This introduces us to the relation of soul and body, or, more generally stated, to the relation of mind and matter. It is the theory of co-operation, of harmonious activity, which Leibniz substitutes for the theory which Descartes had formulated, according to which there are two opposed substances which can affect each other only through the medium of a deus ex machina. Locke, on the other hand, took the Cartesian principle for granted, and thus enveloped himself in all the difficulties which surround the question of “mind and matter.” Locke wavers between two positions, one of which is that there are two unknown substances,—the soul and the object in itself,—which, coming in contact, produce sensations; while the other takes the hypothetical attitude that there may be but one substance,—matter,—and that God, out of the plenitude of his omnipotence, has given matter a capacity which does not naturally belong to it,—that of producing sensations. In either case, however, the final recourse is to the arbitrary power of God. There is no natural—that is, intrinsic and explicable—connection between the sensation and that which produces it. Sensation occupied the hard position which the mechanical school of to-day still allots it. It is that “inexplicable,” “mysterious,” “unaccountable” link between the domains of matter and mind of which no rational account can be given, but which is yet the source of all that we know about matter, and the basis of all that is real in the mind!
Leibniz, recognizing that reality is an organic whole,—not two parts with a chasm between them,—says that “God does not arbitrarily give substances whatever qualities may happen, or that he may arbitrarily determine, but only such as are natural; that is, such as are related to one another in an explicable way as modifications of the substance.” Leibniz feels sure that to introduce the idea of the inexplicable, the purely supernatural, into the natural is to give up all the advantages which the modern mechanical theory had introduced, and to relapse into the meaningless features of scholasticism. If the “supernatural”—that is, the essentially inexplicable—is introduced in this one case, why should it not be in others; why should we not return outright to the “fanatic philosophy which explains all facts by simply attributing them to God immediately or by way of miracle, or to the barbarian philosophy, which explains phenomena by manufacturing, ad hoc, occult qualities or faculties, seemingly like little demons or spirits capable of performing, without ceremony, whatever is required,—as if watches marked time by their horodeictic power, without wheels, and mills ground grain, without grindstones, by their fractive power”? In fact, says Leibniz, by introducing the inexplicable into our explanations “we fall into something worse than occult qualities,—we give up philosophy and reason; we open asylums for ignorance and laziness, holding not only that there are qualities which we do not understand (there are, indeed, too many such), but qualities which the greatest intelligence, if God gave it all the insight possible, could not understand,—that is, such as are in themselves without rhyme or reason. And indeed it would be a thing without rhyme or reason that God should perform miracles in the ordinary course of nature.” And regarding the whole matter of introducing the inconceivable and the inexplicable into science, he says that “while the conception of men is not the measure of God’s power, their capacity of conception is the measure of nature’s power, since everything occurring in the natural order is capable of being understood by the created intelligence.” Such being the thought of Leibniz regarding the virtual attempt to introduce in his day the unknowable into philosophy, it is evident that he must reject, from the root up, all theories of sensation which, like Locke’s, make it the product of the inexplicable intercourse of two substances.
For this doctrine, then, Leibniz substitutes that of an infinite number of substances, all of the same kind, all active, all developing from within, all conspiring to the same end, but of various stages of activity, or bearing various relations of completeness to the one end.
Indeed, one and the same monad has various degrees of activity in itself; that is, it represents more or less distinctly the universe according to its point of view. Its point of view requires of it, of course, primarily, a representation of that which is about it. Thus an infinity of states arises, each corresponding to some one of the multitude of objects surrounding the monad. The soul has no control, no mastery, over these states. It has to take them as they come; with regard to them, the soul appears passive. It appears so because it does not as yet clearly distinguish them. It does not react upon them and become conscious of their meaning or thoroughly rational character. We shall afterwards see that “matter” is, with Leibniz, simply this passive or confused side of monads. It is the monad so far as it has not brought to light the rational activity which is immanent in it. At present we need only notice that the body is simply the part of matter or of passivity which limits the complete activity of any monad. So Leibniz says, “in so far as the soul has perfection, it has distinct thoughts, and God has accommodated the body to the soul. So far as it is imperfect and its perceptions are confused, God has accommodated the soul to the body in such a way that the soul lets itself be inclined by the passions, which are born from corporeal representations. It is by its confused thoughts (sensations) that the soul represents the bodies about it,” just as, we may add, its distinct thoughts represent the monads or souls about it, and, in the degree of their distinctness, God, the monad which is purus actus.
Following the matter into more detail, we may say that since God alone is pure energy, knowing no limitation, God alone is pure spirit. Every finite soul is joined to an organic body. “I do not admit,” says Leibniz, “that there are souls entirely separate from matter, nor created spirits detached from body. . . . It is this body which the monad represents most distinctly; but since this body expresses the entire universe by the connection of all matter throughout it, the soul represents the entire universe in representing the body which belongs to it most particularly.” But according to the principle of continuity there must be in the least apparent portion of matter still “a universe of creatures, of souls, of entelechies. There is nothing sterile, nothing dead in the universe. It is evident from these considerations that every living body has a dominant entelechy, which is the soul in that body, but that the members of this living body are again full of other living beings and souls,” which, however, since not of so high a grade, that is, not representing the universe so fully, appear to be wholly material and subject to the “dominant” entelechy; namely, to the one which gives the law to the others by expressing more adequately the idea at which they only confusedly aim. Owing to the constant change of activity, however, these particles do not remain in constant subordination to the same entelechy (that is, do not form parts of the same body), but pass on to higher or lower degrees of “evolution,” and have their places taken by others undergoing similar processes of change. Thus “all bodies are in a perpetual flux, like rivers, with parts continually leaving and entering in.” Or, interpreting this figurative language, each monad is continually, in its process of development, giving law to new and less developed monads, which therefore appear as its body. The nature of matter in itself, and of its phenomenal manifestation in the body, are, however, subjects which find no explanation here, and which will demand explanation in another chapter.
We may sum up Leibniz’s theory of sensation by saying that it is a representative state developed by the self-activity of the soul; that in itself it is a confused or “involved” grade of activity, and in its relation to the world represents the confused or passive aspects of existence; that this limitation of the monad constitutes matter, and in its necessary connection with the monad constitutes the body which is always joined to the finite soul; that to this body are joined in all cases an immense number of monads, whose action is subordinate to that of this dominant monad, and that it is the collection of these which constitute the visible animal body. Thus if we look at sensation with regard to the monad which possesses it, it is a product of the body of the monad; if we look at it with reference to other monads, it represents or reflects their passive or material side. This is evidently one aspect again of the pre-established harmony,—an aspect in which some of the narrower of Leibniz’s critics have seen the whole meaning of the doctrine exhausted. It is, however, simply one of the many forms in which the harmony, the union of spiritual and mechanical, ideal and material, meets us. In