In any detailed exposition of the critical system it would be requisite in the first place to state with some fullness the precise nature of the problems immediately before Kant, and in the second place to follow with some closeness the successive stages of the system as presented in the three main works, the Critique of Pure Reason, the Critique of Practical Reason and the Critique of Judg ment, with the more important of the minor works, the Metaphysic of Nature and the Metaphysic of Ethics. It would be necessary, also, in any such expanded treatment, to bring out clearly the Kantian classification of the philosophical sciences, and to indicate the relation between the critical or transcendental investigation of the several faculties and the more developed sciences to which that investigation serves as introduction. As any detailed statement of the critical system, however compressed, would be beyond the limits of the present article, it is proposed here to select only the more salient doctrines, and to point out in connection with them what advance had been effected by Kant, and what remained for subsequent efforts at full solution of the problems raised by him.
The doctrine from which Kant starts in his critical or transcen dental investigation of knowledge is that to which the slow devel opment of his thought had led him. The essence of cognition or knowledge was a synthetic act, an act of combining in thought the detached elements of experience. Now synthesis was explicable neither by reference to pure thought, the logical or elaborative faculty, which in Kant's view remained analytic in function, nor by reference to the effects of external real things upon our faculties of cognition. For, on the one hand, analysis or logical treatment applied only to objects of knowledge as already given in synthetic forms, and, on the other hand, real things could yield only isolated effects and not the combination of these effects in the forms of cognitive experience. If experience is to be matter of knowledge for the conscious subject, it must be regarded as the conjoint prod Uct of given material and synthetic combination. Form and mat ter may indeed be dealt with in isolation for purposes of critical inquiry, but in experience they are inseparably united. The prob lem of the Kritik thus becomes the complete statement of the ele ments necessarily involved in synthesis, and of the processes by which these elements are realized in individual consciousness. He is not asking, with Locke, whence the details of experience arise; but he is endeavouring to state exhaustively what conditions are necessarily involved in knowledge, i.e., in any synthetic combina tion of parts of experience by the conscious subject.
So far as the elements necessarily involved in conscious expe rience are concerned, these may be enumerated briefly thus : given data of sense, inner or outer : the forms of perception, i.e.,
space and time ; the forms of thought, i.e., the categories ; the ulti mate condition of knowledge, the identity of the pure ego or self. The self is the central unity in reference to which alone is any part of experience cognizable. But the consciousness of self is the foundation of knowledge only when related to given material. The ego has not in itself the element of difference, and the essence of knowledge is the consciousness of unity in difference. For knowl edge, therefore, it is necessary that difference should be given to the ego. The modes under which the isolated data can be syntheti cally combined so as to form a cognizable whole make up the form of cognition, and upon this form rests the possibility of any a priori or rational knowledge.
Data of sense-affection do not contain in themselves synthetic combination. The first conditions of such combination are found by Kant in the universal forms under which alone sense-phe nomena manifest themselves in experience. These universal forms of perception, space and time, are necessary, a priori, and in char acteristic features resembling intuitions, not notions. They occupy, therefore, a peculiar position, and one section of the Kritik, the Aesthetik, is entirely devoted to the consideration of them. It is only through a priori character of these perceptive forms that rational science of nature is at all possible. Kant is here able to resume, with fresh insight, his previous discussions regarding the synthetic character of mathematical propositions. In his early essays he had rightly drawn the distinction between mathematical demonstration and philosophic proof, referring the certainty of the first to the fact that the constructions were synthetic in char acter and entirely determined by the action of constructive imagi nation. It had not then occurred to him to ask, With what right do we assume that the conclusions arrived at from arbitrary con structions in mathematical matter have applicability to objects of experience? Might not mathematics be a purely imaginary science? To this question he is now enabled to return an answer. Space and time, the two essential conditions of sense-perception, are not data given by things, but universal forms of intellect into which all data of sense must be received. Hence, whatever is true of space and time regarded by imagination as objects, i.e., quanti tative constructions, must be true of the objects making up our sense-experience. The same forms and the same constructive ac tivity of imagination are involved in mathematical synthesis and in the constitution of objects of sense-experience. The foundation for pure or rational mathematics, there being included under this the pure science of movement, is thus laid in the critical doctrine of space and time.