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A careful study of the various theories which have been held concerning sensation would be of as much interest and importance as an investigation of any one point in the range of philosophy. In the theory of a philosopher about sensation we have the reflex of his fundamental category and the clew to his further doctrine. Sensation stands on the border-line between the world of nature and the realm of soul; and every advance in science, every development of philosophy, leaves its impress in a change in the theory of sensation. Apparently one of the simplest and most superficial of questions, in reality it is one of the most difficult and far-reaching. At first sight it seems as if it were a sufficient account of sensation to say that an object affects the organ of sense, and thus impresses upon the mind the quality which it possesses. But this simple statement arouses a throng of further questions: How is it possible that one substance,—matter,—should affect another,—mind? How can a causal relation exist between them? Is the mind passive or active in this impression? How can an object convey unchanged to the mind a quality which it possesses? Or is the sensational quale itself a product of the mind’s activity? If so, what is the nature of the object which excites the sensation? As known, it is only a collection of sensuous qualities; if these are purely mental, what becomes of the object? And if there is no object really there, what is it that excites the sensation? Such questionings might be continued almost indefinitely; but those given are enough to show that an examination of the nature and origin of sensation introduces us to the problems of the relation of intelligence and the world; to the problem of the ultimate constitution of an object which is set over against a subject and which affects it; and to the problem of the nature of mind, which as thus affected from without must be limited in its nature, but which as bearer of the whole known universe must be in some sense infinite. If we consider, not the mode of production of sensation, but its relation to knowledge, we find philosophical schools divided into two,—Sensationalists, and Rationalists. If we inquire into its functions, we find that the empiricist sees in it convincing evidence of the fact that all knowledge originates from a source extra mentem; that the intellectual idealist finds in it evidence of the gradual transition of nature into spirit; that the ethical idealist, like Kant and Fichte, sees in it the material of the phenomenal world, which is necessary in its opposition to the rational sphere in order that there may occur that conflict of pure law and sensuous impulse which alone makes morality possible. We thus realize that as we look at the various aspects of sensation, we are taken into the discussion of ontology, of the theory of knowledge and of ethics.

Locke virtually recognizes the extreme importance of the doctrine of sensation, and his second book might almost be entitled “Concerning the Nature and Products of Sensation.” On the other hand, one of the most characteristic and valuable portions of the reply of Leibniz is in his development of a theory of sensation which is thoroughly new, except as we seek for its germs in its thoughts of Plato and Aristotle. According to Locke, knowledge originates from two sources,—sensation and reflection. Sensations are “the impressions made on our senses by outward objects that are extrinsic to the mind.” When the mind “comes to reflect on its own operations about the ideas got by sensation, and thereby stores itself with a new set of ideas,” it gets ideas of reflection.

If we leave out of account for the present the ideas of reflection, we find that the ideas which come through sensation have two main characteristics. First, in having sensations, the mind is passive; its part is purely receptive. The objects impress themselves upon the mind, they obtrude into consciousness, whether the mind will or not. There is a purely external relation existing between sensation and the understanding. The ideas are offered to the mind, and the understanding cannot refuse to have them, cannot change them, blot them out, nor create them, any more than a mirror can refuse, alter, or obliterate the images which objects produce in it. Sensation, in short, is a purely passive having of ideas. Secondly, every sensation is simple. Locke would say of sensations what Hume said of all ideas,—every distinct sensation is a separate existence. Every sensation is “uncompounded, containing nothing but one uniform appearance, not being distinguishable into different ideas.” Knowledge is henceforth a process of compounding, of repeating, comparing, and uniting sensation. Man’s understanding “reaches no further than to compound and divide the materials that are made to his hand.”

It hardly need be said that Locke has great difficulty in keeping up this thoroughly atomic theory of mind. It is a theory which makes all relations external; they are, as Locke afterwards says, “superinduced” upon the facts. It makes it impossible to account for any appearance of unity and connection among ideas, and Locke quietly, and without any consciousness of the contradiction involved, introduces certain inherent relations into the structure of the ideas when he comes to his constructive work. “Existence and unity are two ideas,” he says, “that are suggested to the understanding by every object without, and every idea within.”

At other places he introduces the idea of quality of a substance, effect of a cause, continued permanence or identity into a sensation, as necessary constituents of it; thus making a sensation a unity of complex elements instead of an isolated bare notion. How far he could have got on in his account of knowledge without this surreptitious qualifying of a professedly simple existence, may be seen by asking what would be the nature of a sensation which did not possess existence and unity, and which was not conceived as the quality of a thing or as the effect of an external reality.

