Immanuel Kant was the paradigmatic philosopher of the European Enlightenment. He eradicated the last traces of the medieval worldview from modern philosophy, joined the key ideas of earlier rationalism and empiricism into a powerful model of the subjective origins of the fundamental principles of both science and morality, and laid the ground for much in the philosophy of the nineteenth and twentieth centuries. Above all, Kant was the philosopher of human autonomy, the view that by the use of our own reason in its broadest sense human beings can discover and live up to the basic principles of knowledge and action without outside assistance, above all without divine support or intervention.

Kant laid the foundations of his theory of knowledge in his monumental Critique of Pure Reason (1781). He described the fundamental principle of morality in the Groundwork of the Metaphysics of Morals (1785) and the Critique of Practical Reason (1788), in the conclusion of which he famously wrote:

Kant tried to show that both the laws of nature and the laws of morality are grounded in human reason itself. By these two forms of law, however, he is often thought to have defined two incommensurable realms, nature and freedom, the realm of what is and that of what ought to be, the former of which must be limited to leave adequate room for the latter. Kant certainly did devote much space and effort to distinguishing between nature and freedom. But as he also says, in the Critique of Judgment (1790), it is equally important 'to throw a bridge from one territory to the other'. Ultimately, Kant held that both the laws of nature and the laws of free human conduct must be compatible because they are both products of human thought imposed by us on the data of our experience by the exercise of our own powers. This was clearly stated in his last book, The Conflict of the Faculties (1798):

Thus, Kant derived the fundamental principles of human thought and action from human sensibility, understanding, and reason, all as sources of our autonomy; he balanced the contributions of these principles against the ineliminable inputs of external sensation and internal inclination beyond our own control; and he strove both to demarcate these principles from each other and yet to integrate them into a single system with human autonomy as both its foundation and its ultimate value and goal. These were the tasks of Kant's three great critiques. In the Critique of Pure Reason, the essential forms of space, time and conceptual thought arise in the nature of human sensibility and understanding and ground the indispensable principles of human experience. He then argued that reason, in the narrow sense manifest in logical inference, plays a key role in systematizing human experience, but that it is a mistake to think that reason offers metaphysical insight into the existence and nature of the human soul, an independent world, and God. In the Critique of Practical Reason and Groundwork, however, he argued that reason as the source of the ideal of systematicity is the source of the fundamental law of morality and our consciousness of our own freedom, which is the source of all value, and that we can postulate the truth of the fundamental dogmas of Christianity, our own immortality and the existence of God, as practical presuppositions of our moral conduct but not as theoretical truths of metaphysics. In the Critique of Judgment, Kant argued that the unanimity of taste and the systematic organization of both individual organisms and nature as a whole could be postulated, again not as metaphysical dogmas but rather as regulative ideals of our aesthetic and scientific pursuits; he then went on to argue that it is through these ideals that we can tie together the realms of nature and freedom, because aesthetic experience offers us a palpable image of our moral freedom, and a scientific conception of the world as a system of interrelated beings makes sense only as an image of the world as the sphere of our own moral efforts. In many of his last writings, from Religion within the Limits of Reason Alone (1793) to his final manuscripts, the Opus postumum , Kant refined and radicalized his view that our religious conceptions can be understood only as analogies for the nature of human reason itself.

The Enlightenment began by attempting to bring even God before the bench of human reason - at the turn of the eighteenth century, both Shaftesbury in Great Britain and Wolff in Germany rejected voluntarism, the theory that God makes eternal truths and moral laws by fiat, and argued instead that we ourselves must know what is right and wrong before we could even recognize supposedly divine commands as divine. Kant completed their argument, concluding that the human being 'creates the elements of knowledge of the world himself, a priori, from which he, as, at the same time, an inhabitant of the world, constructs a world-vision in the idea' ( Opus postumum , 21: 31).