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We have seen the necessity and nature of matter as deductions from the fundamental principles of Leibniz. We have seen that matter is a phenomenon or manifestation of spirit in an imperfect and confused way. But why should it appear as moving, as extended, as resisting, as having cohesion, with all the concrete qualities which always mark it? Is there any connection between these particular properties of matter as physical, and its “metaphysical” or ideal character? These are the questions which now occupy us. Stated more definitely, they take the following form: Is there any essential connection between the properties of matter as a metaphysical element, and its properties as a sensible fact of experience? Leibniz holds that there is. He does not, indeed, explicitly take the ground that we can deduce a priori all the characteristics of matter as a fact of actual experience from its rational notion, but he thinks we can find a certain analogy between the two, that the sensible qualities are images or reflexes of the spiritual qualities, witnessing, so far as possible, to their origin in pure energy.

His position is as follows: that which in the monad is activity or substantial, is, in sensible matter, motion. That which in the monad is lack of a given activity, that which constitutes its subordinate position in the hierarchy of monads, is, in the sphere of material things, inertia. That which in the spiritual world is the individuality of monads, making each forever ideally distinct from every other, is, in the phenomenal realm, resistance or impenetrability. The perfect continuity of monads in the mundus intelligibilis has also its counterpart in the mundus sensibilis in the diffusion or extension of physical things.

Instead of following out this analogy directly, it will rather be found convenient to take up Leibniz’s thought in its historical connection. We have already alluded to the fact that he began as a Cartesian, and that one of the first ideas which repelled him from that system of thought was the notion that the essence of matter is extension. His earliest philosophical writings, as he was gradually coming to the thoughts which thereafter dominated him, are upon this point. In general, his conclusions are as follows: If matter were extension, it would be incapable of passion or of action. Solidity, too, is a notion entirely opposed to the conception of mere extension. The idea of matter as extension contradicts some of the known laws of motion. It requires that the quantity of motion remain unchanged whenever two bodies come in contact, while as matter of fact it is the quantity of energy, that which the motion is capable of effecting, that remains unchanged; or, as he more often puts the objection, the Cartesian notion of matter requires that matter be wholly indifferent to motion, that there be nothing in it which resists motion when imparted. But, says Leibniz, there is something resisting, that to which Keppler gave the name “inertia.” It is not found to be true if one body impacts upon another that the second moves without diminishing the velocity or changing the direction of the first. On the other hand, just in proportion to the size of the second body, it resists and changes the motion of the first, up to the point of causing the first to rebound if small in comparison. And when it was replied that the retardation was due to the fact that the force moving the first body had now to be divided between two, Leibniz answered that this was simply to give up the contention, and besides the notion of extension to use that of force. If extension were the essence of matter, it should be possible to deduce all the properties of matter, or at least to account for them all, from it. But since, as just seen, this does not enable us to account for any of them, since for any of its concrete qualities we have to fall back on force, it is evident where the true essence of matter is to be found.

Leibniz has another argument of a logical nature, as those already referred to are of a physical: “Those who claim that extension is a substance, reverse the order of words as well as of thoughts. Besides extension there must be a subject which is extended; that is to say, something to which it belongs to be repeated or continued. For extension is nothing but a repetition or continued multiplication of that which is spread out,—it is a plurality, a continuity, a co-existence of parts. Consequently, extension does not suffice to explain the nature of the repeated or manifold substance, of which the notion is anterior to that of its repetition.” Extension, in other words, is nothing substantial, it is not something which can exist by itself; it is only a quality, a property, a mode of being. It is always relative to something which has extension. As Leibniz says elsewhere: “I insist that extension is only an abstraction, and requires something which is extended. It presupposes some quality, some attribute, some nature in a subject which is extended, diffused, or continued. Extension is a diffusion of this quality. For example, in milk there is an extension or diffusion of whiteness; in the diamond an extension or diffusion of hardness; in body in general a diffusion of antitypia or materiality. There is accordingly in body something anterior to extension.”

