Immanuel Kant (1724-1804) was a German philosopher and scientist from East Prussia, regarded as one of the most influential thinkers of modern Europe and the last major philosopher of the Enlightenment. In 1781, his Critique of Pure Reason was published. Although now uniformly recognized as one of the greatest works in the history of philosophy, this Critique was largely ignored upon its initial publication. Kant developed his moral philosophy in three main works: Fundamental Principles of the Metaphysic of Morals (1785), The Critique of Practical Reason (1788), and The Metaphysics of Morals (1798). He is known for his theory that there is a single moral obligation, which he called the Categorical Imperative, from which all other moral obligations are generated. He believed that the moral law is a principle of reason itself, and is not based on contingent facts about the world (e. g., what would make us happy). Accordingly, he believed that moral obligation applies to all and only rational agents.