end he was obliged to take in hand the important separation of our _a priori_ from our _a posteriori_ knowledge, which before him had never been carried out with adequate strictness and completeness, nor with distinct consciousness. Accordingly this now became the principal subject of his profound investigations. Now here we would at once remark that Kant’s philosophy has a threefold relation to that of his predecessors. First, as we have just seen, to the philosophy of Locke, confirming and extending it; secondly, to that of Hume, correcting and making use of it, a relation which is most distinctly expressed in the “_Prolegomena_” (that most beautiful and comprehensible of all Kant’s important writings, which is far too little read, for it facilitates immensely the study of his philosophy); thirdly, a decidedly polemical and destructive relation to the Leibnitz-Wolfian philosophy. All three systems ought to be known before one proceeds to the study of the Kantian philosophy. If now, according to the above, the distinction of the phenomenon from the thing in itself, thus the doctrine of the complete diversity of the ideal and the real, is the fundamental characteristic of the Kantian philosophy, then the assertion of the absolute identity of these two which appeared soon afterwards is a sad proof of the saying of Goethe quoted above; all the more so as it rested upon nothing but the empty boast of intellectual intuition, and accordingly was only a return to the crudeness of the vulgar opinion, masked under bombast and nonsense, and the imposing impression of an air of importance. It became the fitting starting-point for the still grosser nonsense of the clumsy and stupid Hegel. Now as Kant’s separation of the phenomenon from the thing in itself, arrived at in the manner explained above, far surpassed all that preceded it in the depth and thoughtfulness of its conception, it was also exceedingly important in its results. For in it he propounded, quite originally, in a perfectly new way, found from a new side and on a new path, the same truth which Plato never wearies of repeating, and in his language generally expresses thus: This world which appears to the senses has no true being, but only a ceaseless becoming; it is, and it is not, and its comprehension is not so much knowledge as illusion. This is also what he expresses mythically at the beginning of the seventh book of the Republic, the most important passage in all his writings, which has already been referred to in the third book of the present work. He says: Men, firmly chained in a dark cave, see neither the true original light nor real things, but only the meagre light of the fire in the cave and the shadows of real things which pass by the fire behind their backs; yet they think the shadows are the reality, and the determining of the succession of these shadows is true wisdom. The same truth, again quite differently presented, is also a