=2. In What does the Validity of the Understanding Consist?= Kant’s discussion of the synthesis of the understanding is given in the _Analytic_, the second part of his _Critique_. His treatment of the understanding is similar to that of perception. The understanding, be it remembered, is regarded by Kant as the second stage in the process of a complete synthesis of knowledge. It is synthesis on a higher level than perception. Indeed, perception is the material which the understanding synthesizes. As in the _Æsthetic_ Kant seeks to show: (1) the _a priori_ factors of the understanding and (2) that these _a priori_ factors give to knowledge its validity. The unifying principle of perception is the mathematical; but physical nature, which is the subject-matter of the study of the understanding, is more than mathematical, more than an aggregate of space and time forms, more than shapes and motions. Nature exists as a connected system of substances, causes, etc. Natural science possesses besides its mathematical basis a number of general _a priori_ principles for the validity of its conclusions. Kant’s task was therefore only begun by showing that perception possesses the universal and synthetic principles of space and time. Perception is only the beginning of knowledge. It is not knowledge, but only subjective consciousness. On the other hand, the understanding is the faculty of knowledge, and therefore Kant seeks to point out its _a priori_ or universal elements, and by their presence prove its validity.