Letter to Simon Foucher
(1675)

by Gottfried Wilhelm Leibniz

I agree with you that it is important once and for all to examine all our presuppositions in order to establish something sound. For I hold that it is only when we can prove everything we assert that we understand perfectly the thing being considered. I know that such studies are not very popular, but I also know that to take the pains to understand matters to their roots is not very popular. As I see it, your purpose is to examine those truths which affirm that there is something outside of us. You seem to be most fair in this, for thus you will grant us all hypothetical truths which affirm, not that something does exist outside of us, but only what would happen if anything existed there. So we at once save arithmetic, geometry, and a large number of propositions in metaphysics, physics, and morals, whose convenient expression depends on arbitrarily chosen definitions, and whose truth depends on those axioms which I am wont to call identical; such, for example, as that two contradictories cannot exist and that at any given time a thing is as it is; that it is, for example, equal to itself, as great as itself, similar to itself, etc.

But although you do not enter explicitly into an examination of hypothetical propositions, I am still of the opinion that this should be done and that we should admit none without having entirely demonstrated and resolved it into identities. It is the truths which deal with what is in fact outside of us which are the primary subject of your investigations. Now in the first place, we cannot deny that the very truth of hypothetical propositions themselves is something outside of us and independent of us. For all hypothetical propositions assert what would be or would not be, if something or its contrary were posited; consequently, they assume two things at the same time which agree with each other, or the possibility or impossibility, necessity or indifference, of something. But this possibility, impossibility, or necessity (for the necessity of one thing is the impossibility of its contrary) is not a chimera which we create, since all that we do consists in recognizing them, in spite of ourselves and in a constant manner. So of all the things which actually are, the possibility or impossibility of being is itself the first. But this possibility and this necessity form or compose what are called the essences or natures and the truths which are usually called eternal. And we are right in calling them this, for there is nothing so eternal as what is necessary. Thus the nature of the circle with its properties is something which exists and is eternal, that is, there is some constant cause outside of us which makes everyone who thinks carefully about a circle discover the same thing, not merely in the sense that their thoughts agree with each other, for this could be attributed solely to the nature of the human mind, but also in the sense that phenomena or experiences confirm them when some appearance of a circle strikes our senses. These phenomena necessarily have some cause outside of us.

But although the existence of necessities comes before all others in itself and in the order of nature, I nevertheless agree that it is not first in the order of our knowledge. For you see that in order to prove its existence, I have taken for granted that we think and that we have sensations. So there are two absolute general truths; truths, that is, which tell of the actual existence of things. One is that we think; the other, that there is a great variety in our thoughts. From the former it follows that we are; from the latter, that there is something other than us, that is to say, something other than that which thinks, which is the cause of the variety of our experiences. Now one of these truths is just as incontestable and as independent as the other, and having stressed only the former in the order of his meditations, Descartes failed to attain the perfection to which he had aspired. If he had followed with exactness what I call a filum meditandi, I believe that he would really have achieved the first philosophy. But not even the greatest genius can force things; we must of necessity enter through the openings which nature has made, in order to avoid being lost. What is more, one man alone cannot do everything all at once, and for myself, when I think of all that Descartes has said that is excellent and original, I am more amazed at what he has done than at some things which he failed to do. I admit that I have not yet been able to read his writings with all the care that I had intended to give them, and as my friends know, it happened that I read most of the other modern philosophers before I read him. Bacon and Gassendi were the first to fall into my hands. Their familiar and easy style was better adapted to a man who wanted to read everything. It is true that I have often glanced through Galileo and Descartes, but since I have only recently become a geometrician, I was soon repelled by their style of writing, which requires deep meditation. Personally, though I have always loved to think by myself, I have always found it hard to read books which one cannot understand without much meditation, for in following one's own thoughts one follows a certain natural inclination and so gains profit with pleasure. One is violently disturbed, in contrast, when compelled to follow the thoughts of someone else. I always liked books which contained some good thoughts, but which I could run through without stopping, for they aroused ideas in me which I could follow up in my own fancy and pursue as far as I pleased. This also prevented me from reading the books on geometry carefully; I freely admit that I have not yet been able to make myself read Euclid in any other way than one usually reads history. I have learned from experience that this method is good in general, yet I have recognized nevertheless that there are authors for whom one must make an exception, such as Plato and Aristotle among ancient philosophers, and Galileo and Descartes among our own. Yet what I know of the metaphysical and physical meditations of Descartes has come almost entirely from the reading of a number of books written in a more popular style which report his opinions. And perhaps I have not as yet understood him well. To the extent that I have read him over myself, however, it seems to me that I have at least been able to discover what he has not done or tried to do, and among other things, this is to analyze all our assumptions. This is why I am inclined to applaud all who examine even the smallest truth to the end, for I know that it is much to understand something perfectly, no matter how small or easy it may seem. One can go very far in this way and finally establish the art of discovery, which depends on knowledge of the simplest things, but on a distinct and perfect knowledge of them. It is for this reason that I have found no fault with the plan of De Roberval, who tried to demonstrate everything in geometry, even some of the axioms. I grant that we should not enforce such exactness upon others, but I believe that it is good to demand it of ourselves.

