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 the understanding. His letters and notes enable us to follow the gradual development of this deeper insight. We are active in the operations of our own thought, i. e. we act in a manner peculiar to our mind; but how can the products of our own mental activity retain their validity when applied to the perceptions which are objectively produced?—As to the nature of this mental activity, an investigation of the fundamental concepts of our understanding, especially the causal concept, reveals the fact that the understanding is likewise a uniting, synthetizing faculty like sense-perception. The uniting principle (Hume's), which was the stumbling-block of Kant's English predecessor, now became Kant's fundamental presupposition of knowledge. He could now say of the fundamental concepts of the understanding (categories), after the analogy of what he had previously said of the forms of intuition: Knowledge exists only when what is given (the matter) in the forms of our thought is united. The concept of synthesis is therefore the fundamental concept of all knowledge and the profoundest thought of the Kantian philosophy. This constitutes Kant's real discovery, which will justify its value, even if Kant's particular theories are to a considerable degree subject to criticism. We must apply his own method in the study of Kant. We must penetrate the finished forms in which his philosophy is cast and discover their primary principles—realities.

According to his own statement, Kant wrote out the results of his reflections covering a period of twelve years quite hastily. His chief work, Kritik der reinen Vernunft (1781), is therefore a very difficult book.—In presenting its contents we shall follow a clearer order than that given by Kant himself.