This digression has been introduced at this point because the next character of a sensation which Locke discusses is its objective character,—its relation to the object which produces it. To discourse of our ideas intelligibly, he says, it will be convenient to distinguish them as they are ideas in our minds and as they are modifications of matter in the bodies that cause them. In other words, he gives up all thought of considering ideas as simply mental modifications, and finds it necessary to take them in their relations to objects.

Taking them in this way, he finds that they are to be divided into two classes, of which one contains those ideas that are copies and resemblances of qualities in the objects, ideas “which are really in the object, whether we take notice of them or no,”—in which case we have an idea of the thing as it is in itself; while the other class contains those which are in no way resemblances of the objects which produce them, “having no more similitude than the idea of pain and of a sword.” The former are primary qualities, and are solidity, extension, figure, motion or rest, and number; while the secondary qualities are colors, smells, and tastes. The former ideas are produced by impulse of the bodies themselves, which simply effect a transference of their qualities over into the mind; while the secondary qualities are arbitrarily annexed by the power of God to the objects which excite them.

It will be noticed that there are two elements which make the sensation of Locke what it is. With reference to its production, it is the effect which one substance, matter, has upon another substance, mind, which is unlike it in nature, and between which whatever relations exist, are thoroughly incomprehensible, so that, indeed, their connections with each other can be understood only by recourse to a tertium quid, an omnipotent power which can arbitrarily produce such collocations as please it. With reference to its function, it is the isolated and “simple” (that is, non-relational) element out of which all actual forms of knowledge are made by composition and re-arrangement.

Leibniz, without entering into explicit criticism of just these two points, develops his own theory with reference to them. To Leibniz, reality constitutes a system; that is, it is of such a nature that its various portions have an essential and not merely external relation to one another. Sensation is of course no exception. It is not a mere accident, nor yet a supernatural yoking of things naturally opposed. It has a meaning in that connection of things which constitute the universe. It contributes to the significance of the world. It is one way in which those activities which make the real express themselves. It has its place or reason in the totality of things, and this whether we consider its origin or its position with regard to knowledge. In a word, while the characteristic of Locke’s theory is that he conceives sensation as in external relation both to reality, as mechanically produced by it, and to knowledge, as being merely one of the atomic elements which may enter into a compound, Leibniz regards reality as organic to sensation, and this in turn as organic to knowledge. We have here simply an illustration of the statement with which we set out; namely, that the treatment of sensation always reflects the fundamental philosophical category of the philosopher.

All reality exists in the form of monads; monads are simple substances whose nature is action; this action consists in representing, according to a certain law of succession, the universe. Various monads have various degrees of activity; that is, of the power of reflecting the world. So much of Leibniz’s general philosophical attitude it is necessary to recall, to understand what he means by “sensation.” The generic name which is applied to this mirroring activity of the monads is “perception,” which, as Leibniz often says, is to be carefully distinguished from apperception, which is the representation become conscious. Perception may be defined, therefore, as the inclusion of the many or multiform (the world of objects) in a unity (the simple substance). It was the great defect of previous philosophy that it “considered only spirits or self-conscious beings as souls,” and had consequently recognized only conscious perceptions. It had been obliged, therefore, to make an impassable gulf between mind and matter, and sensations were thus rendered inexplicable. But Leibniz finds his function as a philosopher in showing that these problems, which seem insoluble, arise when we insist upon erecting into actual separations or differences of kind what really are only stages of development or differences of degree. A sensation is not an effect which one substance impresses upon another because God pleased that it should, or because of an incomprehensible incident in the original constitution of things. It is a higher development of that representative power which belongs to every real being.

Certain monads reach a state of development, or manifestation of activity, which is characterized by the possession of distinct organs. Such monads may be called, in a pre-eminent sense, “souls,” and include all the higher animals as well as man. This possession of differentiated organs finds its analogue in the internal condition of the monad. What appears externally as an organ of sense appears ideally as a conscious representative state which we call “sensation.” “When,” Leibniz says, “the monad has its organs so developed that there is relief and differentiation in the impressions received, and consequently in the perceptions which represent them, we have feeling or sensation; that is, a perception accompanied by memory,” to which at other times he adds “attention.” Life, he says, “is a perceptive principle; the soul is sensitive life; mind is rational soul.” And again he says in substance that when the soul begins to have interests, and to regard one representation as of more value than others, it introduces relief into its perceptions, and those which stand out are called “sensations.”

This origin of sensations as higher developments of the representative activities of a monad conditions their relation to further processes of knowledge. The sensations are confused knowledge; they are ideas in their primitive and most undifferentiated form. They constitute, as Leibniz somewhere says, the vertigo of the conscious life. In every sentient organism multitudes of sensations are constantly thronging in and overpowering its distinct consciousness. The soul is so flooded with ideas of everything in the world which has any relation to its body that it has distinct ideas of nothing. Higher knowledge, then, does not consist in compounding these sensations; that would literally make confusion worse confounded. It consists in introducing distinctness into the previously confused sensations,—in finding out what they mean; that is, in finding out their bearings, what they point to, and how they are related. Knowledge is not an external process performed upon the sensations, it is the development of their internal content.

