cannot be denied to the theories which derive art from the playing impulse. The notion of a sportive activity involves precisely that freedom from external, consciously utilitarian motives which, according to the consensus of almost all writers on æsthetic, is required in every genuine manifestation of art. It is not surprising, therefore, that it was by reference to the play-impulse that Schiller tried to distinguish artistic production from all “unfree” forms of activity.[32] It is true that the notion of “play” as used by Schiller and Spencer—who has given the theory a physiological foundation—is chiefly important as a negative demarcation. But even Schiller brings in a positive factor when he speaks of the force by which “overflowing life itself urges the animal to action” (“wenn das überflüssige Leben sich selbst zur Thätigkeit stachelt”).[33] In Spencer’s theory, on the other hand, the “excessive readiness” to nervous discharge which accompanies every surplus of vigour, and which, in his view, accounts for play, represents a motor element, the impelling force of which must be considered as very strong.[34] As is well known, Spencer, Wallace, and Hudson have applied this principle of surplus energy to explain so-called animal art, rejecting the theory which ascribes æsthetic judgment to the female.[35] As formulated by the last-mentioned authors, the play-theory is, however, open to objection from a physiological point of view. It has been remarked by Dr. Wallaschek that, in speaking of animals, the phrase “surplus of vigour” ought to be superseded by “inapplicability of energy” or “unemployed energy.”[36] And still more explicitly Professor Groos has shown that a stored-up supply of energy is by no means a necessary condition for play.[37] But these criticisms have by no means deprived the play-instinct of its importance as a dynamic factor. Since Groos by his epoch-making researches has been able to prove that the majority of games—especially the games of youth—are based upon instincts, we can adduce as an impelling force “the demon instinct that urges and even compels to activity not only if and so long as the vessel overflows (to use a figure of speech), but even when there is but a last drop left in it.”[38] By considering artistic activity as a kind of play, one is therefore able to account for its great attractiveness, even when no “surplus of vigour” can be shown to exist.