It was not only on Philosophy, but also on the most varied branches of science that Leibnitz expended toil and trouble and energy; it was to mathematics, however, that he specially devoted his attention, and he is the inventor of the methods of the integral and differential calculus. His great services in regard to mathematics and physics we here leave out of consideration, and pay attention to his philosophy alone. None of his books can be exactly looked on as giving a complete systematic account of his philosophy. To the more important among them belongs his work on the human understanding (_Nouveaux essais sur l’entendement humain_) in reply to Locke; but this is a mere refutation. His philosophy is therefore scattered through various little treatises which were written in very various connections, in letters, and replies to objections which caused him to bring out one aspect of the question more strongly than another; we consequently find no elaborated systematic whole, superintended or perfected by him. The work which has some appearance of being such, his _Théodicée_, better known to the public than anything else he wrote, is a popular treatise which he drew up for Queen Sophia Charlotte in reply to Bayle, and in which he took pains not to present the matter in very speculative form. A Wurtemberg theologian, Pfaff by name, and others who were correspondents of Leibnitz and were themselves only too well versed in philosophy, brought it as a charge against Leibnitz—a charge which he never denied—that his philosophy was written in popular form.[243] They laughed very much afterwards at Wolff, who had taken them to be quite in earnest; his opinion was that if Leibnitz were not perfectly serious in this sense with his _Théodicée_, yet he had unconsciously written his best therein. Leibnitz’s _Théodicée_ is not what we can altogether appreciate; it is a justification of God in regard to the evil in the world. His really philosophic thoughts are most connectedly expressed in a treatise on the principles of Grace (_Principes de la Nature et de la Grace_),[244] and especially in the pamphlet addressed to Prince Eugène of Savoy.[245] Buhle (Geschichte der neuern Philosophie, vol. iv. section 1, p. 131) says: “His philosophy is not so much the product of free, independent, original speculation, as the result of well-tested earlier” and later “systems, an eclecticism whose defects he tried to remedy in his own way. It is a desultory treatment of Philosophy in letters.”