Book: Critique of Pure Reason

Overview
Kant's Critique of Pure Reason (1781) examines the scope and limits of human reason to determine what can be known independently of experience. It proposes a “Copernican revolution” in philosophy: objects conform to the mind's way of knowing, not the other way around. By investigating the mind's a priori contributions to experience, the book explains how mathematics and natural science achieve necessity and universality, while curbing traditional metaphysics that overreaches beyond possible experience.

Aims and Method
The central question asks how synthetic a priori knowledge is possible, judgments that extend knowledge yet are known prior to experience, such as those in mathematics and fundamental physics. Kant undertakes a “transcendental” inquiry, not into objects themselves, but into the conditions that make experience of objects possible. Pure reason, left uncritically examined, generates illusions; disciplined, it grounds objective knowledge within proper limits.

Transcendental Aesthetic
Sensibility supplies intuitions through the pure forms of space and time. Space structures outer sense; time structures inner sense. They are not properties of things-in-themselves but subjective forms that organize appearances, explaining why geometry and arithmetic have apodictic certainty about objects of possible experience. This doctrine, called transcendental ideality and empirical reality, holds that space and time are necessary for experience while not belonging to things as they are in themselves.

Transcendental Analytic
Understanding thinks through concepts and brings unity to intuitions via pure concepts called categories (such as causality, substance, and unity). The Transcendental Deduction argues that these categories are objectively valid for all appearances because they are conditions of the possibility of experience, grounded in the unity of apperception, the self-conscious "I think" that must accompany representations. Through schematism, the categories receive temporal rules that allow their application to sensible data.

Principles of the Pure Understanding
From the categories Kant derives principles that govern experience: the permanence of substance, lawful causality, and community among substances. These “Analogies of Experience” secure objective succession and coexistence, underpinning natural science. He also refutes problematic idealisms by arguing that outer experience is a necessary condition for inner self-awareness across time. The result is a robust account of objectivity that depends on the joint contribution of sensibility and understanding.

Transcendental Dialectic
Reason seeks the unconditioned, generating ideas of the soul, the world as a totality, and God. When treated as knowledge, these ideas produce paralogisms about an immortal simple soul, antinomies about the world's beginning and spatial limits, and illusory proofs of a supreme being. Kant shows that these conflicts arise from illegitimately applying categories beyond possible experience. Properly used, the ideas of reason have a regulative role: they guide inquiry toward systematic unity without asserting transcendent facts.

Phenomena and Noumena
A decisive boundary is drawn between phenomena (things as they appear) and noumena (things as they are in themselves). The noumenon is a limiting concept that prevents claims about the supersensible; it is not an object of knowledge. This boundary denies speculative access to the soul, the cosmos as a whole, and God, while leaving room for practical considerations of freedom and moral faith that do not rest on theoretical cognition.

Legacy
By explaining the conditions that make experience and science possible while restricting metaphysical speculation, the Critique reorients philosophy. It secures the objectivity of mathematics and physics, dissolves dogmatic metaphysics, and situates reason's proper tasks within the finite limits of human cognition, inaugurating a new critical era.