published his most important contribution to philosophy, The Critique of Pure Reason (1781). Pure Reason means the faculty by which ideas are obtained independently of all experience, and the critic's object is to ascertain how far such ideas are valid. As a preliminary to that inquiry the question is also mooted, How is experience possible? It is answered by a critique of the understanding or faculty of conception; and as conception implies perception, this again is prefaced by a section in which Kant's theory of space and time is repeated and reinforced.
It will be remembered that what started the whole of the new criticism was Hume's sceptical analysis of Causation; and the central interest of The Critique of Pure Reason lies in the effort to reconstitute the causal law in the light of the new theory of knowledge; but so enormous is the mass of technicalities piled up for this purpose as largely to conceal it from view, and, on its disclosure, to give the idea of a gigantic machine set in motion to crack a nut. And the nut after all is not cracked; the shell slips from between the grappling surfaces long before they meet.
We have seen how Kant interpreted every judgment as a synthesis of subject and predicate. Now, whether the synthesis be à priori or à posteriori, a study of the forms of judgment as enumerated in the common logic shows that there are four, and only four, ways in which it can be effected. All judgments fall under the following classes: Quantity, Quality, Relation, and Modality—terms whose meaning will be presently explained. And each of these again is tripartite. We may say (i.) that one A is B, or that some A's are B, or that all A's are B; (ii.) that A is B, that A is not B, that not all A's are B; (iii.) that A is B, that A
is B if C is D, that A is either B, C, or D; or (iv.) that A may be B, that A is B, or that A must be B. The reason why there are four and only four classes is that judgment has to do with the subject in reference to the predicate, which gives Quantity; with the predicate in reference to the subject, which gives Quality; with the connection between the two, which gives Relation; and with the synthesis between them in reference to our knowledge of it, which gives Modality.
Now, according to Kant, that there should be so many kinds of judgment and no more implies that our understanding contributes a formal element to the constitution of all knowledge, consisting of four combining principles, without which experience would be impossible. He calls these Categories, and they are enumerated in the following table:—
(i.) Quantity.
Unity, Plurality, Totality.
(ii.) Quality.
Reality, Negation, Limitation.
(iii.) Relation.
Substance and Accident; Cause and Effect; Action and Reaction (Reciprocity).
(iv.) Modality.
Possibility and Impossibility; Existence and Non-Existence; Necessity and Contingency.
A study of the Categories suggests some rather obvious criticisms on the Critical Philosophy itself. (i.) The first two terms in each triad evidently form an antithetical couple, of which the third term is the synthesis. Here we have the first germ of a disease by which the systems of Kant's successors were much more seriously infected. In the table it is shown by
the intrusion of Limitation, a wholly superfluous adjunct to Reality and Negation; in the conversion of Reciprocity into a wholly fictitious synthesis of Substantiality with Causation; and in the complete absurdity of making Necessity a combination of Possibility with Existence. (ii.) Innate ideas, after they had been exploded by Locke, are reintroduced into philosophy by a sufficiently transparent piece of legerdemain. For assuming that the human intelligence possesses a power of organising and drilling the sensuous appearances which without its control would appear only as a disorderly mob, it by no means follows that they must thereby be referred to an extraphenomenal principle. But such a principle is plainly implied by the category of Substance. Used in a scholastic sense, it does not mean the sensuous attributes of a thing taken altogether, but something that underlies and supports them. And Kant himself seems to take his category in that significance. For he claims to deduce from it the law of the indestructibility of matter; as if I could not say snow is white without committing myself to the assertion that the ultimate particles of snow have existed and will exist for ever. (iii.) The substitution of Causation for logical sequence, as implicated in the hypothetical judgment of Relation, is perfectly scandalous; and still more scandalous is substitution of Reciprocity or Action and Reaction for Disjunction. The last points require to be examined a little more in detail.
The sequence of an effect to its cause has only a verbal resemblance to the sequence of a logical consequent to its reason. We declare categorically that every change has a cause which precedes it. Logical sequence is, on the other hand, as the very name of the
judgment shows, hypothetical, and may possibly not represent any actual occurrence, besides being, what causation is not, independent of time. A particular case of causation may be hypothetical in respect to our belief that it actually occurred; never the law of causation itself as a general truth. And the same distinction applies with even greater force to the alleged connection between a logical disjunction and a physical reaction. When I say A is either B or C, but not both, there is only this much resemblance, that both cases involve the ideas of equality and of opposition. From the admission that A is not B, I infer that it is C, or, contrariwise, from the admission that it is B, I infer that it is not C, and in both instances with the same certainty; but this does not prove that the earth attracts the moon as much as the moon attracts the earth, only in opposite directions; nor yet that in certain instances all the heat lost by one body is gained by another.