Such knowledge is called a priori and is distinguished from empirical knowledge.[214]—Throughout the Introduction, in both editions equally, Kant fails to state the problems of the Critique in a sufficiently comprehensive manner. He speaks as if the Critique dealt only with the absolutely a priori, in its two forms, as immanent scientific knowledge and as transcendent speculation. It also deals with the equally important and still more fundamental problem of accounting for the possibility of experience.[215] Our empirical knowledge involves an a priori element, and may not therefore be opposed to a priori knowledge in the manner of the passage before us.
This term a priori is not yet definite enough.[216]—It is frequently employed in a merely relative sense. Thus we can say of a person who undermines the foundations of his house that he might have known a priori that it would collapse, that is, that he need not wait for the experience of its actual fall. But still he could not know this entirely a priori; he had first to learn from experience that bodies are heavy, and will fall when their supports are taken away. But as dealt with in the Critique the term a priori is used in an absolute sense, to signify that knowledge which is independent, not of this or that experience only, but of all impressions of the senses. Thus far Kant’s position is comparatively clear; but he proceeds to distinguish two forms within the absolutely a priori, namely, mixed and pure. The absolutely a priori is mixed when it contains an empirical element, pure when it does not. (“Pure” is no longer taken in the meaning which it has in the title of the section.[217] It signifies not the a priori as such, but only one subdivision of it.) Thus after defining absolutely a priori knowledge as independent of all experience, Kant takes it in one of its forms as involving empirical elements. The example which he gives of an absolutely a priori judgment, which yet is not pure, is the principle: every change has its cause. “Change” is an empirical concept, but the synthetic relation asserted is absolutely a priori. In the next section[218] this same proposition is cited as a pure judgment a priori—“pure” being again used in its more general meaning as synonymous with a priori. This confusion results from Kant’s exclusive preoccupation with the a priori, and consequent failure to give due recognition to the correlative problem of the empirical judgment. The omitted factor retaliates by thus forcing its way into Kant’s otherwise clean-cut divisions. Also, it is not true that the relative a priori falls outside the sphere of the Critical enquiry. Such judgment expresses necessity or objectivity, and for that reason demands a transcendental justification no less urgently than the absolutely a priori. The finding of such justification is, indeed, the central problem of the Analytic.[219]
The subdivisions of the a priori may be tabulated thus:
| A priori knowledge— | Relative, e.g. every unsupported house mustfall. | |
| Absolute— | Mixed, e.g. every change has its cause. Pure, e.g. a straight line is the shortest distance between two points. | |
The term pure (rein) thus acquires a second meaning distinct from that defined above.[220] It is no longer employed as identical with a priori, but as a subdivision of it, meaning unmixed. Its opposite is no longer the empirical, but the impure or mixed. Owing, however, to the fact that “pure” (in its first meaning) is identical with the a priori, it shares in all the different connotations of the latter, and accordingly is also employed to denote that which is not relative. But “pure” has yet another meaning peculiar to itself. The phrase “independent of experience” has in reference to “pure” an ambiguity from which it does not suffer in its connection with “a priori” (since mathematical knowledge, whether pure or applied, is always regarded by Kant as a priori). It may signify either independence as regards content and validity, or independence as regards scope. The latter meaning is narrower than the former. By the former meaning it denotes that which originates, and can possess truth, independently of experience. By the latter it signifies that which is not only independent of sense but also applies to the non-sensuous. In this latter meaning pure knowledge therefore signifies transcendent knowledge. Its opposite is the immanent. The various meanings of “pure” (four in number) may be tabulated as follows:
All these varied meanings contribute to the ambiguity of the title of the Critique. Kant himself employs the title in all of the following senses:
1. Critique of absolutely pure a priori knowledge, determination of its sources, conditions, scope and limits.
2. Critique of all a priori knowledge, relative as well as absolute, in so far as it depends upon a priori principles, determination, etc.
3. Critique of all knowledge, whether a priori or empirical, determination, etc.