Hague, and there met face to face the other future great philosopher of the time, Spinoza. But our questionings meet no answer. At Hanover, the industries of Leibniz were varied. An extract from one of his own letters, though written at a somewhat later date, will give the best outline of his activities.
"It is incredible how scattered and divided are my occupations. I burrow through archives, investigate old writings, and collect unprinted manuscripts, with a view to throwing light on the history of Brunswick. I also receive and write a countless number of letters. I have so much that is new in mathematics, so many thoughts in philosophy, so many literary observations which I cannot get into shape, that in the midst of my tasks I do not know where to begin, and with Ovid am inclined to cry out: 'My riches make me poor.' I should like to give a description of my calculating-machine; but time fails. Above all else I desire to complete my Dynamics, as I think that I have finally discovered the true laws of material Nature, by whose means problems about bodies which are out of reach of rules now known may be solved. Friends are urging me to publish my Science of the Infinite, containing the basis of my new analysis. I have also on hand a new Characteristic, and many general considerations about the art of discovery. But all these works, the historical excepted, have to be done at odd moments. Then at the court all sorts of things are expected. I have to answer questions on points in international law; on points concerning the rights of the various princes in the Empire: so far I have managed to keep out of questions of private law. With all this I have had to carry on negotiations with the bishops of Neustadt and of Meaux [Bossuet], and with Pelisson and others upon religious matters."
It is interesting to note how the philosophic spirit, the instinct for unity and generality, showed itself even in the least of Leibniz's tasks. The Duke of Brunswick imposed upon Leibniz the task of drawing up a genealogical table of his House. Under Leibniz's hands this expanded into a history of the House, and this in turn was the centre of an important study of the German Empire. It was impossible that the philosopher, according to whom every real being reflected the whole of the universe from its point of view, should have been able to treat even a slight phase of local history without regarding it in its relations to the history of the world. Similarly some mining operations in the Harz Mountains called the attention of Leibniz to geological matters. The result was a treatise called "Protogäa," in which Leibniz gave a history of the development of the earth. Not content with seeing in a Brunswick mountain an epitome of the world's physical formation, it was his intention to make this an introduction to his political history as a sort of geographical background and foundation. It is interesting to note that the historical studies of Leibniz took him on a three years' journey, from 1687 to 1690, through the various courts of Europe,--a fact which not only had considerable influence upon Leibniz himself, but which enabled him to give stimulus to scientific development in more ways and places than one.
His philosophical career as an author begins for the most part with his return to Hanover in 1690. This lies outside of the scope of the present chapter, but here is a convenient place to call attention to the fact that for Leibniz the multitude of his other duties was so great that his philosophical work was the work "of odd moments." There is no systematic exposition; there are a vast number of letters, of essays, of abstracts and memoranda published in various scientific journals. His philosophy bears not only in form, but in substance, traces of its haphazard and desultory origin. Another point of interest in this connection is the degree to which, in form, at least, his philosophical writings bear the impress of his cosmopolitan life. Leibniz had seen too much of the world, too much of courts, for his thoughts to take the rigid and unbending form of geometrical exposition suited to the lonely student of the Hague. Nor was the regular progression and elucidation of ideas adapted to the later Germans, almost without exception university professors, suited to the man of affairs. There is everywhere in Leibniz the attempt to adapt his modes of statement, not only to the terminology, but even to the ideas, of the one to whom they are addressed. There is the desire to magnify points of agreement, to minimize disagreements, characteristic of the courtier and the diplomat. His comprehensiveness is not only a comprehensiveness of thought, but of ways of exposition, due very largely, we must think, to his cosmopolitan education. The result has been to the great detriment of Leibniz's influence as a systematic thinker, although it may be argued that it has aided his indirect and suggestive influence, the absorption of his ideas by men of literature, by Goethe, above all by Lessing, and his stimulating effect upon science and philosophy. It is certain that the attempt to systematize his thoughts, as was done by Wolff, had for its result the disappearance of all that was profound and thought-exciting.
