The Metaphysical Problem of the Mechanical Conception of Nature
Gottfried Wilhelm Leibniz‘s philosophy emerges from the tension between the new explanatory power acquired by the natural sciences in the seventeenth century and the ontological principles upon which those explanations depend. The modern conception of nature developed by thinkers such as Descartes, Galileo, Bacon, and Hobbes had largely abandoned the explanations of Aristotelian-scholastic philosophy based on substantial forms and final causes, turning instead toward an understanding of physical phenomena through motion, extension, and mechanical interactions. This transformation established a new research program that made the mathematical understanding of nature possible. Although Leibniz acknowledged the scientific success of this program, he questioned whether mechanical explanation could also account for the ultimate reality of nature. His fundamental objection was not that the laws of motion were incorrect, but that these laws were insufficient to explain the conditions of their own validity. Demonstrating how material bodies move and explaining in what sense the moving thing constitutes a real being were not the same philosophical problem. Leibniz’s metaphysics developed from the conviction that modern mechanism could not answer these two questions without distinguishing between them.
One of the decisive transformations in the modern philosophical understanding of nature was Descartes’s redefinition of substance. In his Principles of Philosophy, Descartes defined substance as that which requires nothing else in order to exist, while acknowledging that this definition could be applied in the strict sense only to God. Created substances, although dependent upon God for their existence, could nevertheless be conceived as independent of other created substances. The fundamental distinction Descartes established within this framework was between material substance and thinking substance. The essence of material substance was extension, while the essence of thinking substance was thought. Material existence was understood through length, breadth, and depth, whereas the existence of mind was determined through intellectual activity. Physical and mental reality thus emerged as two domains of being possessing different essential attributes. Yet this distinction made it difficult to explain how the human being, as a unity of body and mind, should be understood. If the entire nature of the body consists in extension and the entire nature of the mind consists in thinking, it becomes necessary to explain how these two realities, possessing no common substantial attribute, can affect one another. The fundamental difficulty of Cartesian dualism arises from the ontological conditions under which bodily movement and mental activity constitute a unity within the same human being.
The problem opened by Descartes gave rise to different conceptions of substance in modern philosophy. Hobbes rejected the notion of incorporeal substance and maintained that reality has a corporeal structure. Within this approach, there is no independent domain of created substance outside the universe or separate from material existence; nature is explained through the movements and relationships of material beings. Spinoza, meanwhile, brought Descartes’s definition of substance to a different conclusion. By accepting that only God or nature is truly independent, he maintained that there is only one substance and understood individual beings as modes of that substance. The substances Descartes had conceived as distinct were therefore reinterpreted in favor of material reality by Hobbes and within a doctrine of one substance by Spinoza. Leibniz’s metaphysics follows a different path in relation to these alternatives. Without reducing reality to a single substance or accepting material existence as the ultimate ontological principle, he attempts to establish the existence of a plurality of individual substances. Yet for such a plurality to be possible, each individual must constitute a genuine unity in its own right. For Leibniz, Descartes’s concept of extension fails to satisfy precisely this requirement.
The fundamental characteristic of what is extended is its divisibility. However small a material body may be conceived, its spatial characteristics allow it to be divided into smaller parts. This raises the question of the principle upon which the real unity of material being depends. If a body consists only of a combination of extended parts, it must be explained what kind of unity the body itself possesses beyond the sum of its constituent parts. The arrangement of parts in a particular configuration may produce a physical whole, but it does not follow that this whole is automatically an independent substance. Leibniz’s second objection to Descartes is connected with this problem. Geometrical determinations such as extension, shape, and motion are insufficient to explain why a material being is active. Although the movements of bodies can be described mathematically, the principle underlying motion and activity itself requires further investigation. From Leibniz’s perspective, the Cartesian conception of material substance therefore remains incomplete in two fundamental respects: unity and activity. Explaining a genuine substance requires identifying the principle of being and activity that it possesses within itself, beyond its geometrical properties.
