But while in Berkeley what we see is the young man carried away and astounded by the grandeur and simplicity of a “new way of ideas” which he has discovered, what we see in Leibniz is the mature man penetrated throughout his being with an idea which in its unity answers to the unity of the world, and which in its complexity answers, tone to tone, to the complex harmony of the world.
The familiarity of the ideas which we use hides their grandeur from us. The unity of the world is a matter of course with us; the dependent order of all within it a mere starting-point upon which to base our investigations. But if we will put ourselves in the position of Leibniz, and behold, not the new planet, but the new universe, so one, so linked together, swimming into our ken, we shall feel something of the same exultant thrill that Leibniz felt,—an exultation not indeed personal in its nature, but which arises from the expansion of the human mind face to face with an expanding world. The spirit which is at the heart of the philosophy of Leibniz is the spirit which speaks in the following words: “Quin imo qui unam partem materiæ comprehenderet, idem comprehenderet totum universum ob eandem περιχώρησιν quam dixi. Mea principia talia sunt, ut vix a se invicem develli possint. Qui unum bene novit, omnia novit.” It is a spirit which feels that the secret of the universe has been rendered up to it, and which breathes a buoyant optimism. And if we of the nineteenth century have chosen to bewail the complexity of the problem of life, and to run hither and thither multiplying “insights” and points of view till this enthusiastic confidence in reason seems to us the rashness of an ignorance which does not comprehend the problem, and the unity in which Leibniz rested appears cold and abstract beside the manifold richness of the world, we should not forget that after all we have incorporated into our very mental structure the fundamental thoughts of Leibniz,—the thoughts of the rationality of the universe and of the “reign of law.”
What was the origin of these ideas in the mind of Leibniz? What influences in the philosophic succession of thinkers led him in this direction? What agencies acting in the intellectual world about him shaped his ideal reproduction of reality? Two causes above all others stand out with prominence,—one, the discoveries and principles of modern physical science; the other, that interpretation of experience which centuries before had been formulated by Aristotle. Leibniz has a double interest for those of to-day who reverence science and who hold to the historical method. His philosophy was an attempt to set in order the methods and principles of that growing science of nature which even then was transforming the emotional and mental life of Europe; and the attempt was guided everywhere by a profound and wide-reaching knowledge of the history of philosophy. On the first point Leibniz was certainly not alone. Bacon, Hobbes, Descartes, Spinoza, each felt in his own way the fructifying touch of the new-springing science, and had attempted under its guidance to interpret the facts of nature and of man. But Leibniz stood alone in his interest in the history of thought. He stands alone indeed till he is greeted by his compeers of the nineteenth century. To Bacon previous philosophy—the Greek, the scholastic—was an “eidol of the theatre.” The human mind must be freed from its benumbing influence. To Descartes it was useless rubbish to be cleared away, that we might get a tabula rasa upon which to make a fresh start. And shall Locke and the empirical English school, or Reid and the Scotch school, or even Kant, be the first to throw a stone at Bacon and Descartes? It was reserved to Leibniz, with a genius almost two centuries in advance of his times, to penetrate the meaning of the previous development of reflective thought. It would be going beyond our brief to claim that Leibniz was interested in this as a historical movement, or that he specially concerned himself with the genetic lines which connected the various schools of thought. But we should come short of our duty to Leibniz if we did not recognize his conscious and largely successful attempt to apprehend the core of truth in all systems, however alien to his own, and to incorporate it into his own thinking.
Nothing could be more characteristic of Leibniz than his saying, “I find that most systems are right in a good share of that which they advance, but not so much in what they deny;” or than this other statement of his, “We must not hastily believe that which the mass of men, or even of authorities, advance, but each must demand for himself the proofs of the thesis sustained. Yet long research generally convinces that the old and received opinions are good, provided they be interpreted justly.” It is in the profound union in Leibniz of the principles which these quotations image that his abiding worth lies. Leibniz was interested in affirmations, not in denials. He was interested in securing the union of the modern method, the spirit of original research and independent judgment, with the conserved results of previous thought. Leibniz was a man of his times; that is to say, he was a scientific man,—the contemporary, for example, of men as different as Bernouilli, Swammerdam, Huygens, and Newton, and was himself actively engaged in the prosecution of mathematics, mechanics, geology, comparative philology, and jurisprudence. But he was also a man of Aristotle’s times,—that is to say, a philosopher, not satisfied until the facts, principles, and methods of science had received an interpretation which should explain and unify them.
Leibniz’s acquaintance with the higher forms of mathematics was due, as we have seen, to his acquaintance with Huygens. As he made the acquaintance of the latter at the same time that he made the acquaintance of the followers of Descartes, it is likely that he received his introduction to the higher developments of the scientific interpretation of nature and of the philosophic interpretation of science at about the same time. For a while, then, Leibniz was a Cartesian; and he never ceased to call the doctrine of Descartes the antechamber of truth. What were the ideas which he received from Descartes? Fundamentally they were two,—one about the method of truth, the other about the substance of truth. He received the idea that the method of philosophy consists in the analysis of any complex group of ideas down to simple ideas which shall be perfectly clear and distinct; that all such clear and distinct ideas are true, and may then be used for the synthetic reconstruction of any body of truth. Concerning the substance of philosophic truth, he learned that nature is to be interpreted mechanically, and that the instrument of this mechanical interpretation is mathematics. I have used the term “received” in speaking of the relation of Leibniz to these ideas. Yet long before this time we might see him giving himself up to dreams about a vast art of combination which should reduce all the ideas concerned in any science to their simplest elements, and then combine them to any degree of complexity. We have already seen him giving us a picture of a boy of fifteen gravely disputing with himself whether he shall accept the doctrine of forms and final causes, or of physical causes, and as gravely deciding that he shall side with the “moderns;” and that boy was himself. In these facts we have renewed confirmation of the truth that one mind never receives from another anything excepting the stimulus, the reflex, the development of ideas which have already possessed it. But when Leibniz, with his isolated and somewhat ill-digested thoughts, came in contact with that systematized and connected body of doctrines which the Cartesians presented to him in Paris, his ideas were quickened, and he felt the necessity—that final mark of the philosophic mind—of putting them in order.
About the method of Descartes, which Leibniz adopted from him, or rather formulated for himself under the influence of Descartes, not much need be said. It was the method of Continental thought till the time of Kant. It was the mother of the philosophic systems of Descartes, Leibniz, and Spinoza. It was equally the mother of the German Aufklärung and the French éclaircissement. Its fundamental idea is the thought upon which Rationalism everywhere bases itself. It says: Reduce everything to simple notions. Get clearness; get distinctness. Analyze the complex. Shun the obscure. Discover axioms; employ these axioms in connection with the simple notions, and build up from them. Whatever can be treated in this way is capable of proof, and only this. Leibniz, I repeat, possessed this method in common with Descartes and Spinoza. The certainty and demonstrativeness of mathematics stood out in the clearest contrast to the uncertainty, the obscurity, of all other knowledge. And to them, as to all before the days of Kant, it seemed beyond doubt that the method of mathematics consists in the analysis of notions, and in their synthesis through the medium of axioms, which are true because identical statements; while the notions are true because clear and distinct.
And yet the method led Leibniz in a very different direction. One of the fundamental doctrines, for example, of Leibniz is the existence everywhere of minute and obscure perceptions,—which are of the greatest importance, but of which we, at least, can never have distinct