Kant developed his philosophy after thirty years of reflection on foundational problems in contemporary natural science; on the rationalism of René Descartes (1596–1650), Gottfried Wilhelm Leibniz (1646–1716), and Christian Wolff (1679–1754); on the critique of rationalism by Christian August Crusius (c. 1715–1775) and David Hume (1711–1776); and on the political and educational views of Jean-Jacques Rousseau (1712–1778). Kant presented his mature views in the Critique of Pure Reason (1781, substantially revised in 1787), the Critique of Practical Reason (1788), and the Critique of the Power of Judgment (1790); in the Prolegomena to Any Future Metaphysics (1783) and Groundwork of the Metaphysics of Morals (1785); in two detailed works applying his general principles, the Metaphysical Foundations of Natural Science (1786) and the Metaphysics of Morals (1797), comprising a "Doctrine of Right" and a "Doctrine of Virtue"; and in polemical works such as Religion within the Boundaries of Mere Reason (1793), a bold argument that the central concepts of Christianity can have only symbolic value for pure reason, and The Conflict of the Faculties (1798), an argument for the freedom of philosophical thought.
In these works, Kant argued that both ordinary experience and natural science rest on informative but certain principles, or "synthetic a priori cognitions," such as that every event we can place at a determinate position in time must be linked to antecedent events by causal laws, which we can explain only by supposing that they reflect the forms of our own thought, in particular the spatial and temporal forms of our intuitions or perceptions and the logical categories of our understanding that we impose on our experience. But by the same token, these forms of intuition and thought can determine only how things appear to us, not how they are in themselves (Kant called this doctrine "transcendental idealism"). Kant then argued that when we conceive of the soul, the world as a complete whole, or God, we overstep the limits of our sensory perception and can have no genuine knowledge of such things.
However, the gap between appearance and things in themselves that explains the certainty of the application of the fundamental principles of our knowledge to the former also makes it at least possible for us to conceive of, if not know, the latter, particularly to conceive of ourselves as being free to make moral choices that may seem inconsistent with the determinism of our actions in the empirical world and of a God who is the author of laws of nature that are ultimately consistent with the laws of morality. Kant argued that the latter are given by our own pure practical rather than theoretical reason and that we can know them without any appeal to God; the laws of morality are discerned by pure reason as necessary to achieve autonomy, the independence of our actions from determination by the mere inclinations of ourselves or others.
Kant then argued that the fundamental principle of justice is that each person must be allowed the maximal freedom of action consistent with a like allowance for all others and that we have ethical duties to promote the free and effective use of our capacities to set and pursue our own ends and those of others, which cannot be coercively enforced within the political sphere. Finally, Kant argued that our pleasures in the beautiful and the sublime are experiences of the disinterested freedom of the imagination from constraint by the direct demands of science and morality, but also that as an experience of freedom of the imagination, aesthetic experience is indirectly conducive to moral and political development. He also argued that a teleological view of nature as a purposive system, while of merely heuristic value for the pursuit of natural science, is also a morally valuable perspective on the world and our place within it.