Modern Contributions to Theistic Thought: Descartes to Leibniz

When we ask whether reason alone can prove that God exists, we are asking one of the oldest and most contested questions in the philosophy of religion. In the seventeenth century, two of the most brilliant minds in Western thought – René Descartes and Gottfried Wilhelm Leibniz – took up this challenge head on. Rather than appealing to scripture or personal experience, they turned to logic, mathematics, and metaphysics. Their arguments remain among the most discussed and debated contributions to theistic philosophy to this day.

Descartes and the rationalist starting point 🔗

René Descartes (1596-1650) is perhaps best known for his famous statement cogito ergo sum – “I think, therefore I am.” But his philosophical project went far beyond establishing his own existence. In his Meditations on First Philosophy, Descartes set out to rebuild human knowledge from the ground up, and God played a central role in that project. According to the Stanford Encyclopedia of Philosophy, Descartes advanced his arguments for God’s existence not only as a piece of natural theology but also to serve an epistemic function – to guarantee that God was not deceiving him about the external world.

This is why the question of God’s existence was not, for Descartes, merely a theological curiosity. Without a non-deceiving God, all clear and rational knowledge of the world would remain under suspicion. God was the bridge back from radical doubt to justified belief.

Descartes’ ontological argument: existence from perfection 🔗

The most celebrated of Descartes’ proofs for God’s existence appears in the Fifth Meditation and is a version of the ontological argument. Its core logic is strikingly direct.

According to the Stanford Encyclopedia of Philosophy, Descartes’ argument differs from Anselm’s famous eleventh-century version in a crucial respect. While Anselm proceeded from the definition of God as “that than which nothing greater can be conceived,” Descartes grounded his proof in two pillars of his own philosophy: the theory of innate ideas and the doctrine of clear and distinct perception. He argued not from an arbitrary definition, but from an innate idea whose content is simply “given” to the mind.

The argument runs as follows: we have a clear and distinct idea of God as a supremely perfect being. Existence is itself a perfection – a being that does not exist would be lacking something and therefore could not be perfectly perfect. Therefore, God, as the supremely perfect being, must necessarily exist. As Study.com explains it, the reasoning is: if God exists, he must be perfect; a perfect being is perfect in all ways; existence is part of perfection; therefore God must possess the attribute of existence.

Descartes himself compared this proof to a geometric demonstration. Just as a triangle cannot have anything other than three angles summing to 180°, God cannot lack existence without ceasing to be God. The Stanford Encyclopedia notes that Descartes argued necessary existence cannot be separated from the essence of a supremely perfect being without contradiction – making “God does not exist” as logically incoherent as “a four-sided triangle.”

Descartes’ causal argument: the idea of God requires an infinite cause 🔗

Descartes offered a second, distinct proof in the Third Meditation. This is a causal or cosmological argument grounded in a hierarchical theory of reality. SparkNotes’ analysis of Descartes’ Principles of Philosophy outlines this carefully: Descartes distinguishes between formal reality – the reality anything has simply by existing – and objective reality – the reality that an idea has insofar as it represents something. God, as infinite substance, has the highest grade of formal reality; finite substances have less; and modes (properties) have the least.

The key causal principle is: a cause must contain at least as much reality as its effect. Applied to the idea of God – which has infinite objective reality, since it represents an infinitely perfect being – this idea cannot have been produced by Descartes himself, a finite and imperfect being. As the philosophical discussion at Ask a Philosopher puts it, the premise is that a cause must be at least as great (real) as its effect, meaning an idea of a most perfect being cannot originate from an imperfect source. Its cause must therefore be God himself.

This argument also addresses the contingency of the self. Descartes observed that he exists as a thinking, finite being – but he did not create himself. Every moment of his continued existence requires a sustaining cause. That cause cannot be another finite being, which would just push the question back further. Ultimately, only a being that is the reason for its own existence – a self-sufficient, infinite God – can explain why anything contingent exists at all.