Real Unity and the Internal Activity of Substance
Leibniz’s intellectual development was shaped less by a rejection of mechanism than by an investigation into its metaphysical foundations. In a letter to Nicolas Rémond dated 1714, he described his own philosophical development, explaining that he had been interested in Aristotelian and scholastic thought during his youth before turning toward mathematical research under the influence of modern mechanical philosophy. Yet when he began investigating the ultimate grounds of the laws of motion, he concluded that they could not be established through mathematical relations alone. The quantitative calculation of natural movements did not eliminate the question of the fundamental principles of activity upon which those movements depended. Leibniz therefore returned to metaphysics at the point where mechanical explanations, despite retaining their successful results, failed to provide a sufficient account. The outcome of this transition was not the simple restoration of scholastic substantial forms, but a reconsideration of the concept of form in connection with force and internal activity.
The approach Leibniz developed in his New System of Nature determines the nature of genuine substance through internal activity. If a material being is assumed to move only under external influences, this does not explain how it can be an active being in its own right. The transmission of motion from one body to another transfers activity from one object to another, but this transmission does not itself disclose the ontological foundation of activity. Leibniz therefore locates the essence of substance not in external relations of motion but in the internal force that determines its own states. Force here does not mean merely a measurable quantity of physical motion. It is the principle of activity that makes possible a substance’s own changes. A genuine substance must be understood not as a passive bearer of events that happen to it, but as an active unity that brings about the development of its internal states. This idea explains why Leibniz moved away from explanations based exclusively on the external movements of material particles. For a being to be genuinely individual, it must possess internal determinations of its own rather than merely occupy a location separate from other beings.
At this point, the necessary relationship Leibniz establishes between unity and reality becomes decisive. The principle he defends in his correspondence with Antoine Arnauld holds that what is not truly one is not truly a being. Although this thought may initially appear to be a proposition concerning numerical identity, it contains a metaphysical claim that reconstructs the concept of substance. The difference between a block of marble and a heap of stones is clear in ordinary experience. A block of marble possesses a certain physical continuity, whereas a heap of stones is a collection of separable parts. Yet in both cases the reality in question is extended and divisible. The physical continuity of marble does not prove that it constitutes an indivisible unity in the metaphysical sense. Likewise, describing a human community as a single community does not create a new substance independent of the individuals composing it. The unity of a community arises from the relationships among individuals, whereas the substantial unity of an individual is not simply the result of relationships with other individuals. Leibniz thus establishes a decisive distinction between unity through aggregation and genuine substantial unity.
Real unity cannot arise from the external combination of parts, because the unity of such a whole would remain dependent upon the relationships among its constituent elements. If these parts themselves consist only of further parts, the explanation is indefinitely postponed. To establish the foundation of unity, one must arrive at a reality that is not itself divisible and does not derive its individuality from an external process of aggregation. Leibniz’s conception of simple substance arises from this requirement. Monads are not miniature examples of material bodies or the smallest particles reached at the end of physical division. They are simple substances possessing a kind of unity that extended beings lack. Leibniz’s return to the notion of substantial form must also be understood in this context. The substantial form employed in the scholastic tradition to explain the nature and unity of a being acquires a new function in Leibniz as the principle of internal activity and individuality. The reality of a substance depends upon its capacity to constitute a being in its own right, prior to its forming any material whole.
This ontological requirement acquires a further foundation through Leibniz’s principle of sufficient reason. According to this principle, no fact or true proposition can be accepted without a sufficient reason why it is so rather than otherwise. Human beings may not always know this reason, but the intelligibility of reality requires that there be an explanation of why a particular fact occurs as it does. The principle of the identity of indiscernibles is connected to the application of this same thought to individual beings. If two beings are assumed to be absolutely indiscernible in all their properties, there must be a sufficient ground for regarding them as numerically distinct. A genuine difference cannot be based merely on giving two things different names or conceiving them in separate positions; it requires a determination that accounts for their difference.
This principle has important consequences for the atomistic conception of matter. Atomism attempts to identify the ultimate components of bodies by maintaining that the material world consists of small, indivisible particles. For Leibniz, however, physical indivisibility is not identical with metaphysical simplicity. If two atoms are assumed to possess exactly the same internal properties and their difference is explained solely by their spatial positions, the question remains why these positions are arranged in precisely that way. If exchanging the positions of two particles possessing identical properties within homogeneous space produces no difference in the determinations of reality, the basis for treating them as two distinct individual substances becomes problematic. This criticism is not intended to invalidate the physical investigation of material particles. What Leibniz questions is whether spatial and quantitative properties alone are sufficient to explain individuality. The difference between genuine substances must be grounded in determinations belonging to their internal natures.
