With Leibnitz, on the other hand, the logical problem holds the foremost place in philosophical inquiry.[122] From the purely logical thesis, developed at quite an early stage of his thinking,[123] that in any true proposition the predicate is Leibnitz. contained in the subject, the main principles of his doctrine of Monads are derivable with the minimum of help from his philosophy of dynamics. Praedicatum inest subjecto. All valid propositions express in the last resort the relation of predicate or predicates to a subject, and this Leibnitz holds after considering the case of relational propositions where either term may hold the position of grammatical subject, A = B and the like. There is a subject then, or there are subjects which must be recognized as not possible to be predicated, but as absolute. For reasons not purely logical Leibnitz declares for the plurality of such subjects. Each contains all its predicates: and this is true not only in the case of truths of reason, which are necessary, and ultimately to be exhibited as coming under the law of contradiction, “or, what comes to the same thing, that of identity,” but also in the case of truths of fact which are contingent, though a sufficient reason can be given for them which “inclines” without importing necessity. The extreme case of course is the human subject. “The individual notion of each person includes once for all what is to befall it, world without end,” and “it would not have been our Adam but another, if he had had other events.” Existent subjects, containing eternally all their successive predicates in the time-series, are substances, which when the problems connected with their activity, or dynamically speaking their force, have been resolved, demand—and supply—the metaphysic of the Monadology.
Complex truths of reason or essence raise the problem of definition, which consists in their analysis into simpler truths and ultimately into simple—i.e. indefinable ideas, with primary principles of another kind—axioms, and postulates that neither need nor admit of proof. These are identical in the sense that the opposite contains an express contradiction.[124] In the case of non-identical truths, too, there is a priori proof drawn from the notion of the terms, “though it is not always in our power to arrive at this analysis,”[125] so that the question arises, specially in connexion with the possibility of a calculus, whether the contingent is reducible to the necessary or identical at the ideal limit. With much that suggests an affirmative answer, Leibnitz gives the negative. Even in the case of the Divine will, though it be always for the best possible, the sufficient reason will “incline without necessitating.” The propositions which deal with actual existence are still of a unique type, with whatever limitation to the calculus.
Leibnitz’s treatment of the primary principles among truths of reason as identities, and his examples drawn inter alia from the “first principles” of mathematics, influenced Kant by antagonism. Identities some of them manifestly were not. The formula of identity passed in another form to Herbart and therefore to Lotze. In recognizing, further, that the relation of an actual individual fact to its sufficient ground was not reducible to identity, he set a problem diversely treated by Kant and Herbart. He brought existential propositions, indeed, within a rational system through the principle that it must be feasible to assign a sufficient reason for them, but he refused to bring them under the conception of identity or necessity, i.e. to treat their opposites as formally self-contradictory. This bore interest in the Kantian age in the treatment alike of cause and effect, and of the ontological proof of existence from essence. Not that the Law of Sufficient Reason is quite free from equivoque. Propositions concerning the possible existence of individuals put Leibnitz to some shifts, and the difficulty accounts for the close connexion established in regard to our actual world between the law of sufficient reason and the doctrine of the final cause. This connexion is something of an afterthought to distinguish from the potential contingency of the objectively possible the real contingency of the actual, for which the “cause or reason” of Spinoza[126] could not account. The law, however, is not invalidated by these considerations, and with the degree of emphasis and the special setting that Leibnitz gives the law, it is definitely his own.
If we may pass by the doctrine of the Identity of Indiscernibles, which played a part of some importance in subsequent philosophy, and the Law of Continuity, which as Leibnitz represents it is, if not sheer dogma, reached by something very like a fallacy, we have as Leibnitz’s remaining legacy to later logicians the conception of Characteristica Universalis and Ars Combinatoria, a universal denoting by symbols and a calculus working by substitutions and the like. The two positions that a subject contains all its predicates and that all non-contingent propositions—i.e. all propositions not concerned with the existence of individual facts ultimately analyse out into identities—obviously lend themselves to the design of this algebra of thought, though the mathematician in Leibnitz should have been aware that a significant equation is never an identity. Leibnitz, fresh from the battle of the calculus in the mathematical field, and with his conception of logic, at least in some of its aspects, as a generalized mathematic,[127] found a fruitful inspiration, harmonizing well with his own metaphysic, in Bacon’s alphabet of nature. He, too, was prepared to offer a new instrument. That the most important section, the list of forms of combination, was never achieved—this too was after the Baconian example while the mode of symbolization was crude with a = ab and the like—matters little. A new technique of manipulation—it is, of course, no more—had been evolved.
