The Complete Philosophical Corpus of a Universal Thinker
Preliminary Discourse on the Conformity of Faith with Reason (Discours préliminaire de la conformité de la foi avec la raison)
We are at the opening of Leibniz’s *Theodicy*, where he establishes the methodological foundation for the entire work by arguing that reason and revealed faith cannot ultimately contradict one another.
Leibniz begins by distinguishing between two kinds of truths: truths of reasoning, which are necessary and whose opposite implies a contradiction, and truths of fact, which are contingent and depend on God’s free choice. Revelation belongs to the latter category. The central problem is whether a truth revealed by God could ever contradict a truth clearly demonstrated by reason. Leibniz insists that this is impossible, because God is the author of both nature and grace. “For since faith is the gift of God, the same God who gives reason cannot contradict himself.” The apparent conflicts arise only when we mistake mere opinions or prejudices for the deliverances of either faith or reason.
He then introduces a crucial distinction between what is above reason and what is against reason. Mysteries such as the Trinity, the Incarnation, and the Eucharist are *above* reason: they cannot be fully comprehended by the natural light of the mind, but they can be shown to contain no contradiction. Nothing that is *against* reason — that is, demonstrably contradictory or false — can be an article of faith. The task of the philosopher is therefore not to derive these mysteries from first principles, but to defend them against the charge of absurdity.
“I hold that there is an infinite distance between the comprehension of a thing and the knowledge that it is true. In mathematics we have knowledge of numbers and figures, but we do not comprehend them, for we cannot perceive all that is contained in them.”
This passage is central to Leibniz’s strategy. He argues that the human mind is finite and cannot penetrate the infinite complexity of God’s works. To demand that a mystery be *comprehended* before it is believed is to demand the impossible. What is required is that the mystery not be *demonstrably contradictory*. And this, Leibniz claims, can be shown for the central mysteries of Christianity.
The significance of this opening is that it clears the ground for the *Theodicy* proper. If faith and reason were fundamentally opposed, the entire project of justifying God’s ways to man would be impossible from the start. By establishing their harmony, Leibniz gives himself permission to use rational arguments to defend divine justice, while still preserving the authority of revelation for truths that surpass human understanding. The reader is thus prepared to encounter a work that is simultaneously philosophical and theological, where reason operates within its proper limits, referring to faith for its ultimate articles while governing the interpretation of those articles by the law of non-contradiction.
Discourse on Metaphysics (Discours de métaphysique)
We move now to one of Leibniz’s earliest systematic expositions of his mature philosophy, composed in 1686, where he lays out the metaphysical foundations of his system in thirty-seven compact sections.
The *Discourse* opens with a bold declaration about the nature of God. Leibniz asserts that God is an absolutely perfect being, and that perfection is nothing other than the magnitude of positive reality. From this follows a principle that will govern the entire work: God always acts in the most perfect manner. This is not merely a moral claim about God’s goodness; it is a metaphysical principle about how the world must be structured. Every action of God is ordered toward the maximum of perfection, and this includes the creation of the universe itself.
Leibniz then turns to the question of why God created this particular world rather than any other. He introduces the concept of compossibility: not all possible things can coexist in a single universe, because some are incompatible with others. God surveys all possible worlds in his understanding — an infinite array of possibilities — and chooses the one that maximizes perfection according to the simplest laws. “God chooses the most perfect world, that is, the one which is at the same time the simplest in hypotheses and the richest in phenomena.” This is the first clear statement of Leibniz’s famous doctrine that this is the best of all possible worlds.
“God does nothing which is not orderly, and it is not even possible to imagine events which are not regulated by some order.”
This sentence encapsulates Leibniz’s rejection of any genuine contingency or randomness in the divine operation. Even miracles, which are exceptions to the subordinate rules of nature, are themselves governed by higher-order laws that belong to the general order of the universe. There is no chaos in God’s creation, because chaos would be an imperfection, and God, being perfect, cannot produce anything imperfect in that sense.
