The history of philosophy is a study which proposes to itself two somewhat different objects, of which the first is mainly historical, while the second is mainly philosophical. From this cause it is apt to result that, where we look for history of philosophy, we find rather history and philosophy. Questions concerning the influence of the times or of other philosophers, concerning the growth of a philosopher's system, and the causes which suggested his leading ideas -- all these are truly historical: they require for their answer a considerable knowledge of the prevailing education, of the public to whom it was necessary to appeal, and of the scientific and political events of the period in question. But it may be doubted how far the topics dealt with in works where these elements predominate can be called properly philosophical. There is a tendency -- which the so-called historical spirit has greatly increased -- to pay so much attention to the relations of philosophies that the philosophies themselves are neglected. Successive philosophies may be compared, as we compare successive forms of a pattern or design, with little or no regard to their meaning: an influence may be established by documentary evidence, or by identity of phrase, without any comprehension of the systems whose causal relations are under discussion. But there remains always a purely philosophical attitude towards previous philosophers -- an attitude in which, without regard to dates or influences, we seek simply to discover what are the great types of possible philosophies, and guide ourselves in the search by investigating the systems advocated by the great philosophers of the past. There is still, in this inquiry -- what is, after all, perhaps the most important of the historical questions -- the problem as to the actual views of the philosopher who is to be investigated. But these views are now examined in a different spirit. Where we are inquiring into the opinions of a truly eminent philosopher, it is probable that these opinions will form, in the main, a closely connected system, and that, by learning to understand them, we shall ourselves acquire knowledge of important philosophic truths. And since the philosophies of the past belong to one or other of a few great types -- types which in our own day are perpetually recurring -- we may learn, from examining the greatest representative of any type, what are the grounds for such a philosophy. We may even learn, by observing the contradictions and inconsistencies frorn which no system hitherto propounded is free, what are the fundamental objections to the type in question, and how these objections are to be avoided. But in such inquiries the philosopher is no longer explained psychologically: he is examined as the advocate of what he holds to be a body of philosophic truth. By what process of development he came to this opinion, though in itself an important and interesting question, is logically irrelevant to the inquiry how far the opinion itself is correct; and among his opinions, when these have been ascertained, it becomes desirable to prune away such as seem inconsistent with his main doctrines, before those doctrines themselves are subjected to a critical scrutiny. Philosophic truth and falsehood, in short, rather than historical fact, are what primarily demand our attention in this inquiry.

      It is this latter task, and not the more strictly historical one, that I have endeavoured to perform towards Leibniz. The historical task has been admirably performed by others, notably Professor Stein, in works to which I have nothing to add; but the more philosophical task appears to be still unperformed. Erdmann's excellent account of Leibniz in his larger history (1842), from which I have learnt more than from any other commentary, was written in ignorance of the letters to Arnauld, and of much other important material which has been published since the date of Erdmann's edition of Leibniz (1840). And since his day, the traditional view of our philosopher's system appears to have been so deeply rooted in the minds of commentators that the importance of new manuscripts has not, I think, been duly recognized. Dillmann, it is true, has written a book whose object is similar to that of the present work, and has emphasized -- rightly as it seems to me -- the danger of obtaining our opinions of Leibniz from the Monadology. But it may be doubted whether Dillmann has succeeded as well in understanding the meaning of Leibniz as in mastering the text of his writings.

      A few personal remarks may serve to explain why I believe a book on Leibniz to be not wholly uncalled for. In the Lent Term of 1899 I delivered a course of lectures on the Philosophy of Leibniz at Trinity College, Cambridge. In preparing these lectures, I found myself, after reading most of the standard commentators and most of Leibniz's connected treatises, still completely in the dark as to the grounds which had led him to many of his opinions. Why he thought that monads cannot interact; how he became persuaded of the Identity of Indiscernibles; what he meant by the law of Sufficient Reason -- these and many other questions seemcd to demand an answer, but to find none. I felt -- as many others have felt -- that the Monadology was a kind of fantastic fairy tale, coherent perhaps, but wholly arbitrary. At this point I read the Discours de Metaphysique and the letters to Arnauld. Suddenly a flood of light was thrown on all the inmost recesses of Leibniz's philosophical edifice. I saw how its foundations were laid, and how its superstructure rose out of them. It appeared that this seemingly fantastic system could be deduced from a few simp premisses, which, but for the conclusions which Leibniz had drawn from them, many, if not most, philosophers would have been willing to admit. It seemed not unreasonable to hope that the passages which had seemed illuminating to me would seem so also to others. I have therefore, in what follows, begun with the doctrines contained in these passages, and endeavoured as far as possible to exhibit the theory of monads as a rigid deduction from a small number of premises. The monad thus appears, not at the beginning of the exposition, but after a long preliminary chain of reasoning. And it must, I think, be allowed that, if this account be correct, Leibniz's value as a philosopher is very much greater than that which would result from the customary expositions.

      I have added an Appendix of classified extracts, in which it has been my object to include at least one definite pronouncement, wherever one could be found, on every point in Leibniz's philosophy. On moot points, or points on which he is inconsistent, I have in general given several quotations. I have given the date of a passage whenever it is not later than 1686, or seems important for some other reason. Passages referred to in the text are generally quoted in the corresponding paragraph of the Appendix, except when they have been already referred to and quoted in an earlier paragraph; but passages quoted in the text are in general not repeated in the Appendix. For convenience of reference, I have made an index of the Appendix, so that any passage contained in it can be found at once by the reference. I have translated all passages quoted, and have nowhere assumed any knowledge of a foreign language. I have also endeavoured to assume no previous acquaintance with Leibniz beyond what can be obtained from Mr. Latta's excellent translations. In quoting passages translated by him I have in general followed his translation; but the translations of Mr. Duncan and Mr. Langley I have usually found it necessary to correct. In quoting from the papers against Clarke, I have followed Clarke's translation wherever this is not seriously inaccurate.

      I have to thank Mr. G. E. Moore, of Trinity College, Cambridge, for reading the proofs and for many valuable suggestions, as also for the serious labour of revising all translations from the Latin, both in the text and in the appendix. I have also to thank Professor James Ward for reading a portion of the work in manuscript and for several important criticisms.

September, 1900.