From the physical side, therefore, we find it impossible to account for the concrete properties of material phenomena from extension; on the logical we find that the idea of extension is always relative to that which is extended. What is that which is to be considered as the bearer of extension and the source of physical qualities? We are led back to the point at which we left the matter in the last chapter. It is force, and force both passive and active. Leibniz uses the term “matter” in at least three senses: it is the metaphysical element of passive force in the monad; it is the monad itself considered as, upon the whole, externally conditioned or unconscious; and it is the phenomenon resulting from the aggregation of the monads in the second sense. The first is naked matter, and is a pure abstraction; the second is the monad as material, as opposed to the monad, as soul; the third is clothed, or second matter, or, concretely, body, corpus. The first is unreal by itself; the second is one phase of substance; the third is not substantial, but is a reality, though a phenomenal one. It is from the substantial monad that we are to explain the two things now demanding explanation,—that element in bodies (matter in third sense) which is the source of their physical properties, and that which is the subject, the carrier, so to speak, of extension.

That of which we are in search as the source of the physical qualities of bodies is motion. This is not force, but its “image.” It is force, says Leibniz, that “is the real element in motion; that is to say, it is that element which out of the present state induces a change in the future state.” As force, in other words, is the causal activity which effects the development of one “representation” of a monad out of another, so motion, in the realm of phenomena, is not only change, but change which is continuous and progressive, each new position being dependent upon the foregoing, and following out of it absolutely without break.

Motion, therefore, is the manifestation of the ideal unity of substance,—a unity not of mere static inherence, but of a continuous process of activity. It is from this standpoint that Leibniz accounts for the so-called transference of motion from one body to another upon contact. The ordinary view of this, which looks at it as if one body loses the motion which another body gains, Leibniz ridicules, saying that those who hold this view seem to think that motion is a kind of thing, resembling, perchance, salt dissolved in water. The right view, on the other hand, does away with all appearance of mystery in the carrying over of motion from one body to another, for it recognizes that continuity is the very essence of motion, and that we do not have two things and a third process, but that the two bodies are phases or elements in one and the same system of movement.

Starting from this idea of motion, then, Leibniz is to account for the actual qualities of matter as found in experience. These are the form, magnitude, cohesion, resistance, and the purely sensible qualities of objects. “First” matter, that is, abstract matter, may be conceived, according to Leibniz, as perfectly homogeneous, a “subtle fluid,” in his words, without any distinction of parts or of solidity. But this is an abstract notion. It is what matter would be without motion. Motion necessarily differentiates this plenum of homogeneity, and thus causes distinctions of figure (that is, boundaries of parts) and varieties of cohesion, or the varying solidity and fluidity of bodies. The latter difference is indeed the ultimate one. The principle of continuity or gradation, as applied to motion, makes it necessary that motions should not be in any two places of exactly the same energy. The result is that the original fluid matter is everywhere differently divided. Motion, entering into the uniform plenum, introduces distinction; it causes so much of the matter as is affected by a given movement to collect together and form in appearance a coherent body, as opposed to surrounding bodies which are affected by different degrees of energy. But even this is only approximate; the same principle of continuity must be applied within any apparently coherent body; its parts, while, in relation to other bodies, they have the same amount of motion, are in relation to one another differently affected. There are no two having exactly the same motion; if they had, there would be no distinction between them; and thus, according to the principle of Leibniz, they would be the same.

It follows at once from this that there is in the universe no body of absolute hardness or solidity, nor of entire softness or fluidity. A perfectly solid body would be one whose system of motions could not be affected by any other system,—a body which by motion had separated itself from motion, or become absolute. This is evidently an idea which contradicts itself, for the very essence of motion is continuity or relation. A body perfectly fluid, on the other hand, would be one in which there was no resistance offered to other motions,—a body, in other words, in which there are no movements that, entering into connection with one another, form a relative opposition to other movements. It would be a body isolated or out of relation with the general system of motions, and hence an impossibility. There is no last term either of solidity or of fluidity.