The explanation of the monad through the concepts of perception and appetition acquires its significance at this stage of the problem of unity and activity. Leibniz defines perception as the representation of multiplicity within unity. Although a monad has no spatial parts within itself, it expresses the multiplicity of the universe through its own distinctive order of representation. There is therefore no contradiction between the indivisibility of simple substance and its ability to contain a multiplicity of relations. Multiplicity does not exist within the monad as an assemblage of material parts, but as a differentiation of representational contents. Appetition, meanwhile, is the internal activity that brings about the transition from one perceptual state to another. Change within a monad is not explained through the addition of a part from outside or the transmission of physical influence from another substance; it occurs through a principle of activity belonging to its own nature. Perception and appetition are therefore not accidental psychological characteristics of the monad, but concepts explaining the structure of its individual existence and activity. Nevertheless, perception must not be identified with conscious thought. Leibniz attributes perception to all monads without maintaining that every monad is conscious of its own perceptions. Conscious awareness constitutes a particular level of perception, while the fundamental activity of simple substance possesses a metaphysical meaning broader than conscious experience.
The Foundation of Material Reality: Monads and Well-Founded Phenomena
One of the most important consequences of Leibniz’s conception of monads concerns the redefinition of the ontological status of the material world. If genuine substances are indivisible, non-extended unities possessing internal activity, it becomes necessary to explain in what sense the bodies encountered in ordinary experience exist. This problem cannot be solved by assuming that monads are the smallest components of the material world. The parts of an extended object are conceived as elements participating in its spatial structure. A monad, however, is not extended and cannot be treated as a geometrical component of a material whole. Consequently, to say that a table, a stone, or a human body consists of monads misrepresents Leibniz’s thought if this is understood as a composition produced by physically assembling parts. The relationship between the material world and monads is one of ontological dependence, fundamentally different from physical composition.
The distinction Leibniz emphasizes in his correspondence with De Volder makes this problem particularly clear: matter does not arise through the physical combination of monads as genuine unities; rather, it results from them. This formulation prevents Leibniz’s metaphysics from being interpreted as a simple form of atomism. In an atomistic explanation, a whole is composed through the combination of its material particles. For Leibniz, by contrast, the reality of material phenomena depends upon non-extended simple substances. Monads are not the smallest physical elements that could be discovered within material phenomena; they are the fundamental realities upon which those phenomena depend for their being. The progressive division of a material object into smaller parts therefore does not eventually reveal a monad. Physical division continues within the domain of physical phenomena. What is metaphysically simple is not a tiny body found at the end of the process of division, but a substance to which spatial division cannot be applied.
This understanding should not be confused with the claim that material reality is merely an illusion. Leibniz does not entirely abolish the reality of bodies and motion; he establishes their mode of reality at a different level. Material objects are understood as well-founded phenomena (phenomena bene fundata). Here, the concept of phenomenon does not signify a subjective fantasy corresponding to no reality whatsoever. There is an ontological foundation that accounts for the regularity, intelligibility, and compatibility of appearances among different perceivers. Leibniz’s analogy of the rainbow is important for explaining this distinction. The visible unity of a rainbow does not constitute an independent substance in its own right, yet it is not entirely devoid of reality as an ordered appearance grounded in particular conditions. Similarly, the fact that material objects can be experienced does not establish that they are ultimate substances in themselves. Their reality depends upon a substantial foundation beyond material appearance and upon the ordered relationships arising from that foundation.
The ontological transformation Leibniz accomplishes here proceeds in a direction different from that of modern reductionist explanations. Within physical reductionism, mental or complex phenomena are generally explained in relation to more fundamental material structures and processes. Leibniz, however, does not accept extended material reality as the ultimate foundation possessing substantial unity in its own right. The ontological status of material objects is explained not through increasingly smaller physical objects, but through non-extended simple substances possessing internal activity. Nevertheless, this approach does not require the invalidation of mechanical explanations of the material world. Leibniz accepts that natural phenomena can be explained through the laws of motion, but he distinguishes the level of phenomena governed by these laws from the level of ultimate reality upon which phenomena depend. Explaining the physical conditions under which a body moves is different from asking why that body does not constitute a genuine substance. Physical explanation reveals the ordered relationships among particular events, while metaphysical grounding investigates the structure of being upon which these relationships depend.