The *Discourse* then introduces the revolutionary concept of individual substance. Leibniz argues that each substance has a complete concept that contains all of its predicates, past, present, and future. From the concept of Alexander the Great, for example, one could in principle deduce his victory at Gaugamela, his death in Babylon, and every other event of his life. This is the famous predicate-in-subject principle: for any true proposition, the predicate is in some sense contained in the concept of the subject. This leads to the startling conclusion that each substance is like a mirror of the entire universe, expressing all other things from its own unique point of view.
The significance of these claims cannot be overstated. If every substance contains all its predicates in its concept, then all actions of a substance are ultimately derivable from its own nature, and there is no genuine causal influence between substances. This prepares the ground for Leibniz’s doctrine of pre-established harmony: the appearance of interaction between mind and body, or between any two substances, is not caused by real influence, but by a divinely pre-arranged parallelism. The *Discourse on Metaphysics* thus lays down, in compressed form, the core ideas that Leibniz will develop and defend for the rest of his career: the principle of perfection, the best of all possible worlds, the complete concept of substance, and the harmony that makes the universe a single, unified system under God.
On the Correction of Metaphysics and the Concept of Substance (De primae philosophiae emendatione, et de notione substantiae)
We turn now to a Latin essay from 1694, where Leibniz explicitly announces his intention to reform the foundations of metaphysics and offers a new definition of substance.
Leibniz begins by diagnosing the failure of previous philosophy. The Scholastics, he argues, rendered the concept of substance empty by defining it merely as “that which can exist by itself,” without giving any positive account of what this means. The Cartesians, meanwhile, reduced substance to extension, which cannot explain the phenomena of motion, force, or life. What has been missing is the dynamical element: substance must be understood not as static being, but as active force (vis activa).
He proposes his new definition: substance is a being capable of action. More precisely, a substance is that which contains within itself the source of its own changes, a principle of activity that Leibniz calls entelechy, borrowing the term from Aristotle. This entelechy is not merely a potentiality waiting to be actualized by an external cause; it is an inherent striving or appetition that drives the substance from one perceptual state to the next. “I recognized that the concept of force, or of that which is called power in the schools, brings great light to philosophy, and that it is the key to the true notion of substance.”
“I think that the first or fundamental notion of substance is that it is an active principle, that is, a source of actions, or a form, or an entelechy.”
This passage marks a decisive break with the mechanistic philosophy of the seventeenth century. Mechanism, Leibniz argues, can explain the *how* of natural phenomena — the size, shape, and motion of particles — but it cannot explain the *why*. Why does motion persist? Why does one body communicate motion to another? The mechanist must appeal to God’s will at every turn, which reduces physics to a series of miracles. The true account, Leibniz insists, must recognize that bodies themselves possess an internal principle of force, something analogous to the soul in living beings.
The significance of this essay is that it reorients metaphysics around the concept of force rather than extension or matter. For Leibniz, the fundamental realities are not inert bits of stuff pushed around by external causes, but active, self-moving centers of force. This leads directly to his later doctrine of monads, which are nothing other than these substantial forms understood as simple, immaterial substances. The *Correction of Metaphysics* thus marks the moment when Leibniz’s philosophy becomes explicitly dynamic, transforming the static ontology of the Cartesians into a world of living activity, where every substance is a perpetual source of its own changes and a mirror of the universe from its own point of view.
On the Ultimate Origination of Things (De rerum originatione radicali)
We move now to a short but dense treatise, likely composed in 1697, in which Leibniz addresses a question that lies at the foundation of all philosophy: why does anything exist at all, rather than nothing? The piece is a metaphysical explanation of the ultimate reason for the existence of the universe.
Leibniz opens with a striking formulation of the problem. He writes that the first question we are entitled to ask is: “Why is there something rather than nothing?” For nothing, he observes, is simpler and easier than something. If there were no beings, that state would require no explanation at all. But since there are beings, there must be a sufficient reason why they exist rather than not. This reason cannot be found within the series of contingent things themselves, for each contingent thing depends on another, and the series as a whole stands in need of an external ground. The ultimate reason must lie in a necessary being that carries the reason for its own existence within itself.