It equally follows as matter of course that there is no indivisible particle of matter,—no atom. The infinity of degrees of motion implies a corresponding division of matter. As already said, it is only in contrast with other relatively constant systems of motion that any body is of uniform motion; in reality there is everywhere throughout it variety of movement, and hence complete divisibility, or rather, complete division. If Leibniz were to employ the term “atom” at all, it could be only in the sense of the modern dynamical theory (of which, indeed, he is one of the originators), according to which the atom is not defined by its spatial position and outlines, but, by the range of its effects, as the centre of energies of infinite circumference. Correlative to the non-existence of the atom is the non-existence of the vacuum. The two imply each other. The hard, limited, isolated body, having no intrinsic relations with other bodies, must have room to come into external relations with them. This empty space, which is the theatre of such accidental contacts as may happen, is the vacuum. But if bodies are originally in connection with one another, if they are in reality but differentiations of varying degrees of motion within one system of motion, then there is no necessity for the vacuum,—nay, there is no place for it. The vacuum in this case could mean only a break, a chasm, in the order of nature. According to the theory of Leibniz, “bodies” are but the dynamic divisions of the one energy that fills the universe; their separateness is not an independent possession of any one of them or of all together, but is the result of relations to the entire system. Their apparent isolation is only by reason of their actual connections. To admit a vacuum anywhere, would thus be to deny the relatedness of the parts separated by it. The theory of the atom and the vacuum are the two phases of the metaphysical assumption of an indefinite plurality of independent separate realities. The theory of Leibniz, resting as it does on the idea of a perfect unity of interrelated members, must deny both of these aspects. Were we making an extended analysis of the opposed view, it would be necessary to point out that it denies itself. For it is only through the vacuum that the atoms are isolated or independent, and the sole function of the vacuum is to serve as the background of the atoms. The atoms are separated only in virtue of their connection, and the vacuum is what it is—pure emptiness—only on account of that which is in it. In short, the theory is only an abstract and incomplete way of grasping the thought of relation or mediated unity.

We have thus discovered that all motions conspire together, or form a system. But in their unity they do not cease to be motions, or variously differentiated members. Through this differentiation, or mutual reaction of motions, there comes about the appearance of boundaries, of separation. From these boundaries or terminations arise the form and size of bodies. From motion also proceeds the cohesion of bodies, in the sense that each relative system resists dissolution, or hangs together. Says Leibniz, “The motions, since they are conspiring, would be troubled by separation; and accordingly this can be accomplished only by violence and with resistance.” Not only form, size, and stability depend upon motion, but also the sensible, the “secondary” qualities. “It must not be supposed that color, pain, sound, etc., are arbitrary and without relation to their causes. It is not God’s way to act with so little reason and order. There is a kind of resemblance, not entire, but of relation, of order. We say, for example, ‘Light is in the fire,’ since there are motions in the fire which are imperceptible in their separation, but which are sensible in their conjunction or confusion; and this is what is made known in the idea of light.” In other words, color, sound, etc., even pain, are still the perception of motion, but in a confused way. We thus see how thoroughly Leibniz carries back all the properties of bodies to motion. To sum up, motion is the origin of the relative solidity, the divisibleness, the form, the size, the cohesion, or active resistance of bodies, and of their properties as made known to us in immediate sensation.

In all that has been said it has been implied that extension is already in existence; “first matter” is supposed to fill all space, and motion to determine it to take upon itself its actual concrete properties. But this “first matter,” when thus spoken of, has a somewhat mythological sound, even if it be admitted that it is an abstraction. For how can an abstraction be extended in space, and how can it form, as it were, a background upon which motion displays itself? The idea of “first matter” in its relation to extension evidently demands explanation. In seeking this explanation we shall also learn about that “subject” which Leibniz said was necessarily presupposed in extension, as a concrete thing is required for a quality.

The clew to the view of Leibniz upon this point may be derived, I think, from the following quotations:—

“If it were possible to see what makes extension, that kind of extension which falls under our eyes at present would vanish, and our minds would perceive nothing else than simple realities existing in mutual externality to one another. It would be as if we could distinguish the minute particles of matter variously disposed from which a painted image is formed: if we could do it, the image, which is nothing but a phenomenon, would vanish. . . . If we think of two simple realities as both existing at the same time, but distinct from one another, we look at them as if they were outside of one another, and hence conceive them as extended.”

The monads are outside of one another, not spatially, but ideally; but this reciprocal distinction from one another, if it is to appear in phenomenal mode, must take the form of an image, and the image is spatial. But if the monads were pure activity, they would not take phenomenal form or appear in an image. They would always be thought just as they are,—unextended activities realizing the spiritual essence of the universe. But they are not pure activity; they are passive as well. It is in virtue of this passive element that the ideal externality takes upon itself phenomenal or sensible form, and thus appears as spatial externality.

