Eleven years after the inaugural dissertation Kant {93} 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 {94} 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.