Leibniz then introduces a second argument, grounded in the nature of eternal truths. Even if we suppose the world to be eternal, he argues, we would still need to explain why it is *this* world rather than some other possible world. The actual world is distinguished from all other possible worlds by its degree of perfection — that is, by the quantity of essence or positive reality it contains, together with the order and harmony of its parts. The eternal truths, which are the ideas in the divine understanding, contain the possibles; among these possibles, those that are more perfect are actualized by the divine will. Thus the world exists because God chooses to create the best of all possible worlds, a theme that will receive its most famous treatment in the *Theodicy*.
“Hence it is very clearly understood that out of the infinite combinations of possibles, and the infinite number of possible series, that one exists which contains the most reality, i.e., the highest degree of perfection.”
Leibniz then deepens this account by describing the mechanism of creation. The possibles, or essences, tend toward existence by virtue of their own intrinsic reality. This striving (conatus) of essences for existence is not an external force but an internal characteristic of the possibles themselves. They press toward actuality in proportion to their perfection. This is not a blind mechanism but a rational one: the divine intellect perceives the possibles, and the divine will selects the combination that maximizes perfection. The actual world is therefore the one best possible, chosen from an infinity of competing candidates.
Clarify: This argument establishes a tight connection between metaphysics and theology. The existence of the world is not brute fact but rationally necessary, given the nature of God and the possibles. Leibniz rejects the idea that God could have arbitrarily chosen a less perfect world; the divine will is guided by the principles of wisdom and goodness. The *radical origination* of things is thus a process of optimal selection, in which the possibles compete for existence and the most perfect combination wins. This sets the stage for Leibniz’s theodicy: if the world is the best possible, then evil must be a necessary condition for the greater good.
New System of the Nature of Substances and of the Communication between Them (Système nouveau de la nature des substances et de la communication entre elles)
This work, published in 1695 in the *Journal des savants*, is one of Leibniz’s most important and widely read pieces. It presents for the first time in a systematic form his mature theory of substances and their pre-established harmony, designed to solve the problem of mind-body interaction that had plagued Cartesian philosophy.
Leibniz begins by recounting his own intellectual journey. He tells the reader that he was long stuck in the Cartesian view, which held that animals are mere machines, or automata, and that the soul is a thinking substance radically distinct from matter. But he came to see that this view could not account for the unity and activity of living things. He was drawn to the substantial forms of the Scholastics, which seemed to provide a principle of unity and action, yet he was also dissatisfied with the way they were used, since they were often invoked as occult qualities that explained nothing. What he needed was a way to rehabilitate substantial forms by making them intelligible within a modern mechanical framework.
The solution Leibniz proposes is to reconceive substance as a simple, indivisible unit — a unit of force or activity, not of extension. Each substance is a living mirror of the universe, expressing the whole cosmos from its own unique point of view. These substances he will later call monads. There can be no interaction between them in the ordinary causal sense, because each substance is self-contained: all its states arise from its own internal principle. Yet they are perfectly correlated, not by direct influence but by a pre-established harmony set in place by God at creation.
“This is why I have said elsewhere that, properly speaking, there is no influence of one substance upon another, but that the modifications of each substance arise from its own internal nature, in a way that corresponds to the modifications of all other substances.”
Leibniz then applies this principle to the specific case of the soul and the body. The soul does not cause movements in the body, nor does the body produce ideas in the soul. Rather, the soul’s perceptions and the body’s motions are two series of phenomena that run in perfect parallel, like two clocks that keep the same time without any causal connection. God, at the moment of creation, set each substance on its own path, such that the series of each substance’s states unfolds in exact correspondence with all the others. This is the pre-established harmony, Leibniz’s alternative to the influx theory (direct causal influence) and the occasionalism of Malebranche (where God intervenes at every moment to produce the correspondence).