But I return to these truths which are primary with respect to ourselves, and first to those which assert that there is something outside of us; namely, that we think and that there is a great variety in our thoughts. This variety cannot come from that which thinks, since one thing by itself cannot be the cause of the changes occurring in it. For everything remains in the state in which it is, unless there is something which changes it. And since it has not been determined by itself to undergo certain changes rather than others, we cannot begin to attribute any variety to it without saying something which admittedly has no reason, which is absurd. Even if we tried to say that our thoughts have no beginning, we should be obliged to assert that each of us has existed from all eternity; yet we should not escape the difficulty, for we should always have to admit that there is no reason for this variety which would have existed from all eternity in our thoughts, since there is nothing in us which determines us to one variety rather than another. Thus there is some cause outside of us for the variety of our thoughts. And since we agree that there are some subordinate causes of this variety which themselves still need a cause, we have established particular beings or substances to whom we ascribe some action, that is, from whose change we think that some change follows in us. So we make great strides toward fabricating what we call matter and body.

But at this point you are right in stopping us for a while and renewing the criticisms of the ancient Academy. For at bottom all our experiences assure us of only two things: first, that there is a connection among our appearances which provides the means to predict future appearances successfully; and, second, that this connection must have a constant cause. But it does not follow strictly from this that matter or bodies exist but only that there is something which gives us appearances in a good sequence. For if some invisible power were to take pleasure in giving us dreams that are well tied into our preceding life and in conformity with each other, could we distinguish them from reality before we had awakened? Now, what prevents the course of our life from being one long well‑ordered dream, about which we could be undeceived in a moment? Nor do I see that such a power would be imperfect just on this ground, as Descartes asserts, to say nothing of the fact that its imperfection is not involved in the present question. For it might be a kind of subordinate power, or a demon who for some unknown reason could interfere with our affairs and who would have at least as much power over us as that caliph had over the man whom he caused to be carried, drunk, into his palace, and let taste of the paradise of Mohammed after he was awakened; after which he was once more made drunk and returned in that condition to the place where he had been found. When this man came to himself, he naturally interpreted this experience, which seemed inconsistent with the course of his life, as a vision, and spread among the people maxims and revelations which he believed he had learned in his pretended paradise; this was precisely what the caliph wished. Since reality has thus passed for a vision, what is to prevent a vision from passing for reality? The more consistency we see in what happens to us, it is true, the more our belief is confirmed that what appears to us is reality. But it is also true that, the more closely we examine our appearances, the better ordered we find them, as microscopes and other means of observation have shown. This permanent consistency gives us great assurance, but after all, it will be only moral until somebody discovers a priori the origin of the world which we see and pursues the question of why things are as they appear back to its foundations in essence. For when this is done, he will have demonstrated that what appears to us is reality and that it is impossible for us ever to be deceived in it. But I believe that this would very nearly approach the beatific vision and that it is difficult to aspire to this in our present state. Yet we do learn therefrom how confused the knowledge which we commonly have of the body and matter must be, since we believe we are certain that they exist, but eventually find that we could be mistaken. This confirms Mr. Descartes’s excellent thought concerning the proof of the difference between body and soul, since one can doubt the one without being able to question the other. For even if there were only appearances or dreams, we should be nonetheless certain of the existence of that which thinks, as Descartes has very well said. I may add that one could still demonstrate the existence of God by ways different from those of Descartes but, I believe, leading farther. For we have no need to assume a being who guarantees us against being deceived, since it lies in our power to undeceive ourselves about many things, at least about the most important ones.

I wish, Sir, that your meditations on this matter may have all the success you desire; but to accomplish this, it is well to proceed in order and to establish your propositions. This is the way to gain ground and make sure progress. I believe you would oblige the public also by conveying to it, from time to time, selections from the Academy and especially from Plato, for I know that there are things in them more beautiful and substantial than is usually thought.

SOURCE: Leibniz, Gottfried Wilhelm. Letter to Simon Foucher with Notes on Foucher’s Reply to Des Gabets (1675, 1676), in Philosophical Papers and Letters, selection translated and edited with an introduction by Leroy E. Loemker, 2^nd ed. (Dordrecht, Holland; Boston: D. Reidel Pub. Co., 1976 [1969, 1^st ed. 1956]), pp. 151-156. (Synthese Historical Library; v. 2)

Part I: Letter [G., I, 369‑74] (1675), pp. 151-154. Part II: Notes on the Reply of Foucher to the Criticism of His Criticism of the ‘Recherche de la Vérité’ (1676), pp. 154-155. References, pp. 155-156.

Only the letter is reproduced here. The introductory paragraph and the footnotes are omitted.

Note: The passage beginning with “Personally . . .” — i.e. Leibniz’s approach to reading, seems to be the most oft-quoted portion of this letter, which mostly concerns Descartes and Malebranche. From some other source, for some reason, I hand-copied said passage on 20 January 1972. I also engaged in some shameless wordplay back then, as I wrote in my diary on 29 December 1971: “My Leibnitz are killing me.”

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