[Footnote 1: The task of confirming the existence of things in themselves changes under his hands into another, that of proving the existence of external phenomena. "That external objects are real as representations" Berkeley had never disputed.]
On the basis of the inseparability of sensibility and understanding the ideal of knowledge—an extension of knowledge to be attained by a priori means (p. 333)—experiences a remarkable addition in the position that the rational synthesis thus obtained must be a knowledge of reality, must be applied to matter given in intuition. To the question, "How are synthetic judgments a priori possible?" is joined a second equally legitimate inquiry, "How do they become objectively valid, or applicable to objects of experience?" The principle from which their validity is proved—they are applicable to objects of experience because without them experience would not be possible, because they are conditions of experience—like the criterion of apriority (strict universality and necessity), is one of the noëtic assumptions of the critical theory.[1]
[Footnote 1: Cf. Vaihinger, Kommentar, i. pp. 425-430.]
Inasmuch as its investigation relates to the conditions of experience the Kantian criticism follows a method which it itself terms transcendental. Heretofore, when the metaphysical method had been adopted, the object had been the suprasensible; and when knowledge had been made the object of investigation, the method followed had been empirical, psychological. Kant had the right to consider himself the creator of noëtics, for he showed it the transcendental point of view. Knowledge is an object of experience, but its conditions are not. The object is to explain knowledge, not merely to describe it psychologically,—to establish a new science of knowledge from principles, from pure reason. That which lies beyond experience is sealed from our thought; that which lies on this side of it is still uninvestigated, though capable and worthy of investigation, and in extreme need thereof. Criticism forbids the transcendent use of reason (transcending experience); it permits, demands, and itself exercises the transcendental[1] use of it, which explains an experiential object, knowledge, from its conditions, which are not empirically given.
[Footnote A: Kant applies the term transcendental to the knowledge (the discovery, the proof) of the a priori factor and its relation to objects of experience. Unfortunately he often uses the same word not only to designate the a priori element itself, but also as a synonym for transcendent. In all three cases its opposite is empirical, namely, empirico-psychological investigation by observation in distinction from noëtical investigation from principles; empirical origin in distinction from an origin in pure reason, and empirical use in distinction from application beyond the limits of experience.]
There is, apparently, a contradiction between the empiristic result of the Critique of Reason (the limitation of knowledge to objects of experience) and its rationalistic proofs (which proceed metaphysically, not empirically), and, in fact, a considerable degree of opposition really exists. Kant argues in a metaphysical way that there can be no metaphysics. This contradiction is solved by the distinction which has been mentioned between that which is beyond, and that which lies within, the boundary of experience. That metaphysic is forbidden which on the objective side soars beyond experience, but that pure rational knowledge is permissible and necessary which develops from principles the grounds of experiential knowledge existing in the subject. In the Kantian school, however, these complementary elements,—empirical result, transcendental or metaphysical, properly speaking, pro-physical method,—were divorced, and the one emphasized, favored, and further developed at the expense of the other. The empiricists hold to the result, while they either weaken or completely misunderstand the rationalism of the method: the a priori factor, says Fries, was not reached by a priori, but by a posteriori, means, and there is no other way by which it could have been reached. The constructive thinkers, Fichte and his successors, adopt and continue the metaphysical method, but reject the empirical result. Fichte's aim is directed to a system of necessary, unconscious processes of reason, among which, rejecting the thing in itself, he includes sensation. According to Schelling nature itself is a priori, a condition of consciousness. This discrepancy between foundation and result continues in an altered form even among contemporary thinkers—as a discussion whether the "main purpose" of Criticism is to be found in the limitation of knowledge to possible experience, or the establishment of a priori elements—though many, in adherence to Kant's own view, maintain that the metaphysics of knowledge and of phenomena (immanent rationalism) is the only legitimate metaphysics.
%1. Theory of Knowledge.
(a) The Pure Intuitions (Transcendental Aesthetic).%—The first part of the Critique of Reason, the Transcendental Aesthetic, lays down the position that space and time are not independent existences, not real beings, and not properties or relations which would belong to things in themselves though they were not intuited, but forms of our intuition, which have their basis in the subjective constitution of our, the human, mind. If we separate from sensuous intuition all that the understanding thinks in it through its concepts, and all that belongs to sensation, these two forms of intuition remain, which may be termed pure intuitions, since they can be considered apart from all sensation. As subjective conditions (lying in the nature of the subject) through which alone a thing can become an object of intuition for us, they precede all empirical intuitions or are a priori.
Space and time are neither substantial receptacles which contain all that is real nor orders inhering in things in themselves, but forms of intuition. Now all our representations are either pure or empirical in their origin, and either intuitive or conceptual in character. Kant advances four proofs for the position that space and time are not empirical and not concepts, but pure intuitions: (1) Time is not an empirical concept which has been abstracted from experience. For the coexistence or succession of phenomena, i.e., their existence at the same time or at different times (from which, as many believe, the representation of time is abstracted), itself presupposes time—a coexistence or succession is possible only in time. It is no less false that space is abstracted from the empirical space relations of external phenomena, their existence outside and beside one another, or in different places, for it is impossible to represent relative situation except in space. Therefore experience does not make space and time possible; but space and time first of all make experience possible, the one outer, the other inner experience. They are postulates of perception, not abstractions from it. (2) Time is a necessary representation a priori. We can easily think all phenomena away from it, but we cannot remove time itself in view of phenomena in general; we can think time without phenomena, but not phenomena without time. The same is true of space in reference to external objects. Both are conditions of the possibility of phenomena. (3) Time is not a discursive or general concept. For there is but one time. And different times do not precede the one time as the constituent parts of which it is made up, but are mere limitations of it; the part is possible only through the whole. In the same way the various spaces are only parts of one and the same space, and can be thought in it alone. But a representation which can be given only by a single object is a particular representation or an intuition. Because, therefore, of the oneness of space and time, the representation of each is an intuition. The a priori, immediate intuition of the one space is entirely different from the empirical, general conception of space, which is abstracted from the various spaces. (4) Determinate periods of time arise by limitation of the one, fundamental time. Consequently this original time must be unlimited or infinite, and the representation of it must be an intuition, not a concept. Time contains in itself an endless number of representations (its parts, times), but this is never the case with a generic concept, which, indeed, is contained as a partial representation in an endless number of representations (those of the individuals having the same name), and, consequently, comprehends them all under itself, but which never contains them in itself. The general concept horse is contained in each particular representation of a horse as a general characteristic, and that of justice in each representation of a definite just act; time, however, is not contained in the different times, but they are contained in it. Similarly the relation of infinite space to the finite spaces is not the logical relation of a concept to examples of it, but the intuitive relation of an unlimited whole to its limited parts.
The Prolegomena employs as a fifth proof for the intuitive character of space, an argument which had already appeared in the essay On the Ultimate Ground of the Distinction of Positions in Space. There are certain spatial distinctions which can be grasped by intuition alone, and which are absolutely incapable of comprehension through the understanding—for example, those of right and left, above and below, before and behind. No logical marks can be given for the distinction between the object and its image in the mirror, or between the right ear and the left. The complete description of a right hand must, in all respects (quality, proportionate position of parts, size of the whole), hold for the left as well; but, despite the complete similarity, the one hand cannot be exactly super-imposed on the other; the glove of the one cannot be worn on the other. This difference in direction, which has significance only when viewed from a definite point, and the impossibility mentioned of a congruence between an object (right hand) and its reflected image (left hand) can be understood only by intuition; they must be seen and felt, and cannot be made clear through concepts, and, consequently, can never be explained to a being which lacks the intuition of space.