Clarify: The *New System* solves the mind-body problem without resorting to miracles or to a denial of the reality of material things. Each substance is autonomous, yet the whole universe is a perfect unity of coordinated activity. This is Leibniz’s most original and far-reaching contribution to metaphysics: it replaces causal interaction with a divine pre-arrangement that guarantees the harmony of all phenomena. The work also marks a definitive break with the Cartesian notion of substance as extended thoughtless matter, replacing it with a world of active, perceiving, self-sufficient units.
On the Nature of Body and the Laws of Motion (Specimen dynamicum)
This text, published in 1695, is Leibniz’s most detailed presentation of his dynamics — the science of force and motion that he developed as a correction and improvement upon Cartesian physics. It is not a mere appendix to the *New System* but a foundational work in its own right, showing how the metaphysical principles of substance and activity translate into a new physics.
Leibniz begins by distinguishing two kinds of force: passive force and active force. Passive force corresponds to the resistance that bodies offer to motion — what the Cartesians called inertia — and is linked to the impenetrability and extension of bodies. Active force, on the other hand, is the principle of motion itself, and it comes in two forms: primitive active force, which is inherent in every substance and corresponds to the substantial form or entelechy, and derivative active force, which is the observable manifestation of this primitive force in the phenomena of motion, impact, and action.
Leibniz then launches a critique of the Cartesian laws of motion. Descartes had asserted that the total quantity of motion (mass times velocity) in the universe is conserved. Leibniz shows that this is false: what is conserved is not quantity of motion but quantity of vis viva (living force), which is measured by mass times the square of velocity. This is the famous Leibnizian measure of force, which later became the foundation for the principle of conservation of energy. Leibniz demonstrates this by considering falling bodies: a body falling from a height of four units has double the velocity of a body falling from a height of one unit, but its effect (e.g., the depth of an indentation in clay) is four times greater, not two times. This shows that force is proportional to the square of velocity.
“Therefore, the force of a body is proportional to the effect it can produce, and the effect of a falling body is proportional to the height from which it falls, not to the velocity it acquires in falling.”
Leibniz then generalizes this into a system of dynamics that treats bodies as centers of active and passive force. The laws of motion are not merely empirical regularities but necessary truths grounded in the metaphysical principles of sufficient reason and equivalence of cause and effect. Every change in motion requires a reason, and that reason is ultimately the distribution of active forces among substances. The apparent collisions of bodies are not real interactions but unfold according to the pre-established harmony, where each body’s internal force dictates its response to the perceived approach of another.
Clarify: The *Specimen dynamicum* is Leibniz’s attempt to unify metaphysics and physics. It shows that the world of bodies is not a collection of inert, extended bits of matter but a manifestation of active and passive forces that are ultimately grounded in the nature of simple substances. This work decisively refutes the Cartesian equation of body with extension and replaces it with a view of body as a phenomenon bene fundatum (well-founded phenomenon) whose reality is derived from the activity of monads. The conservation of *vis viva* becomes a key principle of modern physics, though Leibniz’s own metaphysical interpretation of it remains distinct from later formulations.
Monadology (La Monadologie)
LOCATE: This is the most famous and systematic exposition of Leibniz’s mature metaphysical system, written in 1714 as a concise summary of his philosophy.
PRESENT: The work opens with a definition of its fundamental unit: a monad is nothing other than a simple substance, which enters into compounds. Simple means without parts; compounds are collections of simples. Since monads have no parts, they can have no extension, no figure, and no divisibility—they are the true atoms of nature, the elements of all things. Their nature is purely perception and appetition: perception is the internal state that represents external things, and appetition is the tendency to pass from one perception to another.
Leibniz argues that monads must be created directly by God, and they can only perish by annihilation—they cannot be destroyed by natural means. The entire universe is a plenum of monads, each perceiving the whole universe from its own unique point of view. But these perceptions are mostly confused and indistinct. What we call a body is the well-founded appearance of a collection of monads that are united under a dominant monad, which gives the aggregate its unity. The soul is a dominant monad that possesses apperception—conscious reflection on its own perceptions—unlike simple monads, which have only unconscious perceptions.
