The Critique of Pure Reason (German: Kritik der reinen Vernunft), often known as the First Critique, is a 1781 work by Immanuel Kant. It is a foundational text of modern Western philosophy, proposing a 'Copernican turn' in the approach to central questions posed by previous thinkers. Rather than assuming that the mind must conform to its objects, Kant posited that objects must conform to our minds. Objects must conform to the conditions of possible experience to be experienced at all, and so we can know that they will conform to them, but that knowledge does not extend beyond our experience, to things as they are in themselves, limiting our ability to make many traditional metaphysical claims.
Early Modern Texts: The Critique of Pure Reason, adapted and translated into modern English, by Jonathan Bennett. PDF format.
Gutenberg: The Critique of Pure Reason. Multiple formats.
Internet Archive: The Critique of Pure Reason, translated by J.M.D. Meiklejohn (Everyman's Library edition, 1934). Multiple formats.
University of Adelaide (Internet Archive): The Critique of Pure Reason, translated by J.M.D. Meiklejohn. Multiple formats.