If his philosophy thus reflects the manner of his daily life, the occupations of the latter were informed by the spirit of his philosophy. Two of the dearest interests of Leibniz remain to be mentioned,--one, the founding of academies; the other, the reconciling of religious organizations. The former testifies to his desire for comprehensiveness, unity, and organization of knowledge; the latter to his desire for practical unity, his dislike of all that is opposed and isolated. His efforts in the religions direction were twofold. The first was to end the theological and political controversies of the time by the reunion of the Protestant and Roman Catholic Churches. It was a plan which did the greatest honor to the pacific spirit of Leibniz, but it was predestined to failure. Both sides made concessions,--more concessions than we of to-day should believe possible. But the one thing the Roman Catholic Church would not concede was the one thing which the Protestant Church demanded,--the notion of authority and hierarchy. Indeed, it may be questioned whether the terms on which Leibniz conceived of their reunion do not point to the greatest weakness in his philosophy,--the tendency to overlook oppositions and to resolve all contradiction into differences of degree. Hardly had this plan fallen through when Leibniz turned to the project of a union of the Lutheran and Reformed branches of the Protestant Church. This scheme was more hopeful, and while unrealized during the life of our philosopher, was afterwards accomplished.
It is noteworthy that even before Leibniz went to Paris and to London he had conceived the idea of a society of learned men for the investigation, the systematization, and the publication of scientific truth in all its varied forms,--a society which should in breadth include the whole sphere of sciences, but should not treat them as so many isolated disciplines, but as members of one system. This idea was quickened when Leibniz saw the degree in which it had already been realized in the two great world-capitals. He never ceased to try to introduce similar academies wherever he had influence. In 1700 his labors bore their fruit in one instance. The Academy at Berlin was founded, and Leibniz was its first, and indeed life-long, president. But disappointment met him at Vienna, Dresden, and St. Petersburg, where he proposed similar societies.
Any sketch of Leibniz's life, however brief, would be imperfect which did not mention the names at least of two remarkable women,--remarkable in themselves, and remarkable in their friendship with Leibniz. These were Sophia, grand-daughter of James I. of England (and thus the link by which the House of Brunswick finally came to rule over Great Britain) and wife of the Duke of Brunswick, and her daughter Sophia Charlotte, wife of the first king of Prussia. The latter, in particular, gave Leibniz every encouragement. She was personally deeply interested in all theological and philosophical questions. Upon her death-bed, in 1705, she is said to have told those about her that they were not to mourn for her, as she should now be able to satisfy her desire to learn about things which Leibniz had never sufficiently explained.
Her death marks the beginning of a period in Leibniz's life which it is not pleasant to dwell upon. New rulers arose that knew not Leibniz. It cannot be said that from this time till his death in Hanover in 1716 Leibniz had much joy or satisfaction. His best friends were dead; his political ambitions were disappointed; he was suspected of coldness and unfriendliness by the courts both of Berlin and Hanover; Paris and Vienna were closed to him, so far as any wide influence was concerned, by his religious faith; the controversy with the friends of Newton still followed him. He was a man of the most remarkable intellectual gifts, of an energy which could be satisfied only with wide fields of action; and he found himself shut in by narrow intrigue to a petty round of courtly officialism. It is little wonder that the following words fell from his lips: "Germany is the only country in the world that does not know how to recognize the fame of its children and to make that fame immortal. It forgets itself; it forgets its own, unless foreigners make it mindful of its own treasures." A Scotch friend of Leibniz, who happened to be in Hanover when he died, wrote that Leibniz "was buried more like a robber than what he really was,--the ornament of his country." Such was the mortal end of the greatest intellectual genius since Aristotle. But genius is not a matter to be bounded in life or in death by provincial courts. Leibniz remains a foremost citizen in that "Kingdom of Spirits" in whose formation he found the meaning of the world.
CHAPTER II.
THE SOURCES OF HIS PHILOSOPHY.
