Therefore from the doctrine of the Transcendental Æsthetic I knew of nothing to take away, only of something to add. Kant did not carry out his thought to the end, especially in this respect, that he did not reject Euclid’s whole method of demonstration, even after having said on p. 87; V. 120, that all geometrical knowledge has direct evidence from perception. It is most remarkable that one of Kant’s opponents, and indeed the acutest of them, G. E. Schulze (_Kritik der theoretischen Philosophie_, ii. 241), draws the conclusion that from his doctrine an entirely different treatment of geometry from that which is actually in use would arise; and thus he thought to bring an apagogical argument against Kant, but, in fact, without knowing it, he only began the war against the method of Euclid. Let me refer to § 15 of the first book of this work. After the full exposition of the universal _forms_ of perception given in the Transcendental Æsthetic, one necessarily expects to receive some explanation as to its _content_, as to the way in which the _empirical_ perception comes into our consciousness, how the knowledge of this whole world, which is for us so real and so important, arises in us. But the whole teaching of Kant contains really nothing more about this than the oft-repeated meaningless expression: “The empirical element in perception is _given_ from without.” Consequently here also from the _pure forms of perception_ Kant arrives with one spring at _thinking_ at the _Transcendental Logic_. Just at the beginning of the Transcendental Logic (Critique of Pure Reason, p. 50; V. 74), where Kant cannot avoid touching upon the content of the empirical perception, he takes the first false step; he is guilty of the πρωτον ψευδος. “Our knowledge,” he says, “has two sources, receptivity of impressions and spontaneity of conceptions: the first is the capacity for receiving ideas, the second that of knowing an object through these ideas: through the first an _object_ is given us, through the second it is thought.” This is false; for according to it the _impression_, for which alone we have mere receptivity, which thus comes from without and alone is properly “given,” would be already an _idea_, and indeed an _object_. But it is nothing more than a mere _sensation_ in the organ of sense, and only by the application of the _understanding_ (_i.e._, of the law of causality) and the forms of perception, space and time, does our _intellect_ change this mere _sensation_ into an _idea_, which now exists as an object in space and time, and cannot be distinguished from the latter (the object) except in so far as we ask after the thing in itself, but apart from this is identical with it. I have explained this point fully in the essay on the principle of sufficient reason, § 21. With this, however, the work of the understanding and of the faculty of perception is completed, and no conceptions and no