After a sketch of Leibniz" epistemological views, we are now able to briefly discuss the properly mathematical texts that Leibniz devoted to the theory of parallels and the proof of the Parallel Postulate. Given the epistemological relevance of the system of (real) definitions in Leibniz" geometry, however, it may be of some use to mention the most important characterizations that Leibniz employed in defining the basic geometrical terms.