What is true of all men is true of philosophers, and of Leibniz among them. Speaking generally, what they are unconsciously and fundamentally, they are through absorption of their antecedents and surroundings. What they are consciously and reflectively, they are through their reaction upon the influence of heredity and environment. But there is a spiritual line of descent and a spiritual atmosphere; and in speaking of a philosopher, it is with this intellectual heredity and environment, rather than with the physical, that we are concerned. Leibniz was born into a period of intellectual activity the most teeming with ideas, the most fruitful in results, of any, perhaps, since the age of Pericles. We pride ourselves justly upon the activity of our own century, and in diffusion of intellectual action and wide-spread application of ideas the age of Leibniz could not compare with it. But ours _is_ the age of diffusion and application, while his was one of fermentation and birth.
Such a period in its earlier days is apt to be turbid and unsettled. There is more heat of friction than calm light. And such had been the case in the hundred years before Leibniz. But when he arrived at intellectual maturity much of the crudity had disappeared. The troubling of the waters of thought had ceased; they were becoming clarified. Bacon, Hobbes, Descartes, each had crystallized something out of that seething and chaotic mass of new ideas which had forced itself into European consciousness. Men had been introduced into a new world, and the natural result had been feelings of strangeness, and the vagaries of intellectual wanderings. But by the day of Leibniz the intellectual bearings had been made out anew, the new mental orientation had been secured.
The marks of this "new spiritual picture of the universe" are everywhere to be seen in Leibniz. His philosophy is the dawning consciousness of the modern world. In it we see the very conception and birth of the modern interpretation of the world. The history of thought is one continuous testimony to the ease with which we become hardened to ideas through custom. Ideas are constantly precipitating themselves out of the realm of ideas into that of ways of thinking and of viewing the universe. The problem of one century is the axiom of another. What one generation stakes its activity upon investigating is quietly taken for granted by the next. And so the highest reach of intellectual inspiration in the sixteenth century is to-day the ordinary food of thought, accepted without an inquiry as to its source, and almost without a suspicion that it has a recent historic origin. We have to go to Bacon or to Leibniz to see the genesis and growth of those ideas which to-day have become materialized into axiomatic points of view and into hard-and-fast categories of thought. In reading Leibniz the idea comes over us in all its freshness that there was a time when it was a discovery that the world is a universe, made after one plan and of one stuff. The ideas of inter-relation, of the harmony of law, of mutual dependence and correspondence, were not always the assumed starting-points of thought; they were once the crowning discoveries of a philosophy aglow and almost intoxicated with the splendor of its far-reaching generalizations. I take these examples of the unity of the world, the continuity and interdependence of all within it, because these are the ideas which come to their conscious and delighted birth in the philosophy of Leibniz. We do not put ourselves into the right attitude for understanding his thought until we remember that these ideas--the commonest tools of our thinking--were once new and fresh, and in their novelty and transforming strangeness were the products of a philosophic interpretation of experience. Except in that later contemporary of Leibniz, the young and enthusiastic Irish idealist, Berkeley, I know of no historic thinker in whom the birth-throes (joyous, however) of a new conception of the world are so evident as in Leibniz. 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 ~perichôrêsin~ 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 consciousness. How is this factor of his thought, which almost approaches mysticism, to be reconciled with the statements just made? It is found in the different application which is made of the method. The object of Descartes is the _erection of a new structure of truth_ upon a _tabula rasa_ of all former doctrines. The object of Leibniz is the _interpretation of an old body of truth_ by a method which shall reveal it in its clearest light. Descartes and Spinoza are "rationalists" both in their method and results. Leibniz is a "rationalist" in his method; but his application of the method is everywhere controlled by historic considerations. It is, I think, impossible to over-emphasize this fact. Descartes was profoundly convinced that past thought had gone wrong, and that its results were worthless. Leibniz was as profoundly convinced that its instincts had been right, and that the general idea of the world which it gave was correct. Leibniz would have given the heartiest assent to Goethe's saying, "Das Wahre war schon längst gefunden." It was out of the question, then, that he should use the new method in any other than an interpreting way to bring out in a connected system and unity the true meaning of the subject-matter.
