The monad, which is pure activity, that which culminates the scale of reality, has no confused ideas, and to it all knowledge is eternally rational, having no sensible traces about it. But every other monad, having its activity limited, has ideas which come to it at first in a confused way, and which its activity afterwards differentiates. Thus it is that Leibniz can agree so heartily with the motto of the Sensationalist school,—that there is nothing in the intellect which was not first in the sensory. But Leibniz uses this phrase as Aristotle would have done, having in mind the distinction between potentiality and actuality. In posse, sensation is all knowledge; but only in posse. And he, like Aristotle, interprets the relation between potentiality and actuality as one of a difference of activity. The potential is that which becomes real through a dynamic process. The actual is capacity plus action. Sensation, in short, is spiritual activity in an undeveloped and hence partial and limited condition. It is not, as Locke would have it, the real factor in all knowledge.
The marks of sensation which Locke lays down,—their passivity, their simplicity, their position as the real element in knowledge,—Leibniz either denies, therefore, or accepts in a sense different from that of Locke. Strictly speaking, sensation is an activity of the mind. There are no windows through which the soul receives impressions. Pure passivity of any kind is a myth, a scholastic fiction. Sensation is developed from the soul within; it is the activity of reality made manifest to itself. It is a higher kind of action than anything we find in minerals or in plants. If we look at sensation ideally, however, that is, according to the position which it holds in the system of knowledge, it is properly regarded as passive. It represents the limitation, the unrealized (that is, the non-active) side of spiritual life.
“Efficient causality” is a term which has its rightful and legitimate use in physical science. Simply from the scientific point of view we are correct in speaking of objects as affecting the body, and the body, through its nervous system, as affecting the soul and producing sensations. But philosophy does not merely use categories, it explains them. And Leibniz contends that to explain the category of causality in a mechanical sense, to understand by it physical influence actually transferred from one thing to another, is to make the idea inexplicable and irrational. The true meaning of causality is ideal. It signifies the relative positions which the objects concerned have in the harmonious system of reality. The body that is higher in the scale impresses the other; that is to say, it dominates it or gives its law. There is no energy or quality which passes physically from one to the other. But one monad, as higher in the stage of development than another, makes an ideal demand upon that one. It places before the other its own more real condition. The less-developed monad, since its whole activity consists in representing the universe of reality, answers to this demand by developing the corresponding quality in itself. The category of harmonious or co-operative action is thus substituted for that of external and mechanical influence. Physical causality when given a philosophic interpretation means organic development. The reality of a higher stage is the more active: the more active has a greater content in that it mirrors the universe more fully; it manifests accordingly more of the law of the universe, and hence has an ideal domination over that which is lower in the scale. It is actually (that is, in activity) what the other is potentially. But as the entire existence of the latter is in representing or setting forth the relations which make the world, its activity is aroused to a corresponding production. Hence the former is called “cause,” and the latter “effect.”
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
