In the essay on the principle of sufficient reason, § 23, I have fully explained the incorrectness of the following proof of the a priori nature and of the necessity of the law of causality from the mere succession of events in time; I must, therefore, content myself here by referring to that passage.6 This is precisely the case with the proof of reciprocity also, the concept of which I was obliged to explain above as invalid. What is necessary has also been said of modality, the working out of the principles of which now follows.
There are still a few points in the further course of the transcendental analytic which I should have to refute were it not that I am afraid of trying the patience of the reader; I therefore leave them to his own reflection. But ever anew in the “Critique of Pure Reason” we meet that principal and fundamental error of Kant's, which I have copiously denounced above, the complete failure to distinguish abstract, discursive knowledge from intuitive. It is this that throws a constant obscurity over Kant's whole theory of the faculty of knowledge, and never allows the reader to know what he is really speaking about at any time, so that instead of understanding, he always merely conjectures, for he alternately tries to understand what is said as referring to thought and to perception, and remains always in suspense. In the chapter “On the Division of all Objects into Phenomena and Noumena,” Kant carries that incredible want of reflection as to the nature of the idea of perception and the abstract idea, as I shall explain more fully immediately, so far as to make the monstrous assertion that without thought, that is, without abstract conceptions, there is no knowledge of an object; and that perception, because it is not thought, is also not knowledge, and, in general, is nothing but a mere affection of sensibility, mere sensation! Nay, more, that perception without conception is absolutely void; but conception without perception is yet always something (p. 253; V. 309). Now this is exactly the opposite of the truth; for concepts obtain all significance, all content, only from their relation to ideas of perception, from which they have been abstracted, derived, that is, constructed through the omission of all that is unessential: therefore if the foundation of perception is taken away from them, they are empty and void. Perceptions, on the contrary, have in themselves immediate and very great significance (in them, indeed, the thing in itself objectifies itself); they represent themselves, express themselves, have no mere borrowed content like concepts. For the principle of sufficient reason governs them only as the law of causality, and determines as such only their position in space and time; it does not, however, condition their content and their significance, as is the case with concepts, in which it appears as the principle of the ground of knowing. For the rest, it looks as if Kant really wished here to set about distinguishing the idea of perception and the abstract idea. He objects to Leibnitz and Locke that the former reduced everything to abstract ideas, and the latter everything to ideas of perception. But yet he arrives at no distinction; and although Locke and Leibnitz really committed these errors, Kant himself is burdened with a third error which includes them both – the error of having so mixed up knowledge of perception and abstract knowledge that a monstrous hybrid of the two resulted, a chimera of which no distinct idea is possible, and which therefore necessarily only confused and stupefied students, and set them at variance.
Certainly thought and perception are separated more in the chapter referred to “On the Division of all Objects into Phenomena and Noumena” than anywhere else, but the nature of this distinction is here a fundamentally false one. On p. 253; V. 309, it is said: “If I take away all thought (through the categories) from empirical knowledge, there remains absolutely no knowledge of an object, for through mere perception nothing at all is thought, and that this affection of sensibility is in me establishes really no relation of such ideas to any object.” This sentence contains, in some degree, all the errors of Kant in a nutshell; for it brings out clearly that he has falsely conceived the relation between sensation, perception, and thought, and accordingly identifies the perception, whose form he yet supposes to be space, and indeed space in all its three dimensions, with the mere subjective sensation in the organs of sense, but only allows the knowledge of an object to be given through thought, which is different from perception. I, on the contrary, say: Objects are first of all objects of perception, not of thought, and all knowledge of objects is originally and in itself perception. Perception, however, is by no means mere sensation, but the understanding is already active in it. The thought, which is added only in the case of men, not in the case of the brutes, is mere abstraction from perception, gives no fundamentally new knowledge, does not itself establish objects which were not before, but merely changes the form of the knowledge already won through perception, makes it abstract knowledge in concepts, whereby its concrete or perceptible character is lost, but, on the other hand, combination of it becomes possible, which immeasurably extends the range of its applicability. The material of our thought is, on the other hand, nothing else than our perceptions themselves, and not something which the perceptions did not contain, and which was added by the thought; therefore the material of everything that appears in our thought must be capable of verification in our perception, for otherwise it would be an empty thought. Although this material is variously manipulated and