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[.—''Am. J. Ps.=The American Journal of Psychology; Ar. f. G. Ph.=Archiv für Geschichte der Philosophie; Int. J. E.=International Journal of Ethics; Phil. Stud=Philosophische Studien; Rev. Ph.=Revue Philosophique; R. I. d. Fil.=Rivista Italiana di Filosofia; V. f. w. Ph.=Vierteljahrschrift für wissenschaftliche Philosophie; Z. f. Ph.=Zeitschrift für Philosophie und philosophische Kritik; Z. f. Ps. u. Phys. d. Sinn.=Zeitschrift für Psychologie und Physiologie der Sinnesorgane; Phil. Jahr.=Philosophisches Jahrbuch; Rev. de Mét.=Revue de Métaphysique et de Morale; Ar. f. sys. Ph.=Archiv für systematische der Philosophie.''—Other titles are self-explanatory.]

The first stage of knowledge is very humble: it is that of everyday, practical knowledge. Primum vivere, deinde philosophare. The earliest work of the mind is to form things. What one ordinarily calls a fact is an adaptation of the real to practical interests and to the demands of social life. In order to overcome the original confusion, we separate, form groups, establish uniformities, fix permanencies, and thus create separate things, centres of convergence for action, places of repose for thought. As we act principally by contact, tactual and muscular impressions most commonly determine the limits of objects. An object of common experience, far from being a pure datum, is already an abstraction, a symbol of our power of acting. The human mind is like a prism which separates elements of sensation that are undecomposable for the non-reflective consciousness. The principal elements or aspects of sensation are intensity, tonality, purity, and extensity. Extensity expresses the amplitude of the acts which sensation proposes to us to execute, and defines the zone of indeterminateness for such reaction; hence it is a property of our reflective view of matter. The opinion is gaining ground among psychologists that extensity belongs, not merely to certain kinds of sensations, but to all. Much is to be said for this view; but we attend to extensity only in sensations coördinated with useful motions, with the result that visual or tactual extensity—more convenient, stable, and interesting—makes us neglect other extensities These, together with muscular extensity, fuse together to form what may be called 'imaginable' space. This last presents itself as a convenient and suggestive notation, through which we learn to perceive a 'logical function,' geometrical space, which we otherwise construct, and which corresponds to our unqualified power of differentiation. We necessarily think by means of common ideas, and are never able, in the ensemble of these latter, to examine thoroughly more than a few isolated points.