It may be said that among Leibnitz’s successors there is no Leibnitzian. The system as a whole is something too artificial to secure whole-hearted allegiance. Wolff’s formalism is the bastard outcome of the speculation of Leibnitz, and is related to it as remotely as Scholasticism is to Aristotle. Wolff found a sufficient reason for everything and embodied the results of his inquiries in systematic treatises, sometimes in the vernacular. He also, by a transparent petitio principii, brought the law of the sufficient reason under that of non-contradiction. Wolff and his numerous followers account for the charge of dogmatism against “the Leibnitzio-Wolffian school.” They are of importance in the history of logic for two reasons only: they affected strongly the German vocabulary of philosophy and they constituted the intellectual environment in which Kant grew to manhood.
A truer continuator of Leibnitz in the spirit was Herbart.
iii. Kant’s Logic.
Herbart’s admitted allegiance, however, was Kantian with the qualification, at a relatively advanced stage of his thinking, that it was “of the year 1828”—that is, after controversy had brought out implications of Kant’s teaching not wholly contemplated by Kant himself. The critical philosophy had indeed made it impossible to hark back to Leibnitz or any other master otherwise than with a difference.
Yet it is not a single and unambiguous logical movement that derives from Kant. Kant’s lesson was variously understood. Different moments in it were emphasized, with a large diversity of result. As interpreted it was acquiesced in or revolted from and revolt ranged from a desire for some modifications of detail or expression to the call for a radical transformation. Grounds for a variety of developments are to be found in the imperfect harmonization of the rationalistic heritage from the Wolffian tradition which still dominates Kant’s pure general logic with the manifest epistemological intention of his transcendental theory. Or again, within the latter in his admission of a duality of thought and “the given” in knowledge, which within knowledge was apparently irreducible, concurrently with hints as to the possibility, upon a wider view, of the sublation of their disparateness at least hypothetically and speculatively. The sense in which there must be a ground of the unity of the supersensible[128] while yet the transcendent use of Reason—i.e. its use beyond the limits of experience was denied theoretical validity—was not unnaturally regarded as obscure.
Kant’s treatment of technical logic was wholly traditional, and in itself is almost negligible. It is comprised[129] in an early essay on the mistaken subtlety of the syllogistic figures, and a late compilation by a pupil from the introductory matter and Formal Logic. running annotations with which the master had enriched his interleaved lecture-room copy of Meyer’s Compendium of 1752. Wolff’s general logic, “the best,” said Kant, “that we possess,” had been abridged by Baumgarten and the abridgment then subjected to commentation by Meyer. With this traditional body of doctrine Kant was, save for matters of minor detail, quite content. Logic was of necessity formal, dealing as it must with those rules without which no exercise of the understanding would be possible at all. Upon abstraction from all particular methods of thought these rules were to be discerned a priori or without dependence on experience by reflection solely upon the use of the understanding in general. The science of the form of thought abstracted in this way from its matter or content was regarded as of value both as propaedeutic and as canon. It was manifestly one of the disciplines in which a position of finality was attainable. Aristotle might be allowed, indeed, to have omitted no essential point of the understanding, what the moderns had achieved consisted in an advance in accuracy and methodical completeness. “Indeed, we do not require any new discoverers in logic,”[130] said the discoverer of a priori synthesis, “since it contains merely the form of thought.” Applied logic is merely psychology, and not properly to be called logic at all. The technical logic of Kant, then, justifies literally a movement among his successors in favour of a formal conception of logic with the law of contradiction and the doctrine of formal implication for its equipment. Unless the doctrine of Kant’s “transcendental logic” must be held to supply a point of view from which a logical development of quite another kind is inevitable, Kant’s mantle, so far as logic is concerned, must be regarded as having fallen upon the formal logicians.