Leibniz, in a passage already quoted, refers to the diffusion of materiality or antitypia. This word, which is of frequent occurrence in the discussions of Leibniz, he translates generally as “impenetrability,” sometimes as “passive resistance.” It corresponds to the solidity or resistance of which Locke spoke as forming the essence of matter. Antitypia is the representation by a monad of the passive element in other monads. Leibniz sometimes speaks as if all created monads had in themselves antitypia, and hence extension; but he more accurately expresses it by saying that they need (exigent) it. This is a technical term which he elsewhere uses to express the relation of the possible to the actual. The possible “needs” the actual, not in the sense that it necessarily requires existence, but in the sense that when the actual gives it existence, it is the logical basis of the actual,—the actual, on the other hand, being its real complement. The passivity of the monad is therefore at once the logical basis and the possibility of the impenetrability of matter. It is owing to the passivity of the monad that it does not adequately reflect (that it is not transparent to, so to speak) the activities of other monads. In its irresponsiveness, it fails to mirror them in itself. It may be said, therefore, to be impenetrable to them. They in turn, so far as they are passive, are impenetrable to it. Now the impenetrable is, ex vi terminis, that which excludes, and that which excludes, not in virtue of its active elasticity, but in virtue of its mere inertia, its dead weight, as it were, of resistance. But mutual exclusion of this passive sort constitutes that which is extended. Extension is the abstract quality of this concrete subject. Such, in effect, is the deduction which Leibniz gives of body, or physical matter, from matter as metaphysical; of matter as sensible or phenomenal, from matter as ideal or as intelligible.

If we put together what has been said, it is clear that material phenomena (bodies, corpora, in Leibniz’s phrase) simply repeat in another sphere the properties of the spiritual monad. There is a complete parallelism between every property, each to each, and this necessarily; for every property of “body” is in logical dependence upon, and a phenomenalization of, some spiritual or ideal quality. Motion is the source of all the dynamic qualities of body, and motion is the reflection of Force, that force which is Life. But this force in all finite forms is conditioned by a passive, unreceptive, unresponsive factor; and this must also have its correlate in “body.” This correlate is primarily impenetrability, and secondarily extension. Thus it is that concrete body always manifests motion, indeed, but upon a background of extension, and against inertia. It never has free play; had it an unrestrained field of activity, extension would disappear, and spatial motion would vanish into ideal energy. On the other hand, were the essence of matter found in resistance or impenetrability, it would be wholly inert; it would be a monotone of extension, without variety of form or cohesion. As Leibniz puts it with reference to Locke, “body” implies motion, or impetuosity, resistance, and cohesion. Motion is the active principle, resistance the passive; while cohesion, with its various grades of completeness, which produce form, size, and solidity, is the result of their union.

Leibniz, like Plato, has an intermediary between the rational and the sensible; and as Plato found that it was mathematical relations that mediate between the permanent and unified Ideas and the changing manifold objects, so Leibniz found that the relations of space and time form the natural transition from the sphere of monads to the world of bodies. As Plato found that it was the possibility of applying mathematical considerations to the world of images that showed the participation of Ideas in them, and constituted such reality as they had, so Leibniz found that space and time formed the element of order and regularity among sense phenomena, and thus brought them into kinship with the monads and made them subjects of science. It is implied in what is here said that Leibniz distinguished between space and time on the one hand, and duration and extension on the other. This distinction, which Leibniz draws repeatedly and with great care, has been generally overlooked by his commentators. But it is evident that this leaves Leibniz in a bad plight. Mathematics, in its various forms, is the science of spatial and temporal relations. But if these are identical with the forms of duration and extension, they are purely phenomenal and sensible. The science of them, according to the Leibnizian distinction between the absolutely real and the phenomenally real, would be then a science of the confused, the imperfect, and the transitory; in fact, no science at all. But mathematics, on the contrary, is to Leibniz the type of demonstrative, conclusive science. Space and time are, in his own words, “innate ideas,” and the entire science of them is the drawing out of the content of these innate—that is, rational, distinct, and eternal—ideas. But extension and duration are sensible experiences; not rational, but phenomenal; not distinct, but confused; not eternal, but evanescent. We may be sure that this contradiction would not escape Leibniz, although it has many of his critics and historians.

