(1724–1804), German philosopher. In the Critique of Pure Reason (1781) he countered Hume's sceptical empiricism by arguing that any affirmation or denial regarding the ultimate nature of reality (‘noumenon’) makes no sense. All we can know are the objects of experience (‘phenomena’), interpreted by space and time and ordered according to twelve key concepts. Kant's Critique of Practical Reason (1788) affirms the existence of an absolute moral law—the categorical imperative.