Immanuel Kant (1724-1804) singlehandly set the stage for German philosophy in the nineteenth century. In this summary, I will emphasize those of his doctrines which were most influential on such nineteenth-century philosophers as Scopenhauer and Hegel.
Philosophy, according to Kant, is the outcome of the use of human reason, which undertakes investigations a priori, or independently of experience. Reason also has both a theoretical and a practical employment. Reason is theoretical when it is concerned with the way things really are, and it is practical when it considers how things ought to be. Thus the two main branches of philosophy are metaphysics , the investigation a priori of the nature of reality, and ethics , which seeks a priori for rules governing the way in which beings with free will ought to decide what to do.
An important difference between the traditional practice of metaphysics and ethics illustrates Kant's fundamental orientation toward philosophy. He believed that traditional metaphysics attempts to describe objects that are completely beyond the scope of the senses. It divides objects into a sensible world and an intelligible world and claims that human reason has insight into the nature of purely intelligible objects. Ethics, on the contrary, treats the practical use of reason as if it were concerned only with sensible objects, most importantly with their relation to pleasure and pain. Kant maintained that metaphysics must be confined solely to the discovery of those rules which govern the sensible world, while ethics has nothing to do with anything sensible.
Kant considered himself to be a revolutionary thinker. He believed that he brought to philosophy a new method, which he called criticism. Other philosophers had brought forth their systems without having examined beforehand the power of human reason to think objects a priori. Criticism reveals the inherent limitations of reason in its theoretical employment, and as a result it repudiates transcendent metaphysics. But it also reveals the power of reason over its own domain of objects, objects of experience. It further reveals that reason dictates to itself the moral law.