He found employment as a family tutor and, during the nine years that he gave to it, worked for three different families. With them he was introduced to the influential society of the city, acquired social grace, and made his farthest travels from his native city—some 60 miles (96 km) away to the town of Arnsdorf. In 1755, aided by the kindness of a friend, he was able to complete his degree at the university and take up the position of Privatdozent, or lecturer.

Three dissertations that he presented on obtaining this post indicate the interest and direction of his thought at this time. In one, Meditationum Quarundam de Igne Succincta Delineation (1755; “A Brief Outline of Some Meditations on Fire”), he argued that bodies operate on one another through the medium of a uniformly diffused elastic and subtle matter that is the underlying substance of both heat and light. His first teaching was in mathematics and physics, and he was never to lose his interest in scientific developments. That it was more than an amateur interest is shown by his publication within the next few years of several scientific works dealing with the different human races, the nature of winds, the causes of earthquakes, and the general theory of the heavens. In the latter work, Allgemeine Naturgeschichte und Theorie des Himmels (1755; Universal Natural History and Theory of the Heavens), Kant proposed a nebular theory of the formation of the solar system, according to which the Sun and the planets condensed from a single gaseous cloud. Independently advanced by Laplace in 1796, it was subsequently known as the Kant-Laplace hypothesis.

At this period Newtonian physics was important to Kant as much for its philosophical implications as for its scientific content. A second dissertation, the Metaphysicae cum Geometria Iunctae Usus in Philosophia Naturali, Cuius Specimen I. Continet Monadologiam Physicam (1756; The Employment in Natural Philosophy of Metaphysics Combined with Geometry, of Which Sample I Contains the Physical Monadology)—also known as the Monodologia Physica—contrasted the Newtonian methods of thinking with those employed in the philosophy then prevailing in German universities. This was the philosophy of Gottfried Wilhelm Leibniz, a universal scholar, as systematized and popularized by Wolff and by Alexander Gottlieb Baumgarten, author of a widely used text, the Metaphysica (1739). Leibniz’s works as they are now known were not fully available to these writers, and the Leibnizian philosophy that they presented was extravagantly rationalistic, abstract, and cut-and-dried. It nevertheless remained a powerful force, and the main efforts of independent thinkers in Germany at the time were devoted to examining Leibniz’s ideas.

In a third dissertation, Principiorum Primorum Cognitionis Metaphysicae Nova Dilucidato (1755; “New Elucidation of the First Principles of Metaphysical Cognition”), Kant analyzed especially the principle of sufficient reason, which in Wolff’s formulation asserts that for everything there is a sufficient reason why it should be rather than not be. Although critical, Kant was cautious and still a long way from challenging the assumptions of Leibnizian metaphysics.

During the 15 years that he spent as a Privatdozent, Kant’s renown as a teacher and writer steadily increased. Soon he was lecturing on many subjects other than physics and mathematics—including logic, metaphysics, and moral philosophy. He even lectured on fireworks and fortifications and every summer for 30 years taught a popular course on physical geography. He enjoyed great success as a lecturer; his lecturing style, which differed markedly from that of his books, was humorous and vivid, enlivened by many examples from his reading in English and French literature and in travel and geography, science and philosophy.

Although he twice failed to obtain a professorship at Königsberg, he refused to accept offers that would have taken him elsewhere—including the professorship of poetry at Berlin that would have brought greater prestige. He preferred the peace and quiet of his native city in which to develop and mature his own philosophy.