I would recall in this connection the very significant conflict, and one which the historians of philosophy for some unknown reason neglect, between Leibniz and the already deceased Descartes. In his letters Descartes several times expresses his conviction that the eternal truths do not exist from all eternity and by their own will, as their eternity would require, but that they were created by God in the same way as He created all that possesses any real or ideal being. “If I affirm,” writes Descartes, “that there cannot be a mountain without a valley, this is not because it is really impossible that it should be otherwise, but simply because God has given me a reason which cannot do other than assume the existence of a valley wherever there is a mountain.” Citing these words of Descartes, Bayle agrees that the thought which they express is remarkable, but that he, Bayle, is incapable of assimilating it; however, he does not give up the hope of someday succeeding in this. Now Leibniz, who was always so calm and balanced and who ordinarily paid such sympathetic attention to the opinions of others, was quite beside himself every time he recalled this judgment of Descartes. Descartes, who permitted himself to defend such absurdities, even though it was only in his private correspondence, aroused his indignation, as did also Bayle whom these absurdities had seduced.
Indeed, if Descartes “is right,” if the eternal truths are not autonomous but depend on the will, or, more precisely, the pleasure of the Creator, how would philosophy or what we call philosophy be possible? How would truth in general be possible? When Leibniz set out on the search for truth, he always armed himself with the principle of contradiction and the principle of sufficient reason, just as, in his own words, a captain of a ship arms himself on setting out to sea with a compass and maps. These two principles Leibniz called his invincible soldiers. But if one or the other of these principles is shaken, how is truth to be sought? There is something here about which one feels troubled and even frightened. Aristotle would certainly have declared on the matter of the Cartesian mountain without a valley that such things may be said but cannot be thought. Leibniz could have appealed to Aristotle, but this seemed to him insufficient. He needed proofs but, since after the fall of the principles of contradiction and of sufficient reason the very notion of proof or demonstrability is no longer anything but a mirage or phantom, there remained only one thing for him to do—to be indignant. Indignation, to be sure, is an argumentum ad hominem [an argument directed at the man]; it ought then to have no place in philosophy. But when it is a question of supreme goods, man is not too choosy in the matter of proof, provided only that he succeeds somehow or other in protecting himself . . .
Leibniz’s indignation, however, is not at bottom distinguished from the Kantian formulas—“reason aspires avidly,” “reason is irritated,” etc. Every time reason greatly desires something, is someone bound immediately to furnish whatever it demands? Are we really obliged to flatter all of reason’s desires and forbidden to irritate it? Should not reason, on the contrary, be forced to satisfy us and to avoid in any way arousing our irritation?
Kant could not resolve to “criticize” reason in this way and the Kantian critique of reason does not ask such questions, just as the pre-critical philosophy never asked them. Plato and Aristotle, bewitched by Socrates, and, after them, modern philosophy—Descartes, Spinoza, Leibniz, as well as Kant—seek, with all the passion of which men are capable, universal and necessary truths—the only thing, according to them, which is worthy of being called “knowledge.” In short, it would hardly be extravagant to say that the problem of knowledge, or more exactly, knowledge as a problem, not only has never drawn the attention of the most notable representatives of philosophical thought but has repelled them. Everyone has been convinced that man needs knowledge more than anything else in the world, that knowledge is the only source of truth, and especially—I emphasize this particularly and insist upon it—that knowledge furnishes us with universal and necessary truths which embrace all being, truths from which man cannot escape and from which there is consequently no need to escape. Leibniz said that the “eternal truths” are not content to constrain but do something still more important: they “persuade.” And it is not, of course, only Leibniz personally whom they persuade but all men; Leibniz would not have ascribed any value to truths capable of persuading him but incapable of persuading others or even of constraining them.
In this respect there is hardly any difference between Leibniz and Kant. The latter has told us that reason avidly aspires to necessary and universal judgments. It is true that, in the case of Kant, the element of constraint seems to play a decisive and definitive role: even if there should be men whom the truths do not persuade, whom they irritate as experience irritates Kant, this would be no great misfortune; the truths would nevertheless constrain them and thus fully succeed in justifying themselves. And, in the last analysis, does not constraint persuade? In other words, truth is truth so long as it has demonstrative proofs at its disposal. As for indemonstrable truths, no one has any need of them and they appear to be incapable of persuading even a Leibniz.
It is this that determines Kant’s attitude towards metaphysics. It is known that according to Kant, who speaks of this more than once in his Critique of Reason, metaphysics has as its object three problems—God, the immortality of the soul, and freedom. But suddenly it appears that the final result of the Kantian critique is that none of these three metaphysical truths is demonstrable and that there can be no scientific metaphysics. One would have thought that such a discovery would have shaken Kant’s soul to its deepest foundations. But it did nothing of the sort. In his Preface to the Second Edition of The Critique of Pure Reason, Kant declares calmly, almost solemnly: “I had to renounce knowledge (Wissen) in order to make room for faith (Glauben).” So Kant speaks in this same Preface, where we read the following lines: “It will always be a scandal for philosophy and human reason in general that we must accept the existence of things outside ourselves merely on faith3 and that, if someone should take it into his head to doubt it, we would be incapable of setting before him any sufficient proof.” It is impossible to prove the existence of God, the immortality of the soul, or free will, but there is nothing offensive or disturbing in this either for philosophy or for human reason; all these will get along without proof and will content themselves with faith, with what Kant and everyone call faith. But when it is a question of the existence of objects outside ourselves, then faith does not suffice, then it is absolutely necessary to have proof. And yet, if one admits Kant’s point of departure, the existence of objects outside ourselves is hardly in a more enviable situation, as far as proof is concerned, than God, the immortality of the soul, or free will. At best, the existence of objects outside ourselves can be postulated or be an object of faith. But it is this that Kant cannot endure, just as Leibniz could not endure Descartes’ mountain without a valley. And Kant, not having at his disposal any convincing demonstration, just like Leibniz, did not recoil before the use of an argumentum ad hominem, before indignation: if we do not succeed in knowing that things exist outside ourselves, then philosophy and reason are forever covered with shame; it is a “scandal! . . .”
Why did Leibniz so passionately defend his eternal truths, and why was be so horrified at the idea of subordinating them to the Creator? Why did Kant take to heart the fate of objects outside ourselves, while the fate of God, of the soul and of freedom left him untouched? Is it not just the opposite which should have happened? The “scandal” of philosophy, one would think, consists in the impossibility of proving the existence of God. One would also think that the dependence of God on the truths would poison man’s mind and fill it with horror. So one would think; but in reality it was the contrary of this that occurred. Reason, which aspires eagerly to necessity and universality, has obtained all that it wished and the greatest representatives of modern philosophy have expelled everything which could irritate reason to the region of the “supra-sensible” from which no echo comes to us and where being is confounded with non-being in a dull and dreary indifference.
Even before The Critique of Pure Reason Kant wrote to Marcus