All that reason can reach, lies within the boundaries of experi ence itself ; it cannot recognize the nature of things in themselves, its sole task in theoretical cognition consists in "spelling phe nomena in order to be able to read them as experiences." The metaphysical rationalism of the dogmatic systems, as it is to be found in Descartes and Spinoza, Leibniz and Wolff, is thin opposed by the new attitude of critical rationalism. The latter, too, retains the fundamental idea of the older rationalism; namely, that reason can recognize completely only that which it can produce according to its own design—"that we know a priori only so much of things as we ourselves put into them." (Critique of Pure Reason, 2nd. ed.; p. 18.) But this kind of "construc tion" is possible only in so far as we deal not with things-in themselves, independently of all possible experience, but with experience itself and the conditions of its possibility. The under standing is able to recognize a priori and, as it were, to anticipate the form of experience in so far as it constitutes this form. It is, itself, "the legislation for nature"—but only in so far as we under stand by "nature" not the subsistence and constitution of abso lute objects, but the order and regularity of empirical phenomena. Thus rational cognition becomes fruitful only where, instead of dwelling in the world of "noumena," it concerns itself with phenomena. The characteristic of these, however, is that they form a spatio-temporal manifold. All efficacy of the "pure con cepts of understanding" is, therefore, necessarily confined to space and time, to the forms of "pure intuition." Only in connection with pure intuition does cognition by understanding or cognition by pure reason receive a real content.
This fundamental idea of Kant's critical rationalism is more explicitly developed in his doctrine of the "schematism of the pure concepts of understanding." The categories of thought are merely
directions for establishing certain relationships ; but these direc tions need to be referred to sensuousness. Thus, the concept of substance, for instance, does not mean the Being-in-itself as such (as conceived by Spinoza), but the persistence of an object in time, and it immediately loses all possibility of application when we abstract from the order of time. In general, the "schemata" of the pure concepts of understanding are nothing but "determina tions a priori of time according to rules," and these schemata are "the true and only conditions for securing for our concepts of understanding a reference to objects, i.e., significance. The categories are, therefore, only of empirical use, inasmuch as they merely serve to subject phenomena to general rules of synthesis and thus make them fit for coherent correlation in experience." (Critique of Pure Reason, and ed., p. 185.) Kantian rationalism, in other words, does not aim at the "existence of things," but at the form of experience itself, at the order and connection of phenomena. (Cf. TRANSCENDENTALISM.) The mistake of Empiricism consists in overlooking the intellec tual factor which is indispensable for any cognition of objects; the mistake of Rationalism consists in over-estimating this factor, in isolating it from the sensuous conditions upon which its ap plicability depends. "Leibniz intellectualized the phenomena, just as Locke altogether sensualized the concepts of understand ing, i.e., regarded them as nothing but empirical or abstracted con cepts of reflection." (Critique of Pure Reason, and ed., p. 327.) In this form of critical rationalism, the original problem from which the scientific rationalism of the Renaissance had started, is restated; but it is now solved in a new sense. The scientific ration alism of Kepler and Galilei, too, asserted that "experience is possible only through the representation of a necessary connection between perceptions." (Critique of Pure Reason, and ed., p. 2 18. ) But it was able to explain this necessity only by founding it on a metaphysical proposition, namely, the contention that the world itself is the work of an infinite intellect. Kant removes this de pendence of rationalism upon a proposition of dogmatic meta physics: he treats the truth of experience as self-sufficient, al though its form and order are based on the general orderliness of the understanding, without which they would be impossible.