  Unlike most of the great philosophers of the period, Leibniz did not write a magnum opus; there is no single work that can be said to contain the core of his thought. While he did produce two books, the Theodicy (1710) and the New Essays Concerning Human Understanding (finished in 1704 but not published until 1765), the student of Leibniz's thought must piece together Leibniz's philosophy from his myriad writings: essays published in scholarly journals and in more popular journals; unpublished works left abandoned by their author; and his many letters. Moreover, many of Leibniz's writings have not yet been published. The authoritative scholarly version of Leibniz's works, the Akademie edition, has thus far only published his philosophical writings from 1663 to 1690; in other words, only half of his writing life has been covered. And the mere act of dating pieces often depends upon careful analysis of the paper Leibniz wrote on and watermarks and so on. (Hence, for example, the important short work, Primary Truths, which, because of its content, was often thought to date to 1686 (as in AG), has recently been redated by the Akademie editors to 1689 because of a watermark.) Piecing together Leibniz's philosophy into a systematic whole is made more difficult because Leibniz seems to have changed or at least refined his views on a number of issues over the course of his career and because he was always very aware (some might say too aware) of the audience for any of his writings.