[Footnote 1:  From the remaining Schopenhauer literature (F.  Laban has published a chronological survey of it, 1880) we may call attention to the critiques of the first edition of the chief work by Herbart and Beneke, and that of the second edition by Fortlage (Jenaische Litteratur Zeitung, 1845, Nos. 146-151); J.E.  Erdmann Herbart und Schopenhauer, eine Antithese (Zeitschrift fuer Philosophie, 1851); Wilh.  Gwinner, Schopenhauers Leben, 1878 (the second edition of Schopenhauer aus persoenlichem Umgang dargestellt, 1862); Fr. Nietzsche, Schopenhauer als Erzieher (Unzeitgemaesse Betrachtungen, Stueck iii., 1874); O. Busch, A.  Schopenhauer, 2d. ed., 1878; K. Peters, Schopenhauer als Philosoph und Schriftsteller, 1880; R. Koeber, Die Philosophie A. Schopenhauers, 1888. [The English reader may be referred to Haldane and Kemp’s translation of The World as Will and Idea, 3 vols., 1883-86; the translation of The Fourfold Root and the Will in Nature in Bohn’s Philosophical Library, 1889; Saunders’s translations from the Parerga and Paralipomena, 1889 seq.; Helen Zimmern’s Arthur Schopenhauer, his Life and his Philosophy, 1876; W. Wallace’s Schopenhauer, Great Writers Series, 1890 (with a bibliography by Anderson, including references to numerous magazine articles, etc.); Sully’s Pessimism, 2d ed., 1882, chap. iv.; and Royce’s Spirit of Modern Philosophy, chap, viii., 1892.—­TR.]]

In regard to subjective idealism Schopenhauer confesses himself a thoroughgoing Kantian.  That sensations are merely states in us has long been known; Kant opened the eyes of the world to the fact that the forms of knowledge are also the property of the subject.  I know things only as they appear to me, as I represent them in virtue of the constitution of my intellect; the world is my idea.  The Kantian theory, however, is capable of simplification, the various forms of cognition may be reduced to a single one, to the category of causality or principle of sufficient reason—­which was preferred by Kant himself—­as the general expression of the regular connection of our representations.  This principle, in correspondence with the several classes of objects, or rather of representations—­viz., pure (merely formal) intuitions, empirical (complete) intuitions, acts of will, abstract concepts—­has four forms:  it is the principium rationis essendi, rationis fiendi, rationis agendi, rationis cognoscendi.  The ratio essendi is the law which regulates the coexistence of the parts of space and the succession of the divisions of time.  The ratio fiendi demands for every change of state another from which it regularly follows as from its cause, and a substance as its unchangeable substratum—­matter.  All changes take place necessarily, all that is real is material; the law of causality is valid for phenomena alone, not beyond them, and holds only for the states