A natural, but very remarkable consequence of this manifold activity and lifelong absorption in public affairs was the failure of so great a thinker to produce a single systematic and elaborate work containing a complete and detailed exposition of his philosophical, and especially his ontological views. For such an exposition Leibnitz could find at no period of his life the requisite time and scope. In the vast multitude of his productions there is no complete philosophic work. The most arduous of his literary labors are historical compilations, made in the service of the State. Such were the "History of the House of Brunswick," already mentioned, the "Accessiones Historiæ," the "Scriptores Rerum Brunsvicensium Illustrationi inservientes," and the "Codex Juris Gentium Diplomaticus";-- works involving an incredible amount of labor and research, but adding little to his posthumous fame. His philosophical studies, after entering the Hanoverian service, which he did in his thirtieth year, were pursued, as he tells his correspondent Placcius, by stealth,--that is, at odd moments snatched from official duties and the cares of state. Accordingly, his metaphysical works have all a fragmentary character. Instead of systematic treatises, they are loose papers, contributions to journals and magazines, or sketches prepared for the use of friends. They are all occasional productions, elicited by some external cause, not prompted by inward necessity. The "Nouveaux Essais," his most considerable work in that department, originated in comments on Locke, and was not published until after his death. The "Monadology" is a series of propositions drawn up for the use of Prince Eugene, and was never intended to be made public. And, probably, the "Théodicée" would never have seen the light except for his cultivated and loved pupil, the Queen of Prussia, for whose instruction it was designed.