It follows, therefore, that sensation is organic to all forms of knowledge whatever. 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 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 truth, while in other systems the fact of sensation is a fact demanding some artificial mode of reconciling “mind” and “matter,” or is else to be accepted as an inexplicable fact, in the system of Leibniz it is itself evidence that the spiritual and the mechanical are not two opposed kinds of existence, but are organically united. It is itself the manifestation of the harmony of the ideal and the material, not something which requires that a factitious theory be invented for explaining their appearance of harmony. Sensation has within itself the ideal element, for it is the manifestation, in its most undeveloped form, of the spiritual meaning of the universe. It has a mechanical element, for it expresses the limitation, the passivity, of the monad.

It is from this standpoint that Leibniz criticises what Locke says about the relation of sensations to the objects which produce them. Leibniz holds that all our sensations have a definite and natural connection with the qualities of objects,—the “secondary” as well as the “primary.” They all represent certain properties of the object. Even the pain which the thrust of a needle gives us, while it does not resemble anything in the needle, does in some way represent or resemble motions going on in our body. This resemblance is not necessarily one of exact form, but just as the ellipse, hyperbola, and parabola are projections of the circle in the sense that there is a natural and fixed law of connection between them, so that every point of one corresponds by a certain relation with every point of the other, so the resemblance between the sensation and the quality of the object is always in the form of a fixed law of order, which, however unknown to us it may now be, is capable of being found out. If we are to make any distinction between “secondary” and “primary” sensations, it should be not that one presents qualities that are in the objects, and the other affections which exist only in us, but that the primary sensations (of number, form, size, etc.) represent the qualities in a distinct way, appealing to the rational activity of intelligence, while the secondary represent the qualities in a confused way, a way not going beyond the effect upon the mind into relations, that is, into distinct knowledge.

This brings regularly before us the question of the relation of sensations to knowledge. We have seen enough already to know that Leibniz does not believe that knowledge begins with the simple (that is, unrelated), and then proceeds by a process of compounding. The sensation is not simple to Leibniz, but thoroughly complex, involving confusedly within itself all possible relations. As relations are brought forth into distinct light out of this confusion, knowledge ends rather than begins with the simple. And again it is evident that Leibniz cannot believe that knowledge begins and ends in experience, in the sense in which both himself and Locke use the word; namely, as meaning the combination and succession of impressions.

“Experience,” as they use the term, consists in sensations and their association,—“consecution” as Leibniz calls it. Experience is the stage of knowledge reached by animals, and in which the majority of men remain,—and indeed all men in the greater part of their knowledge. Leibniz takes just the same position regarding the larger part of our knowledge which Hume takes regarding it all. It consists simply in associations of such a nature that when one part recurs there is a tendency to expect the recurrence of the other member. It resembles reason, but it is based on the accidental experience of events in a consecutive order, and not on knowledge of their causal connection. We all expect the sun to rise to-morrow; but with all of us, excepting the astronomer, such expectation is purely “empirical,” being based on the images of past experiences which recur. The astronomer, however, sees into the grounds, that is, the reasons, of the expectation, and hence his knowledge is rational.

Thus we have two grades of knowledge,—one empirical, consisting of knowledge of facts; the other rational, being of the truths of reason. The former is contingent and particular, the latter is necessary and universal. Leibniz insists, with a pertinacity which reminds us of Kant, that “experience” can give instances or examples only, and that the fact that anything has happened in a given way any number of times in the past, can give no assurance that it will continue to do so in the future. There is nothing in the nature of the case which renders its exact opposite impossible. But a rational truth is necessary, for its opposite is impossible, being irrational or meaningless. This may not always be evident in the case of a complex rational truth; but if it be analyzed into simpler elements, as a geometrical proposition into definitions, axioms, and postulates, the absurdity of its opposite becomes evident. Sensation, in conclusion, is the having of confused ideas,—ideas corresponding to matter. Experience is the association of these confused ideas, and their association according to their accidental juxtaposition in the life of the soul. It therefore is not only thoroughly sensible, but is also phenomenal. Its content is sensations; its form is contingent and particular consecution. Both form and content, accordingly, need to be reconstructed if they are to be worthy of the name of science or of knowledge. This is the position which Leibniz assumes as against the empiricist, Locke. The details of this reconstruction, its method and result, we must leave till we come in the course of the argument again to the subject of knowledge.