present work, but expounded at full length in my prize essay on the freedom of the will, p. 30-33. Two things in nature remain untouched by that chain of causality which stretches into infinity in both directions; these are matter and the forces of nature. They are both conditions of causality, while everything else is conditioned by it. For the one (matter) is that _in_ which the states and their changes appear; the other (forces of nature) is that by virtue of which alone they can appear at all. Here, however, one must remember that in the second book, and later and more thoroughly in “The Will in Nature,” the natural forces are shown to be identical with the will in us; but matter appears as the mere _visibility of the will_; so that ultimately it also may in a certain sense be regarded as identical with the will. On the other hand, not less true and correct is what is explained in § 4 of the first book, and still better in the second edition of the essay on the principle of sufficient reason at the end of § 21, p. 77 (third edition, p. 82), that matter is causality itself objectively comprehended, for its entire nature consists in _acting in general_, so that it itself is thus the activity (ενεργεια = reality) of things generally, as it were the abstraction of all their different kinds of acting. Accordingly, since the essence, _essentia_, of matter consists in _action in general_, and the reality, _existentia_, of things consists in their materiality, which thus again is one with action in general, it may be asserted of matter that in it _existentia_ and _essentia_ unite and are one, for it has no other attribute than _existence itself_ in general and independent of all fuller definitions of it. On the other hand, all _empirically_ given matter, thus all material or matter in the special sense (which our ignorant materialists at the present day confound with matter), has already entered the framework of the _forms_ and manifests itself only through their qualities and accidents, because in experience every action is of quite a definite and special kind, and is never merely general. Therefore pure matter is an object of _thought_ alone, not of _perception_, which led Plotinus (_Enneas II._, lib. iv., c. 8 & 9) and Giordano Bruno (_Della Causa_, dial. 4) to make the paradoxical assertion that matter has no extension, for extension is inseparable from the form, and that therefore it is _incorporeal_. Yet Aristotle had already taught that it is not a body although it is corporeal: “σωμα μεν ουκ αν ειη, σωματικη δε” (_Stob. Ecl._, lib. i., c. 12, § 5). In reality we think under _pure matter_ only action, in the abstract, quite independent of the _kind_ of action, thus _pure causality_ itself; and as such it is not an _object_ but a _condition_ of experience, just like space and time. This is the reason why in the accompanying table of our pure _a priori_ knowledge matter is able to take the place of causality, and therefore