A central thesis of the *Monadology* is the pre-established harmony. Since monads have no windows through which anything can enter or leave, they cannot causally interact. Yet their perceptions correspond perfectly because God has programmed each monad from the beginning to unfold in perfect parallel with every other. As Leibniz writes:
“The soul follows its own laws, and the body likewise follows its own laws; and they agree with each other in virtue of the pre-established harmony between all substances, since they are all representations of one and the same universe.”
This harmony extends to all realms: the realm of final causes (souls) and the realm of efficient causes (bodies) run in parallel without interfering. What appears as causation in the physical world is merely a consequence of the internal perceptual unfolding of monads.
The *Monadology* also defends a version of the principle of sufficient reason: nothing happens without a reason why it should be so rather than otherwise. This principle, combined with God’s perfect goodness, leads Leibniz to the conclusion that this is the best of all possible worlds. God, in creating the universe, considered infinitely many possible worlds in his understanding and chose to actualize the one that maximizes perfection—the maximum variety of phenomena with the greatest order, and the maximum of reality with the minimum of defects. Evil exists only as a privation, a necessary shadow that makes the overall harmony more beautiful, much as dissonance in music makes the resolution sweeter.
CLARIFY: The *Monadology* is Leibniz’s most compressed and definitive statement of his mature philosophy. It synthesizes all his earlier projects—the defense of substance as force, the theory of perception, the pre-established harmony, and theodicy—into a single coherent system. The work is not merely a metaphysical theory; it is a complete vision of reality as a harmony of countless autonomous spiritual points, each mirroring the universe from its own perspective, all united by divine choice. It remains the classic expression of idealism—the view that reality is ultimately mental in nature—and a profound attempt to reconcile mechanism with purpose, science with religion, and individuality with universal order.
Principles of Nature and Grace, Founded on Reason (Principes de la nature et de la grâce, fondés en raison)
LOCATE: Written in the same year as the *Monadology* (1714), this shorter work presents the same system in a more accessible and diplomatic form, addressed to Prince Eugene of Savoy.
PRESENT: Leibniz begins by defining substance as a being capable of action. Simple substances are those without parts; compounds are aggregates of simples. This leads directly to the conclusion that the ultimate elements of things, the monads, must be of a spiritual nature—they are souls or analogies of souls. Their essence consists in perception (the representation of the composite in the simple) and appetition (the striving for new perceptions). Every monad is a living mirror of the universe, but clarity of perception varies immensely.
The work emphasizes the continuity of nature: there is no gap between the simplest monad and the most rational soul. All monads are animate, but only those with distinct perception and memory rise to the level of souls in the strict sense. Animals have souls, but humans have spirits—monads that are capable of knowledge of necessary truths and of God, and that thereby enter into a moral community with the Creator. Spirits are not merely mirrors of the universe but also images of the Divinity itself, capable of entering into a society with God.
Leibniz then outlines three principles that govern the cosmos: the principle of contradiction (for necessary truths), the principle of sufficient reason (for contingent truths), and the principle of the best (which governs God’s choice). These lead to the conclusion that the actual world must be the most perfect possible, because a perfect being cannot act without sufficient reason, and the best reason is the greatest harmony and perfection.
The physical world operates by the laws of motion, but these laws themselves are derived from the principle that action is conserved—specifically, vis viva (living force) is conserved, not merely quantity of motion. This conservation law reflects God’s wisdom: he has chosen the laws that produce the greatest possible outcome. Meanwhile, souls and bodies coexist in pre-established harmony: the soul perceives what happens in the body not because the body acts on it, but because God has tuned both to correspond.
The work closes with an image of the city of God: all spirits are citizens of this universal republic, ruled by God as the most perfect monarch. The happiness of the blessed consists in their perpetual progress toward greater perfection, guided by love of God and neighbor. Death is simply the transformation of the body, not the destruction of the monad. The soul continues to perceive the universe, though with a different degree of clarity.
