Those who adopt this attitude—which seems the more justified now that the arts have become widely differentiated—deny the possibility, not only of all general art-philosophy, but also of any sociological and psychological treatment of artistic activities as a whole. But even if all other hypotheses are banished, æsthetic research cannot possibly dispense with the fundamental assumption of the unity of art. And in point of fact there can be found in most systems, if we do not insist on too minute and positive demarcations, at least one common quality which is ascribed to all its different forms. Notwithstanding the mutual contradictions of art-theories, the believers in a general æsthetic can always appeal to the consent with which the majority of authors have upheld the negative criterion of art. Metaphysicians as well as psychologists, Hegelians as well as Darwinians, all agree in declaring that a work, or performance, which can be proved to serve any utilitarian, non-æsthetic object must not be considered as a genuine work of art. True art has its one end in itself, and rejects every extraneous purpose: that is the doctrine which, with more or less explicitness, has been stated by Kant,[5] Schiller,[6] Spencer,[7] Hennequin,[8] Grosse,[9] Grant Allen,[10] and others. And popular opinion agrees in this respect with the conclusions of science. This distinctive quality of independence seems therefore to afford us a convenient starting-point for the treatment of art in general.