In the spring of 1661, at the age of fourteen, Leibniz began his studies at the university in Leipzig where he came under the influence of Jakob Thomasius, a well-known German philosopher. Thomasius, who became the young man's mentor and adviser, was born in Leipzig in 1622, attended university there, and eventually became Professor of Rhetoric, Dialectic, and Moral Philosophy. Before his death in 1684, he published in all the main areas of philosophy and directed dissertations on a wide range of topics. He was considered an “erudite” historian of philosophy, an important conciliator, and “a most recognized” philosopher (Sturm1686: 72–3). Leibniz calls him “the most celebrated German Peripatetic” (A VI ii, 426) and refers to him as “our most famous Thomasius” (A VI i, 300). In April 1669, Leibniz wrote a letter to Thomasius in which he argues for the reconciliation of the Aristotelian and the mechanical philosophies, and for a conception of substance that would effect that reconciliation. He published the letter the next year, and it, thereby, became the young man's first public presentation of his newly developed theory of substance. The title given to the letter is revealing: “Letter to a Man of the Most Refined Learning Concerning the Reconcilability of Aristotle and the Moderns.” In the remainder of Leibniz's long life, he wrote thousands of letters to hundreds of people.