According to Kant, his reading of David Hume awakened him from his dogmatic slumber and set him on the road to becoming the "critical philosopher," whose position can be seen as a synthesis of the Leibniz-Wolffian rationalism and the Humean skepticism. Kant termed his basic insight into the nature of knowledge "the Copernican revolution in philosophy."
Instead of assuming that our ideas, to be true, must conform to an external reality independent of our knowing, Kant proposed that objective reality is known only insofar as it conforms to the essential structure of the knowing mind. He maintained that objects of experience—phenomena—may be known, but that things lying beyond the realm of possible experience—noumena, or things-in-themselves—are unknowable, although their existence is a necessary presupposition. Phenomena that can be perceived in the pure forms of sensibility, space, and time must, if they are to be understood, possess the characteristics that constitute our categories of understanding. Those categories, which include causality and substance, are the source of the structure of phenomenal experience.
The scientist, therefore, may be sure only that the natural events observed are knowable in terms of the categories. Our field of knowledge, thus emancipated from Humean skepticism, is nevertheless limited to the world of phenomena. All theoretical attempts to know things-in-themselves are bound to fail. This inevitable failure is the theme of the portion of the Critique of Pure Reason entitled the "Transcendental Dialectic." Here Kant shows that the three great problems of metaphysics—God, freedom, and immortality—are insoluble by speculative thought. Their existence can be neither affirmed nor denied on theoretical grounds, nor can they be scientifically demonstrated, but Kant shows the necessity of a belief in their existence in his moral philosophy.
Kant's ethics centers in his categorical imperative (or moral law)—"Act as if the maxim from which you act were to become through your will a universal law." This law has its source in the autonomy of a rational being, and it is the formula for an absolutely good will. However, since we are all members of two worlds, the sensible and the intelligible, we do not infallibly act in accordance with this law but, on the contrary, almost always act according to inclination. Thus what is objectively necessary, i.e., to will in conformity to the law, is subjectively contingent; and for this reason the moral law confronts us as an "ought."
In the Critique of Practical Reason Kant went on to state that morality requires the belief in the existence of God, freedom, and immortality, because without their existence there can be no morality. In the Critique of Judgment Kant applied his critical method to aesthetic and teleological judgments. The chief purpose of this work was to find a bridge between the sensible and the intelligible worlds, which are sharply distinguished in his theoretical and practical philosophy. This bridge is found in the concepts of beauty and purposiveness that suggest at least the possibility of an ultimate union of the two realms.