Leibniz’s criticism of mechanism is therefore not simply an attempt to place metaphysics in opposition to the explanatory power of the natural sciences. Mechanical and metaphysical explanations should not be treated as rival accounts seeking to solve the same problem through different methods. The mechanical sciences can investigate the movements of bodies, their interactions, and their transformations. Yet the validity of mechanical laws does not mean that those laws themselves can be explained solely through motion and extension. Leibniz’s fundamental argument is that mechanical explanation presupposes an ontological order that makes it possible. Within this order, genuine substance is not the combination of extended parts but a unity possessing internal activity. The material world, meanwhile, is the domain of ordered phenomena grounded in these unities. Metaphysics thus ceases to be a form of knowledge that intervenes in individual events unexplained by physics and becomes a discipline investigating the foundations of the concepts of reality, causality, and unity employed by physical explanation.
Pre-Established Harmony and the Irreducibility of Consciousness
Leibniz’s doctrine of pre-established harmony can be understood through the application of this metaphysical distinction to the mind–body problem. Since Cartesian dualism defines thought and extension as distinct substantial attributes, it must explain how the mind affects the body and how the body affects the mind. Leibniz’s commonly presented solution holds that mind and body operate according to their own laws while remaining in complete harmony with one another. The perceptions and desires of the mind follow their internal order, while bodily movements occur in accordance with mechanical laws. Although there is no direct causal transmission between these processes, their correspondence arises through an order established by God from the beginning. The relationship between a person’s willing an action and the body’s moving in accordance with that will is therefore not explained as the mind exerting a material force upon the body. Mental and bodily processes correspond to one another while developing according to their respective laws.
This explanation, however, constitutes only the first level of Leibniz’s system. If mind and body are accepted as two independent created substances in the Cartesian sense, pre-established harmony appears to be a doctrine of coordination intended to solve the problem of their interaction. Yet Leibniz’s more fundamental metaphysical claim is that the extended body is not an independent substance in the first place. What exists in the strict sense consists of simple substances; bodily reality is an ordered domain of phenomena grounded in them. The mind–body problem is therefore transformed from the question of how two different kinds of substance interact into the question of how the internal activities of simple substances are related to the order of material phenomena. The absence of direct physical influence among monads is connected to the fact that each possesses a principle determining the development of its own internal states. Pre-established harmony expresses the general order through which these distinct sequences of development remain mutually consistent.
Leibniz’s account of the organization of living beings makes this distinction more apparent. A living being is not regarded merely as the external combination of extended parts. Its unity is explained through the ordered relationship between a dominant or central monad and other monads. The dominant monad is not a material component situated at a particular physical point within the body. Its central position concerns its function within the order of unity and representation belonging to the living being. Bodily organization likewise does not arise from monads assembling like material particles; it constitutes a phenomenal whole grounded in the relationships among numerous simple substances. Each monad represents the universe from its own distinctive perspective, while harmony exists among these orders of representation. The unity of a living being is therefore understood not simply through the mechanical relationships of its parts, but through a more fundamental ontological order grounded in genuine unities.
The explanatory function of pre-established harmony is to determine the relationship between different levels of reality. Mechanical laws explain the order of bodily events, while perception and appetition account for the internal activity of simple substances. The fact that these two domains of explanation do not conflict does not require their reduction to the same kind of causality. In Leibniz’s thought, the order of efficient causes and the order of final causes perform different functions in this respect. The movements of bodies can be explained through mechanical laws, while transitions between the internal states of substances are understood through inner orientation and activity. The orderly explanation of physical events is thus preserved without being accepted as the sole explanatory form that exhausts the entire reality of substance. Leibniz’s fundamental criticism of mechanism consequently extends from the mind–body problem to a general ontological principle: a causal explanation valid at one level does not automatically establish the conditions of being at another.
