=The Proof of the Validity of Human Knowledge.= If we turn now to review what we have said about Kant, we find that he undertakes to solve the problem, _How can we know?_ by a critical study of the forms of the reason. We have found that the reason is essentially a synthetic power, and is the framework of the world of phenomena to which knowledge is limited. Knowledge is the complex thing, consisting of sensations as its woof and synthesis as its warp. To answer the question, Under what conditions is knowledge possible? we must study not sensations, but synthesis in its several forms. If Kant can show that the mind furnishes the _a priori_, that is, the universal and necessary forms to knowledge, he thinks he has proved his case. He has then explained why human knowledge is valid and thus proved that human knowledge _is_ valid. Now Kant tries to show what the special _a priori_ forms of knowledge are and in what the validity of such forms consists. In the first book of the _Critique of Pure Reason_, the _Æsthetic_, he undertakes to show what the _a priori_ forms of mathematics are and how they make knowledge valid by being forms of mental synthesis. In the next part of the _Critique_, the _Analytic_, he tries to show what the _a priori_ forms of the knowledge of physical science are and how they make physical science valid and objective. In the last part, the _Dialectic_, he discusses the _a priori_ forms of the reason and shows why they have no validity in knowledge. These are three stages in which the knowing activity develops as three different forms of synthesis. The stages are perception, understanding, and reason. Each higher stage has the lower as its content. Finished knowledge involves perceptions, reproductions in the understanding, and a recognition of the whole by a thinking subject. Perception, understanding, and reason are not separate acts, but different levels of one consciousness. These will be taken up in succession. =1. In What does the Validity of Sense-Perception Consist?= Kant points out: (1) Sense-perception has (a) a content of sense qualities, like sound, color, etc., and (b) the relations of space and time. (2) Space and time originally belong to the subject as its forms of sense-perception, and are not introduced from without by experience. (3) By means of space and time _a priori_ knowledge is possible. If there is any validity in perceptual knowledge, it depends upon the constitution of space and time; not upon the character of the empirical content, or the sensations. The question about the validity of sense-perception, then, is a question about the reliability of mathematics.