house of Brunswick; and a journey which he made in order to study for this purpose was extended as far as Vienna and Rome. Upon his return he took charge of the Wolfenbüttel library in addition to his other engagements. The marriage of the Princess Sophie Charlotte with Frederick of Brandenburg, the first king of Prussia, brought Leibnitz into close relations with Berlin. At his suggestion the Academy (Society) of Sciences was founded there in 1700, and he himself became its first president. In Charlottenburg he worked on his principal work, the _New Essays concerning the Human Understanding_, which was aimed at Locke, but the publication of which was deferred on account of the death of the latter in the interim (1704), and did not take place until 1765, in Raspe's collective edition. The death of the Prussian queen in 1705 interrupted for several years the _Theodicy_, which had been undertaken at her request, and which did not appear until 1710. In Vienna, where he resided in 1713-14, Leibnitz composed a short statement of his system for Prince Eugen; this, according to Gerhardt, was not the sketch in ninety paragraphs, familiar under the title _Monadology_, which was first published in the original by J.E. Erdmann in his excellent _Complete Edition of the Philosophical Works of Leibnitz_, 1840, but the _Principles of Nature and of Grace_, which appeared two years after the author's death in _L'Europe Savante_. While Ernst August, as well as the German emperor and Peter the Great, distinguished the philosopher, who was not indifferent to such honors, by the bestowal of titles and preferments, his relations with the Hanoverian court, which until then had been so cordial, grew cold after the Elector Georg Ludwig ascended the English throne as George I. The letters which Leibnitz interchanged with his daughter-in-law, gave rise to the correspondence, continued to his death, with Clarke, who defended the theology of Newton against him. The contest for priority between Leibnitz and Newton concerning the invention of the differential calculus was later settled by the decision that Newton invented his method of fluxions first, but that Leibnitz published his differential calculus earlier and in a more perfect form. The variety of pursuits in which Leibnitz was engaged was unfavorable to the development and influence of his philosophy, in that it hindered him from working out his original ideas in systematic form, and left him leisure only for the composition of shorter essays. Besides the two larger works mentioned above, the _New Essays_ and the _Theodicy_, we have of philosophical works by Leibnitz only a series of private letters, and articles for the scientific journals (the _Journal des Savants_ in Paris, and the _Acta Eruditorum_ in Leipsic, etc.), among which may be mentioned as specially important the _New System of Nature, and of the Interaction of Substances as well as of the Union which exists between the