It is true, however, that he occasionally uses the terms as synonymous; but this where the distinction between them has no bearing on the argument in hand, and where the context determines in what sense the term is used. The distinction which he actually makes, and to which he keeps when space and time are the subject of discussion, is that extension and duration are qualities or predicates of objects and events, while space and time are relations, or orders of existence. Extension and duration are, as he says, the immensity, the mass, the continuation, the repetition, of some underlying subject. But space and time are the measure of the mass, the rule or law of the continuation, the order or mode of the repetition. Thus immediately after the passage already quoted, in which he says that extension in body is the diffusion of materiality, just as whiteness is the diffusion of a property of milk, he goes on to say “that extension is to space as duration to time. Duration and extension are attributes of things; but space and time are to be considered, as it were, outside of things, and as serving to measure them.” Still more definitely he says: “Many confound the immensity or extent of things with the space by means of which this extent is defined. Space is not the extension of body, any more than duration is its time. Things keep their extension, not always their space. Everything has its own extent and duration; but it does not have a time of its own, nor keep for its own a space.” Or, as he expresses the latter idea elsewhere, space is like number, in the sense that it is indifferent to spatial things, just as number is indifferent to res numerata. Just as the number five is not a quality or possession of any object, or group of objects, but expresses an order or relation among them, so a given space is not the property of a thing, but expresses the order of its parts to one another. But extension, on the other hand, is a property of the given objects. While extension, therefore, must always belong to some actual thing, space, as a relation, is as applicable to possible things as to actual existences; so that Leibniz sometimes says that time and space “express possibilities.” They are that which makes it possible for a definite and coherent order of experiences to exist. They determine existence in some of its relations, and as such are logically prior to any given forms of existence; while extent and duration are always qualities of some given form of existence, and hence logically derivative. Since time and space “characterize possibilities” as well as actualities, it follows as a matter of course “that they are of the nature of eternal truths, which relate equally to the possible and to the existing.” Being an eternal truth, space must have its place in that which is simply the active unity of all eternal truths,—the mind of God. “Its truth and reality are based upon God. It is an order whose source is God.” Since God is purus actus, he is the immediate, the efficient source only of that which partakes in some degree of his own nature, or is rational; and here is another clear point of distinction between space and extension, between time and duration.

But we must ask more in detail regarding their nature. Admitting that they are relations, ideal and prior to particular experiences, the question must be asked, What sort of relations are they; how are they connected with the purely spiritual on one hand, and with the phenomenal on the other? Leibniz’s most extended answers to these questions are given in his controversy with Clarke. The latter took much the same position regarding the nature of space (though not, indeed, concerning the origin of its idea) as Locke, and the arguments which Leibniz uses against him he might also have used, for the most part, against Locke. Locke and Clarke both conceived of space and time as wholly without intrinsic relation to objects and events. It is especially against this position that Leibniz argues, holding that space and time are simply orders or relations of objects and events, that space exists only where objects are existing, and that it is the order of their co-existence, or of their possible co-existence; while time exists only as events are occurring, and is the relation of their succession. Clarke, on the other hand, speaks of the universe of objects as bounded by and moving about in an empty space, and says that time existed before God created the finite world, so that the world came into a time already there to receive its on-goings, just as it fell into a space already there to receive its co-existences.

To get at the ideas of Leibniz, therefore, we cannot do better than follow the course of this discussion. He begins by saying that both space and time are purely relative, one being the order of co-existences, the other of successions. Space characterizes in terms of possibility an order of things existing at the same time, so far as they exist in mutual relations (ensemble), without regard to their special modes of existence. As to the alternate doctrine that space is a substance, or something absolute, it contradicts the principle of sufficient reason. Were space something absolutely uniform, without things placed in it, there would be no difference between one part and another, and it would be a matter of utter indifference to God why he gave bodies certain positions in space rather than others; similarly it would be a matter of indifference why he created the world when he did, if time were something independent of events. In other words, the supposed absoluteness of space and time would render the action of God wholly without reason, capricious, and at haphazard. Similarly, it contradicts the principle of “indiscernibles,” by which Leibniz means the principle of specification, or distinction. According to him, to suppose two things exactly alike, is simply to imagine the same thing twice. Absolute uniformity, wholly undifferentiated, is a fiction impossible to realize in thought. “Space considered without objects has nothing in it to determine it; it is accordingly nothing actual. The parts of space must be determined and distinguished by the objects which are in them.” Finally, were space and time absolutely real things in themselves, they would be independent of God, and even limitations upon him. “They would be more substantial than substances. God would not be able to change or destroy them. They would be immutable and eternal in every part. Thus there would be an infinity of eternal things (these parts) independent of God.” They would limit God because he would be obliged to exist in them. Only by existing through this independent time would he be eternal; only by extending through this independent space would he be omnipresent. Space and time thus become gods themselves.