CLARIFY: The *Principles of Nature and Grace* is the more accessible twin of the *Monadology*. It softens the technical rigor of the latter for a courtly audience, emphasizing the moral and religious implications of the system. The work shows that Leibniz’s metaphysics is not merely speculative but has practical consequences: it grounds immortality, divine providence, and the meaning of human life in a rational philosophical framework. The image of spirits as “citizens of the city of God” transforms metaphysics into a kind of spiritual politics, where the highest good is participation in the divine harmony.
Fifth Letter to Samuel Clarke
LOCATE: We are now in the final major text of this collection, Leibniz’s fifth and last letter to Samuel Clarke, written in 1716 and left unfinished at his death — it is the culminating exchange in a famous correspondence with Newton’s spokesman, defending the Leibnizian system against Newtonian absolute space, time, and divine intervention.
PRESENT: The letter opens by restating the core disagreement. Leibniz attacks the Newtonian doctrine of absolute space and absolute time as “idols of the vulgar,” arguing that they make space and time independent of God, which implies either that God is not sovereign over them or that space is an eternal, infinite, uncreated thing — a rival to God. Leibniz writes:
“I have said more than once, that I hold space to be something merely relative, as time is; that I hold it to be an order of coexistences, as time is an order of successions.”
For Leibniz, space is not a real container but the order of coexistence among substances. If one substance is removed, space remains only as an ideal possibility, not as a real extension. The Newtonians treat absolute space as a “hypostasis” — a real, infinite, eternal being — which Leibniz calls a reversion to paganism, for it makes the universe independent of God.
He then takes up the issue of miracles. The Newtonians, particularly through Clarke, argued that God must occasionally correct the motions of the planets (to prevent the solar system from collapsing) and that such corrections are miracles. Leibniz ridicules this as making God a poor clockmaker who cannot make a perfect machine. He writes:
“According to my opinion, the same force and vigor remains always in the world, and only passes from one part of matter to another, agreeably to the laws of nature and the beautiful pre-established order.”
The Newtonian view, Leibniz charges, makes God constantly “wind up” the world like a faulty watch — this implies that the world is in perpetual decline or that God’s work is imperfect. Leibniz’s pre-established harmony is the alternative: God created the world such that every substance follows its own internal law, and all substances harmonize without external intervention. There is no need for God to intervene in the course of nature, except in the original act of creation.
Leibniz then addresses the vis viva (“living force”) controversy, which he had already developed in the *Specimen dynamicum*. Newtonians measure force by momentum (mass times velocity), Leibniz by vis viva (mass times the square of velocity). He argues that the quantity of motion is not conserved in collisions, but the quantity of *force* (i.e., *vis viva*) is. This is a physical argument with metaphysical implications: it shows that the universe has a stable, conserved quantity of power, consistent with God’s wisdom.
The letter also tackles the nature of attraction in Newtonian physics. Leibniz insists that attraction, if it is not reducible to mechanical impulse (i.e., a push or pull through contact), is an “occult quality” — a scholastic absurdity. He will not accept action at a distance without a medium. He proposes instead that all motion propagates through a subtle aether or fluid, in accordance with mechanical principles. Newton’s gravitational attraction, Leibniz says, is a “miracle” if taken as intrinsic to matter.
Toward the end, Leibniz refutes the Newtonian argument that God’s omnipresence implies absolute space. Clarke had argued that since God is everywhere, space must be something real in which God exists. Leibniz responds that God is present to things not by location but by immediate operation — God knows and sustains all things directly, but this does not require a real infinite container. God’s presence is virtual and intentional, not spatial.
The letter breaks off unfinished. Leibniz was writing it during his final illness, and it ends mid-sentence.
CLARIFY: This letter is Leibniz’s most sustained polemic against Newtonianism, and it makes explicit the metaphysical stakes of the rivalry. The correspondence forced Leibniz to clarify his relational theory of space and time, his optimism about the perfection of the created world, and his commitment to mechanical explanation within a broader teleological framework. The exchange with Clarke became a landmark in the history of physics and metaphysics, influencing Kant’s later resolution of the debate in the *Critique of Pure Reason*. Leibniz’s arguments here remain central to philosophical discussions of space, time, and divine action.