One of the clearest consequences of this distinction appears in the problem of reducing consciousness to mechanical processes. In section 17 of the Monadology, Leibniz develops a thought experiment to demonstrate that perception cannot be explained solely through shape and motion. Suppose that a machine capable of thinking or perceiving exists and that it is enlarged to such a degree that someone can enter it. An observer walking inside the machine would be able to examine its moving parts, mechanical connections, and operating mechanisms. Yet what the observer would encounter would still be parts moving and acting upon one another; perception itself would not appear as a mechanical component. The importance of Leibniz’s mill thought experiment does not lie in claiming that a physical machine cannot be complex or that mechanical processes cannot be related to mental life. The problem is whether an explanation consisting of the movements of parts is sufficient to explain what perception is.
For Leibniz, perception is the representation of multiplicity within a genuine unity. Mechanical explanation, meanwhile, concerns the positions, movements, and mutual effects of extended parts. Knowing how every component of a machine operates does not automatically explain why or in what sense this operation constitutes perception. A complete description of the relationships among parts and the representation of those relationships within a unity are not the same explanation. Leibniz’s fundamental objection to physical reductionism is located precisely here. The difficulty encountered in explaining perception does not arise merely from insufficiently detailed knowledge of a mechanical system; there is a conceptual distinction between mechanical description and the ontological nature of perception. The foundation of perception must therefore be sought in a simple substance possessing unity in itself and capable of internal states. Leibniz’s definition of monads through perception and appetition is not a psychological assumption subsequently added to the problem of consciousness, but a direct consequence of his conception of real unity and activity.
It would nevertheless be mistaken to interpret Leibniz’s view as maintaining that every monad is conscious or that all natural beings possess mental lives resembling human experience. A distinction must be made between perception and conscious awareness. The fact that monads possess representational activity does not mean that these representations are consciously apprehended in every case. Conscious perception becomes identifiable through Leibniz’s concept of apperception (apperception), which designates a more specific level of awareness. The fact that perception has a broader meaning than consciousness demonstrates that the internal activity of substance is not confined to moments when human beings become aware of their own thoughts. This distinction establishes an approach that does not reduce mental life in its entirety to the explicit contents of conscious thought. Nevertheless, the fundamental metaphysical principle employed by Leibniz in explaining consciousness remains unchanged: the reality of a representation must be understood within the activity of a substance possessing internal unity, rather than through the external combination of extended parts.
Ontological Grounding and Leibniz’s Contemporary Philosophical Significance
The significance of Leibniz’s metaphysics for contemporary philosophy does not lie in the claim that modern natural science has verified the existence of monads, but in his systematic formulation of the question of the level at which beings are real. To affirm that an object exists and to explain how it depends upon other beings are different philosophical operations. There is little difficulty in speaking of the existence of a table in ordinary life. The existence of its physical components can be accepted just as readily. Yet whether the table constitutes a new substance independent of its components, or at what ontological level it possesses reality, requires further investigation. Leibniz’s distinction between real unity and unity through aggregation represents one historical formulation of this problem. Accepting the reality of material objects does not require accepting that they are ultimate and independent substances.
Contemporary discussions of grounding in analytic metaphysics investigate not merely whether beings exist, but through which relations of ontological dependence they are connected to one another. Jonathan Schaffer’s proposal to redirect the fundamental question of metaphysics from “What exists?” toward “What grounds what?” can be compared with Leibniz’s distinction between genuine substance and well-founded phenomena. This comparison does not mean that the two philosophers endorse the same ontology. While Leibniz accepts simple substances as ultimate reality, contemporary theories of grounding can be developed within different conceptions of fundamental being. What the approaches share, however, is the recognition that the existence of something does not necessarily mean that it is ontologically fundamental. Metaphysics is thus directed less toward an inventory of existing objects than toward an investigation of the relations of dependence among levels of being.
The problem of material composition also represents a contemporary counterpart to Leibniz’s understanding of real unity. Peter van Inwagen’s discussions of the conditions under which material objects genuinely form a composite whole raise the distinction between the mere coexistence of physical parts and the emergence of a new object. Is the arrangement of the parts of a table in a particular configuration sufficient for the table to be accepted as an independent being? Or does the formation of a genuine unity require a stronger ontological criterion than mere aggregation? Leibniz’s approach maintains that such questions cannot be resolved solely by examining physical arrangement. Nevertheless, it does not follow that contemporary debates concerning material composition necessarily lead to Leibnizian monadology. What matters is preserving the relationship between the existence of parts and the substantial reality of the whole as a problem that itself requires explanation.