When Clarke declares that by the absoluteness of space and time he does not mean that they are themselves substances, but only properties, attributes of substance, Leibniz advances the same arguments in different form. If space were the property of the things that are in space, it would belong now to one substance, now to another, and when empty of all material substance, even to an immaterial substance, perhaps to God. “Truly a strange attribute which is handed about from one thing to another. Substances thus leave their accidents as if they were old clothes, and other substances put them on.” Since these finite spaces are in infinite space, and the latter is an attribute of God, it must be that an attribute of God is composed of parts, some of them empty, some full, some round, some square. So, too, whatever is in time would help make one of the attributes of God. “Truly a strange God,” says Leibniz, “this Deity of parts” (ce Dieu à parties). Clarke’s reply to this was that space and time are attributes of God and of God alone, not of things in space and time,—that, indeed, strictly speaking, there are no parts in space or in time; they are absolutely one. This was virtually to give up the whole matter. It was to deny the existence of finite spaces and times, and to resolve them into an indefinite attribute of God. Such a view, as Leibniz points out, not only is contrary to experience, but affords no aid in determining the actual concrete forms and situations of bodies, and durations and successions of events. The absolute space and time, having no parts, are wholly out of relations to these concrete existences. The latter require, therefore, a space and a time that are relations or orders. Clarke’s hypothesis is, as Leibniz says, wholly without use or function, and requires a theory like that of Leibniz to account for the actually determinate forms of experience. In his last reply Clarke shifts his ground again, and says that space and time are effects of God’s existence; “they are the necessary results of his existence.” “His existence is the cause of space and time.” The death of Leibniz prevented any further reply. It is not hard to imagine, however, that in a general way his reply would have been to ask how space and time are at once attributes essential and necessary to God, as constituting his immensity and eternity, and effects dependent upon his existence. To take this latter position, indeed, seems to abandon the position that they are absolute, and to admit that, like the rest of God’s creation, they are relative and finite.

So much for Leibniz’s polemic. Its meaning is that space and time have significance only with reference to things and events, that they are the intellectual, the ideal side of these objects and occurrences, being the relations which give them order and unity. A space which is not the space of objects, which is not space in and through objects, is an inanity; it is not spirit, it is not matter; it is not a relation of either. It is nothingness magnified to infinity, and then erected into existence. And all for nothing; for it does not enable us to account for a single concrete fact of experience. For this we must have recourse to relations and orders of existence. Space is therefore to be defined as the order which makes it possible for objects to have situation; time as that which makes it possible for events to have dating,—not as if they were actually prior to them, and although nothings in themselves, yet capable of giving concrete determination to things, but as actually the relations themselves, and as ideally necessary for the coherent experience of co-existent objects and of connected events. As Leibniz puts it epigrammatically: “Space is the order of possible constants; time the order of inconstant possibilities.”

We have finished the exposition of the views of Leibniz about matter and material facts. One question, however, remains to be discussed,—a question which Leibniz’s contemporary critics would not allow him to pass over in silence, even had he been so disposed. What is the reality of matter, of motion, of space, and of time? Since they are, as Leibniz says, only phenomena, not absolute realities, what distinguishes them from dreams, from illusions? What distinguishes sensible phenomena from capricious fantasies, and gives them reality?

