I now return to the great mistake of Kant, already touched on above, that he has not properly separated perceptible and abstract knowledge, whereby an inextricable confusion has arisen which we have now to consider more closely. If he had sharply separated ideas of perception from conceptions merely thought _in abstracto_, he would have held these two apart, and in every case would have known with which of the two he had to do. This, however, was unfortunately not the case, although this accusation has not yet been openly made, and may thus perhaps be unexpected. His “object of experience,” of which he is constantly speaking, the proper object of the categories, is not the idea of perception; neither is it the abstract conception, but it is different from both, and yet both at once, and is a perfect chimera. For, incredible as it may seem, he lacked either the wisdom or the honesty to come to an understanding with himself about this, and to explain distinctly to himself and others whether his “object of experience, _i.e._, the knowledge produced by the application of the categories,” is the idea of perception in space and time (my first class of ideas), or merely the abstract conception. Strange as it is, there always runs in his mind something between the two, and hence arises the unfortunate confusion which I must now bring to light. For this end I must go through the whole theory of elements in a general way. ‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐ The “Transcendental Æsthetic” is a work of such extraordinary merit that it alone would have been sufficient to immortalise the name of Kant. Its proofs carry such perfect conviction, that I number its propositions among incontestable truths, and without doubt they are also among those that are richest in results, and are, therefore, to be regarded as the rarest thing in the world, a real and great discovery in metaphysics. The fact, strictly proved by him, that a part of our knowledge is known to us _a priori_, admits of no other explanation than that this constitutes the forms of our intellect; indeed, this is less an explanation than merely the distinct expression of the fact itself. For _a priori_ means nothing else than “not gained on the path of experience, thus not come into us from without.” But what is present in the intellect, and has not come from without, is just what belongs originally to the intellect itself, its own nature. Now if what is thus present in the intellect itself consists of the general mode or manner in which it must present all its objects to itself, this is just saying that what is thus present is the intellect’s forms of knowing, _i.e._, the mode, fixed once for all, in which it fulfils this its function. Accordingly, “knowledge _a priori_” and “the intellect’s own forms” are at bottom only two expressions for the same things thus to a certain extent synonyms.