Finally, in 1770, after serving for 15 years as a Privatdozent, Kant was appointed to the chair of logic and metaphysics, a position in which he remained active until a few years before his death. In this period—usually called his critical period, because in it he wrote his great Critiques—he published an astounding series of original works on a wide variety of topics, in which he elaborated and expounded his philosophy.

The Inaugural Dissertation of 1770 that he delivered on assuming his new position already contained many of the important elements of his mature philosophy. As indicated in its title, De Mundi Sensibilis atque Intelligibilis Forma et Principiis: Dissertatio (“On the Form and Principles of the Sensible and Intelligible Worlds”), the implicit dualism of the Träume is made explicit, and it is made so on the basis of a wholly un-Leibnizian interpretation of the distinction between sense and understanding. Sense is not, as Leibniz had supposed, a confused form of thinking but a source of knowledge in its own right, although the objects so known are still only “appearances”—the term that Leibniz also used. They are appearances because all sensing is conditioned by the presence, in sensibility, of the forms of time and space, which are not objective characteristics or frameworks of things but “pure intuitions.” But though all knowledge of things sensible is thus of phenomena, it does not follow that nothing is known of things as they are in themselves. Certainly, humans have no intuition, or direct insight, into an intelligible world, but the presence in them of certain “pure intellectual concepts”—such as those of possibility, existence, necessity, substance, and cause—enables them to have some descriptive knowledge of it. By means of these concepts they can arrive at an exemplar that provides them with “the common measure of all other things as far as real.” This exemplar gives them an idea of perfection for both the theoretical and practical orders: in the first, it is that of the Supreme Being, God; in the latter, that of moral perfection.

After the Dissertation, Kant published virtually nothing for 11 years. Yet, in submitting the Dissertation to a friend at the time of its publication, he wrote:

About a year since I attained that concept which I do not fear ever to be obliged to alter, though I may have to widen it, and by which all sorts of metaphysical questions can be tested in accordance with entirely safe and easy criteria, and a sure decision reached as to whether they are soluble or insoluble.