Leibniz begins his answer by pointing out that the mere fact that bodies are phenomena does not make them unreal. To say that anything is phenomenal is to say that it is sensible; but “the senses make no declaration regarding metaphysical matters” such as truth and reality. The senses, in a word, only inform us that the experiences are there for the senses, that they are sensible. What is the ultimate nature of the sensible or the phenomenal, what is their reality, is a question wholly outside the province of sense. The questions of ultimate nature, of reality, are questions of metaphysics, and hence are to be decided by the reason, not by the senses. And Leibniz goes on to say that the truthfulness of the senses, since it concerns only the sensible, consists in the reciprocal agreement of sensible facts, and in that we are not deceived in reasoning from one to another. An isolated sense-experience could not be said to be either true or false, real or illusory. It would be true that it was experienced, and that is all that could be said about it. But since our experiences are not thus separated, but have a certain order, there arises what we may call sensible reality and illusion. When the order between two facts remains the same “in different times and places and in the experience of different men,” we call these facts real. If, however, our experience cannot be repeated by ourselves or by other men when the same conditions (that is, connections) are present, it is unreal, or false. It is thus “the relation of phenomena which guarantees truth of fact regarding sensible objects.” Constancy, regularity, justify us in ascribing reality; chaotic change and lack of orderly connection are a sign of unreality. Even our dreams have a reality; for they have their connections and place in experience. If we understood their connections we should even be able to explain their apparent lack of connection with the rest of experience. Leibniz thinks that both the Academicians and Sceptics and their opponents erred in attempting to find greater reality in sensible things than that of regular phenomena. Since our observations and judgments upon sensible phenomena are of such a nature that we can predict future phenomena and prepare for them, we have all the reality in them that can be had or asked for. Even if it be granted possible (as it must be on this basis) that, metaphysically speaking, sense-experience is only a connected dream, it yet has a sufficient reality; for we are not deceived in the measures taken with reference to phenomena, provided that we act on the ground of their observed harmonies and relations. Thus while we are obliged to admit that our senses inform us that there are hard, passive, extended, indivisible things, not perfectly continuous and not intellectual in their nature, and we know on metaphysical grounds that this information is not correct, we cannot say that our senses deceive us, for sense makes no statements regarding such matters. It is our reason that errs if it takes the information that the senses give as if it were a declaration of reason itself. Sensible things have all the reality necessary for this range of experience,—practical,—such regularity of co-existence and sequence as allows us to act without being led astray.

But if we regard sense-phenomena not merely in their connection with one another, but in their dependence upon the absolute realities, we have still better justification for their comparative reality. These phenomena are consequences of necessary and eternal truths. One endowed with a perfect knowledge of such truths would be able to deduce, a priori, the phenomena from them. The reality of sensible phenomena thus consists not merely in their connection with one another, but in the fact that they are connected as the laws of the intelligible world require. They follow not only rules of co-existence and sequence; but these rules may be brought under general laws of motion, which in turn may be deduced from geometrical principles. These latter, however, are a priori; they are truths which are grounded in the very intelligence of God. The sensible has its basis in the ideal. To state the same fact in another way, all sensible phenomena occur in time and space; or rather, time and space are the orders, the relations, of phenomena occurring and existing. But, as we have just seen, time and space are ideal. A relation, as Leibniz points out, being neither attribute nor accident, cannot be in the things which it relates, as their possession. In his own words, it cannot be conceived as if it had one leg in one object, the other leg in the other. A relation is not a material bond, running through or cementing objects; it is ideal, existing in the mind. And while it is true that space and time are the relations of objects and events, it is also true that if all objects and events were annihilated, space and time would continue to have their ideal existence in the intelligence of God as the eternal conditions of phenomena. They thus form the links between absolute reality and the reality of sensible existence. The principle of sufficient reason forms another link. It may be recalled that in discussing Leibniz’s theory of volition we found that the will of God in relation to the sensible world is always determined by the choice of the better; that in this consists the controlling reason and regulative principle of all that occurs and exists. Thus for every fact in the sensible world there is connection with “metaphysical,” or absolute, reality, not only through the medium of the intellectual relations of time and space, but through the dynamic intermediary of the divine will acting in accordance with the divine reason. Sensible facts have, then, a reality, but a dependent one. There would be no contradiction involved if they were not what they actually are.

We may sum up the matter by saying that the reality of sensible phenomena consists in the constancy of the mutual order in which they exist, and in the dependence of this order upon the divine Intelligence and Will. In this respect, at least, Leibniz resembles the young Irish idealist, Berkeley, who only seven years after Leibniz wrote the “New Essays” composed his “Principles of Human Knowledge,” urging that the immediate reality of sense-phenomena consists in their “steadiness, order, and coherence,” “in a constant uniform working,” and that this “gives us a foresight which enables us to regulate our actions for the benefit of life.” It was Berkeley also who wrote that their ultimate reality consists in their being ideas of a Divine Spirit. This was six years before the death of Leibniz. Yet it does not appear that Berkeley knew of Leibniz, and the only allusion to Berkeley which I have found in the writings of Leibniz shows that Leibniz knew only of that caricature of his views which has always been current,—that Berkeley was one who denied the existence of any external world. What he writes is as follows: “As for him in Ireland who questions the reality of ‘bodies,’ he seems neither to offer what is rational, nor sufficiently to explain his own ideas. I suspect that he is one of those men who are desirous of making themselves known through paradoxes.”