So much of generality for the method of Leibniz. The positive substance of doctrine which he developed under scientific influence affords matter for more discussion. Of the three influences which meet us here, two are still Cartesian; the third is from the new science of biology, although not yet answering to that name. These three influences are, in order: the idea that nature is to be explained mechanically; that this is to be brought about through the application of mathematics; and, from biology, the idea that all change is of the nature of continuous growth or unfolding. Let us consider each in this order.
What is meant by the mechanical explanation of nature? To answer a question thus baldly put, we must recall the kind of explanations which had satisfied the scholastic men of science. They had been explanations which, however true, Leibniz says, as general principles, do not touch the details of the matter. The explanations of natural facts had been found in general principles, in substantial forces, in occult essences, in native faculties. Now, the first contention of the founders of the modern scientific movement was that such general considerations are not verifiable, and that if they are, they are entirely aside from the point,--they fail to explain any given fact. Explanation must always consist in discovering an immediate connection between some fact and some co-existing or preceding fact. Explanation does not consist in referring a fact to a general power, it consists in referring it to an antecedent whose existence is its necessary condition. It was not left till the times of Mr. Huxley to poke fun at those who would explain some concrete phenomenon by reference to an abstract principle ending in --ity. Leibniz has his word to say about those who would account for the movements of a watch by reference to a principle of horologity, and of mill-stones by a fractive principle.
Mechanical explanation consists, accordingly, in making out an actual connection between two existing facts. But this does not say very much. A connection of what kind? In the first place, a connection of the same order as the facts observed. If we are explaining corporeal phenomena, we must find a corporeal link; if we are explaining phenomena of motion, we must find a connection of motion. In one of his first philosophical works Leibniz, in taking the mechanical position, states what he means by it. In the "Confession of Nature against the Atheists" he says that it must be confessed to those who have revived the corpuscular theory of Democritus and Epicurus, to Galileo, Bacon, Gassendi, Hobbes, and Descartes, that in explaining material phenomena recourse is to be had neither to God nor to any other incorporeal thing, form, or quality, but that all things are to be explained from the nature of matter and its qualities, especially from their magnitude, figure, and motion. The physics of Descartes, to which was especially due the spread of mechanical notions, virtually postulated the problem: given a homogeneous quantity of matter, endowed only with extension and mobility, to account for all material phenomena. Leibniz accepts this mechanical view without reserve.
What has been said suggests the bearing of mathematics in this connection. Extension and mobility may be treated by mathematics. It is indeed the business of the geometer to give us an analysis of figured space, to set before us all possible combinations which can arise, assuming extension only. The higher analysis sets before us the results which inevitably follow if we suppose a moving point or any system of movements. Mathematics is thus the essential tool for treating physical phenomena as just defined. But it is more. The mechanical explanation of Nature not only requires such a development of mathematics as will make it applicable to the interpretation of physical facts, but the employment of mathematics is necessary for the very discovery of these facts. Exact observation was the necessity of the growing physical science; and exact observation means such as will answer the question, _How much?_ Knowledge of nature depends upon our ability to _measure_ her processes,--that is, to reduce distinctions of quality to those of quantity. The only assurance that we can finally have that two facts are connected in such a way as to fulfil the requirements of scientific research, is that there is a complete quantitative connection between them, so that one can be regarded as the other transformed. The advance of physical science from the days of Copernicus to the present has consisted, therefore, on one hand, in a development of mathematics which has made it possible to apply it in greater and greater measure to the discussion and formulation of the results of experiment, and to deduce laws which, when interpreted physically, will give new knowledge of fact; and, on the other, to multiply, sharpen, and make precise all sorts of devices by which the processes of nature may be measured. The explanation of nature by natural processes; the complete application of mathematics to nature,--these are the two thoughts which, so far, we have seen to be fundamental to the development of the philosophy of Leibniz.
