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of being brought into community?” And his answer is that what Leibniz really implied was not a harmony between independent things, but a harmony between modes of knowing, between sense on the one hand and understanding on the other. The “Critique of Pure Reason” carried the discussion farther by pointing out its grounds; namely, that, without the unity of sense and understanding, no experience would be possible. Why there should be this harmony, why we should have experience, this question it is impossible to answer, says Kant,—adding that Leibniz confessed as much when he called it a “pre-established” harmony, thus not explaining it, but only referring it to a highest cause. That Leibniz really means a harmony within intelligence, not a harmony of things by themselves, is made more clear, according to Kant, from the fact that it is applied also to the relation between the kingdom of nature and of grace, of final and of efficient causes. Here the harmony is clearly not between two independently existing external things, but between what flows from our notions of nature (Naturbegriffe) and of freedom (Freiheitsbegriffe); that is, between two distinct powers and principles within us,—an agreement which can be explained only through the idea of an intelligent cause of the world.
If we review these points in succession, the influence of Leibniz upon Kant becomes more marked. As to the first one, it is well known that Kant’s philosophy is based upon, and revolves within, the distinction of analytic and synthetic judgments; and this distinction Kant clearly refers to the Leibnizian distinction between the principles of contradiction and of sufficient reason, or of identity and differentiation. It is not meant that Kant came to this thought through the definitions of Leibniz; on the contrary, Kant himself refers it to Hume’s distinction between matters of fact and relations of ideas. But when Kant had once generalized the thought of Hume, it fell at once, as into ready prepared moulds, into the categories of Leibniz. He never escapes from the Leibnizian distinction. In his working of it out consists his greatness as the founder of modern thought; from his acceptance of it as ultimate result his contradictions. That is to say, Kant did not merely receive the vague idea of sufficient reason: he so connected it with what he learned from Hume that he transformed it into the idea of synthesis, and proceeded to work out the conception of synthesis in the various notions of the understanding, or categories, as applicable to the material of sense. What Leibniz bequeathed him was the undefined idea that knowledge of matters of fact rests upon the principle of sufficient reason. What Kant did with this inheritance was to identify the wholly vague idea of sufficient reason with the notion that every fact of experience rests upon necessary synthetic connection,—that is, connection according to notions of understanding with other facts,—and to determine, so far as he could, the various forms of synthesis, or of sufficient reason. With Leibniz the principle remained essentially infertile, because it was the mere notion of the ultimate reference of experience to understanding. In the hands of Kant, it became the instrument of revolutionizing philosophy, because Kant showed the articulate members of understanding by which experience is constituted, and described them in the act of constituting.
So much for his working out of the thought. But on the other hand, Kant never transcended the absoluteness of the distinction between the principles of synthesis and analysis, of sufficient reason and contradiction. The result was that he regarded the synthetic principle as the principle only of our knowledge, while perfect knowledge he still considered to follow the law of identity, of mere analysis. He worked out the factor of negation, of differentiation, contained in the notion of synthesis, but limited it to synthesis upon material of sense, presupposing that there is another kind of knowledge, not limited to sense, not depending upon the synthetic principle, but resting upon the principle of contradiction, or analysis, and that this kind is the type, the norm, of the only perfect knowledge. In other words, while admitting the synthetic principle of differentiation as a necessary element within our knowledge, he held that on account of this element our knowledge is limited to the phenomenal realm. Leibniz’s error was in supposing that the pure principles of the logical understanding, resting on contradiction, could give us knowledge of the noumenal world; his truth was in supposing that only by such principles could they be known. Thus, in substance, Kant. Like Leibniz, in short, he failed to transcend the absoluteness of the value of the scholastic method; but he so worked out another and synthetic method,—the development of the idea of sufficient reason,—that he made it necessary for his successors to transcend it.
The second point concerns the relations of the sensuous and the super-sensuous. Here, besides setting right the ordinary misconception of Leibniz, Kant did nothing but render him consistent with himself. Leibniz attempted to prove the existence of God, as we have seen, by the principles both of sufficient reason and contradiction. Kant denies the validity of the proof by either method. God is the sufficient cause, or reason, of the contingent sense world. But since Leibniz admits that this contingent world may, after all, be but a dream, how shall we rise from it to the notion of God? It is not our dreams that demonstrate to us the existence of reality. Or, again, sense-knowledge is confused knowledge. How shall this knowledge, by hypothesis imperfect, guarantee to us the existence of a perfect being? On the other hand, since the synthetic principle, or that of sufficient reason, is necessary to give us knowledge of matters of fact, the principle of contradiction, while it may give us a consistent and even necessary notion of a supreme being, cannot give this notion reality. Leibniz, while admitting, with regard to all other matters of fact, that the principles of formal logic can give no unconditional knowledge, yet supposes that, with regard to the one unconditional reality, they are amply sufficient. Kant but renders him self-consistent on this point.
It is, however, with regard to the doctrine of pre-established harmony that Kant’s large measure of indebtedness to Leibniz is most apt to be overlooked. Kant’s claim that Leibniz himself meant the doctrine in a subjective sense (that is, of a harmony between powers in our own intelligence) rather than objective (or between things out of relation to intelligence) seems, at first sight, to go far beyond the mark. However, when we recall that to Leibniz the sense world is only the confused side of rational thought, there is more truth in Kant’s saying than appears at this first sight. The harmony is between sense and reason. But it may at least be said without qualification that Kant only translated into subjective terms, terms of intelligence, what appears in Leibniz as objective. This is not the place to go into the details of Kant’s conception of the relation of the material to the psychical, of the body and the soul. We may state, however, in his own words, that “the question is no longer as to the possibility of the association of the soul with other known and foreign substances outside it, but as to the connection of the presentations of inner sense with the modifications of our external sensibility.” It is a question, in short, of the harmony of two modes of our own presentation, not of the harmony of two independent things. And Kant not only thus deals with the fact of harmony, but he admits, as its possible source, just what Leibniz claims to be its actual source; namely, some one underlying reality, which Leibniz calls the monad, but to which Kant gives no name. “I can well suppose,” says Kant, “that the substance to which through external sense extension is attributed, is also the subject of the presentations given to us by its inner sense: thus that which in one respect is called material being would be in another respect thinking being.”
Kant treats similarly the problem of the relations of physical and final causes, of necessity and freedom. Here, as in the case just mentioned, his main problem is to discover their harmony. His solution, again, is in the union, in our intelligence, of the understanding—as the source of the notions which “make nature”—with the ideas of that reason which gives a “categorical imperative.” The cause of the possibility of this harmony between nature and freedom, between the sense world and the rational, he finds in a being, God, whose sole function in the Kantian philosophy may be said to be to “pre-establish” it. I cannot believe that Kant, in postulating the problems of philosophy as the harmony of sense and understanding, of nature and freedom, and in finding this harmony where he did, was not profoundly influenced, consciously as well as unconsciously, by Leibniz. In fact, I do not think that we can understand the nature either of Kant’s immense contributions to modern thought or of his inconsistencies, until we have traced them to their source in the Leibnizian philosophy,—admitting, on the other hand, that we cannot understand why Kant should have found necessary a new way of approach to the results of Leibniz, until we recognize to the full his indebtedness