Leibniz’s concept of force also requires careful distinction when compared with contemporary conceptions of the physical world. Modern physics does not explain material reality solely through extended and passive particles; it develops much more complex theoretical structures involving force, energy, fields, and interactions. Yet these developments do not establish that Leibniz’s metaphysical concept of force is identical with the concepts of force or energy employed in modern physics. Leibniz investigates the ontological meaning of the internal activity of substance. Modern physics, by contrast, concerns particular quantitative relationships and their experimentally explicable order. The fact that neither approach reduces material reality solely to geometrical extension does not make their concepts identical. The philosophical value of Leibniz’s thought lies not in anticipating particular results of modern physics, but in demonstrating that the metaphysical presuppositions of physical explanation themselves remain objects of investigation.
A similar distinction is important in contemporary philosophy of consciousness. The relationship between mental events and physical or neural processes can be investigated, but determining these relationships and explaining what subjective experience is are not identical problems. The explanatory difficulty highlighted in David Chalmers’s discussions of consciousness raises the question of how the functional and causal structures of physical processes are related to the character of conscious experience. A historical affinity can be established between this discussion and Leibniz’s mill thought experiment. Nevertheless, Leibniz’s explanation of perception as an internal state of simple substance is not identical with the various approaches developed in contemporary philosophy of consciousness. His contribution to these debates lies in offering a reasoned inquiry into why a complete description of physical processes cannot automatically be regarded as a complete solution to the problem of consciousness.
At this point, the question of the metaphysical limits of mechanism becomes apparent once again. For Leibniz, mechanical explanation of nature is valid and necessary within its own domain, but it cannot be identified with the ultimate ontology that explains the entirety of reality. Knowing the laws governing moving bodies does not determine whether those bodies possess substantial unity. Explaining the order of material phenomena does not automatically reveal the level of reality upon which they depend. Similarly, demonstrating that physical processes occur in connection with mental experience does not eliminate every question concerning the ontological nature of perception. Leibniz’s philosophy accepts the validity of scientific explanation while making the concepts of unity, activity, and being upon which that explanation depends the subjects of a more fundamental metaphysical investigation.
Conclusion
The development of Leibniz’s conception of substance is the result of a philosophical investigation that accepts the explanatory power of modern mechanism while questioning its ontological claims. Descartes’s definition of extension as the essence of material substance made the geometrical explanation of motion possible, but left unresolved the problem of explaining the genuine unity and internal activity of extended being. To address this deficiency, Leibniz sought the foundation of reality not in the indivisibility of physical parts, but in substances that are metaphysically simple and active. The principle of sufficient reason and the identity of indiscernibles establish that the differences among individual beings cannot be reduced to external spatial positions, while the concepts of perception and appetition explain how genuine substance possesses an internal principle of representation and change. The monad is thus conceived not as a miniature component of the mechanical universe, but as the fundamental unity upon which phenomena subject to mechanical explanation depend for their being.
This transformation does not abolish the reality of the material world altogether; it repositions it at the level of well-founded phenomena. The movements of bodies and physical interactions can be explained through mechanical laws, but the success of these explanations does not establish that bodies possess ultimate substantial reality. The doctrine of pre-established harmony seeks to explain the relationship between the internal activities of simple substances and the order of material phenomena without appealing to direct physical interaction between substances. The criticism of the mechanical explanation of consciousness, meanwhile, reveals the consequences of this metaphysical approach for mental life. Knowledge of every movement of extended parts does not eliminate the question of what perception means as representation within a unity.
The enduring significance of Leibniz’s philosophy arises not from producing skepticism about the validity of mechanical explanations, but from preserving the distinction between explanation and ontological grounding. The natural sciences can establish the laws according to which particular events occur; metaphysics investigates the principles through which those laws and the reality to which they apply can be understood. Explaining how something operates and determining in what sense it is a real being cannot simply replace one another. Through precisely this distinction, Leibniz’s criticism of mechanism keeps the question of whether real unity, substantial activity, and consciousness can be explained solely through the concepts of extension and motion among the fundamental problems of philosophy.
