What is Critique of Pure Reason about?

The Critique of Pure Reason is Kant's attempt to resolve a crisis in 18th-century philosophy by asking what human reason can and cannot know. The crisis had been produced by Hume's skepticism: if all knowledge comes from experience, then causal necessity, mathematical certainty, and metaphysical claims about God and the soul are all on shaky ground. Kant accepted Hume's challenge but rejected his conclusion. His solution was to invert the problem: instead of asking how the mind conforms to objects, ask how objects conform to the mind.

The first major section establishes that space and time are not features of the external world that we discover but forms of human intuition through which we organize experience. Everything we perceive is already structured spatially and temporally before any concept is applied. This is Kant's "transcendental aesthetic," and it grounds the possibility of mathematics: geometry is certain because it describes the structure of our perception, not contingent facts about the world.

The second section — the transcendental analytic — argues that the understanding applies twelve pure concepts (categories), including causality, substance, and necessity, to the raw material of intuition. These categories make experience possible, but they also set its limits: they apply only within experience, never beyond it. Kant distinguishes between phenomena (things as they appear to us) and noumena (things as they are in themselves). We have access only to phenomena; the thing-in-itself remains permanently beyond our reach.

The longest section, the transcendental dialectic, analyzes the three great objects of traditional metaphysics — the soul, the world as a whole, and God — and shows that reason falls into antinomies and paralogisms when it tries to know these directly. We cannot prove or disprove the existence of God or the immortality of the soul by pure reason alone. This is not skepticism but a critical restriction: these are not objects of knowledge but of rational faith. The Critique is Kant's attempt to save science from Hume's skepticism and save religion and morality from the pretensions of dogmatic metaphysics.