The third factor, and that which brings Leibniz nearer, perhaps, our own day than either of the others, is the growth of physiological science. Swammerdam, Malpighi, Leewenhoek,--these are names which occur and recur in the pages of Leibniz. Indeed, he appears to be the first of that now long line of modern philosophers to be profoundly influenced by the conception of life and the categories of organic growth. Descartes concerned himself indeed with physiological problems, but it was only with a view to applying mechanical principles. The idea of the vital unity of all organs of the body might seem to be attractive to one filled with the notion of the unity of all in God, and yet Spinoza shows no traces of the influence of the organic conception. Not until Kant's famous definition of organism do we see another philosopher moved by an attempt to comprehend the categories of living structure.
But it is the idea of organism, of life, which is radical to the thought of Leibniz. I do not think, however, that it can truly be said that he was led to the idea simply from the state of physiological investigation at that time. Rather, he had already learned to think of the world as organic through and through, and found in the results of biology confirmations, apt illustrations of a truth of which he was already thoroughly convinced. His writings show that there were two aspects of biological science which especially interested him. One was the simple fact of organism itself,--the fact of the various activities of different organs occurring in complete harmony for one end. This presented three notions very dear to the mind of Leibniz, or rather three moments of the same idea,--the factors of activity, of unity brought about by co-ordinated action, and of an end which reveals the meaning of the activity and is the ideal expression of the unity. The physiologists of that day were also occupied with the problem of growth. The generalization that all is developed _ab ovo_ was just receiving universal attention. The question which thrust itself upon science for solution was the mode by which ova, apparently homogeneous in structure, developed into the various forms of the organic kingdom. The answer given was "evolution." But evolution had not the meaning which the term has to-day. By evolution was meant that the whole complex structure of man, for example, was virtually contained in the germ, and that the apparent phenomenon of growth was not the addition of anything from without, but simply the unfolding and magnifying of that already existing. It was the doctrine which afterwards gave way to the epigenesis theory of Wolff, according to which growth is not mere unfolding or unwrapping, but progressive differentiation. The "evolution" theory was the scientific theory of the times, however, and was warmly espoused by Leibniz. To him, as we shall see hereafter, it seemed to give a key which would unlock one of the problems of the universe.
Such, then, were the three chief generalizations which Leibniz found current, and which most deeply affected him. But what use did he make of them? He did not become a philosopher by letting them lie dormant in his mind, nor by surrendering himself passively to them till he could mechanically apply them everywhere. He was a philosopher only in virtue of the active attitude which his mind took towards them. He could not simply accept them at their face-value; he must ask after the source of their value, the royal stamp of meaning which made them a circulatory medium. That is to say, he had to interpret these ideas, to see what they mean, and what is the basis of their validity.
Not many men have been so conscious of just the bearings of their own ideas and of their source as was he. He often allows us a direct glimpse into the method of his thinking, and nowhere more than when he says: "Those who give themselves up to the details of science usually despise abstract and general researches. Those who go into universal principles rarely care for particular facts. But I equally esteem both." Leibniz, in other words, was equally interested in the application of scientific principles to the explanation of the details of natural phenomena, and in the bearing and meaning of the principles themselves,--a rare combination, indeed, but one, which existing, stamps the genuine philosopher. Leibniz substantially repeats this idea when he says: "Particular effects must be explained mechanically; but the general principles of physics and mathematics depend upon metaphysics." And again: "All occurs mechanically; but the mechanical principle is not to be explained from material and mathematical considerations, but it flows from a higher and a metaphysical source."
As a man of science, Leibniz might have stopped short with the ideas of mechanical law, of the application of mathematics, and of the continuity of development. As a philosopher he could not. There are some scientific men to whom it always seems a perversion of their principles to attempt to carry them any beyond their application to the details of the subject. They look on in a bewildered and protesting