Criticisms of Descartes’ arguments 🔗

Descartes’ proofs did not go unchallenged. The most powerful objection to the ontological argument came later from Immanuel Kant, who argued that existence is not a predicate or property like omnipotence or omniscience. As SparkNotes’ analysis explains, to say “God does not have existence” is not logically incoherent in the same way as saying “God is not omnipotent” – if something does not exist, it cannot have or lack any properties at all. Kant’s point was that Descartes had mistakenly treated existence as though it were just another attribute that could be added to a list of perfections.

A second problem noted by commentators is the charge of circular reasoning. Philosophers have pointed out that Descartes validates his own clear and distinct ideas by appealing to a non-deceiving God – but his proof for God’s existence itself relies on clear and distinct ideas. The guarantee of knowledge rests on God, and the proof of God rests on the guarantee of clear ideas.

Leibniz and the principle of sufficient reason 🔗

Gottfried Wilhelm Leibniz (1646-1716) built on and significantly deepened the rationalist tradition. While he shared Descartes’ conviction that God’s existence could be established through reason, his arguments took a different philosophical direction, rooted in what he called the principle of sufficient reason.

According to Britannica, the principle of sufficient reason asserts that there must be an adequate reason to account for the existence and nature of everything that could conceivably not exist. For Leibniz, there are no brute, unexplained facts – everything that exists has a sufficient reason for being the way it is and not otherwise. The Stanford Encyclopedia of Philosophy notes that in the Monadology, Leibniz stated this as one of two foundational principles of all reasoning, alongside the principle of non-contradiction.

Leibniz applied this principle directly to the existence of the universe. The world consists entirely of contingent things – things that exist but could just as well not have existed. No individual thing in the universe explains its own existence, nor does the entire collection of contingent things explain itself as a whole. As the Internet Encyclopedia of Philosophy explains, even an infinitely old universe would not solve the problem, since an infinite series of contingent causes still does not explain why there is any series at all rather than nothing. There must therefore be something beyond the totality of contingent things – a necessary being that carries the reason for its existence within itself. That being, Leibniz concluded, is God.

In his own words from On the Ultimate Origination of the Universe (1697), Leibniz argued that the sufficient reason which needs no further reason must lie outside the series of contingent things and be found in a necessary being – which is what we call God.

God as the monad of monads 🔗

Leibniz’s broader metaphysical system is built around the concept of monads – simple, indivisible, immaterial substances that make up all of reality. Each monad is a kind of self-contained unit of perception, with no windows or doors through which it interacts with other monads. As summarized in discussions of his Monadology, every monad has its own internal principle of being, and any change a monad undergoes is internally determined – not caused externally.

Within this framework, God occupies a unique and supreme position. According to the Wikipedia entry on the Monadology, God is described as a simple substance – but the only necessary simple substance. All other monads are contingent; they exist but need not have existed. God alone exists necessarily. Moreover, while all monads perceive the universe with varying degrees of clarity, Leibniz held that God perceives all monads with complete and perfect clarity – a level of knowledge no finite being can approach.

God is thus not merely the first in a series of beings but the ultimate source and sustainer of all monadic existence. As explained in analyses of the Monadology, God is the sufficient reason for every detail of reality – creatures draw their perfection from God and their imperfections from their own limited nature. God’s existence is grounded in necessity itself: it suffices for God to be possible for him to be actual, since a necessary being cannot merely possibly exist without actually existing.

Leibniz’s ontological argument: improving on Descartes 🔗

Leibniz did not simply repeat Descartes’ ontological argument – he refined it. He recognized a potential gap in the argument: it assumes, without proof, that the concept of a supremely perfect being is coherent and possible. If the idea of God contained a hidden contradiction – like the idea of a largest prime number – then the entire argument would collapse before it began.

As EBSCO’s research guide to the Monadology notes, Leibniz modified Anselm’s ontological argument by adding a proof of God’s possibility. He argued that God, as an unlimited being possessing all positive realities, could not contain a contradiction – since contradictions only arise when positive properties exclude each other, and God’s infinite perfections are all purely positive. Therefore the concept of God is genuinely possible, and a possible necessary being must actually exist.

From analyses of the Monadology, Leibniz argued that God alone has the privilege that he must exist if he is possible – unlike contingent things, which might or might not exist even if possible. This closes the logical gap that Descartes left open.

The best of all possible worlds 🔗

One of the most distinctive aspects of Leibniz’s theology is his claim that God, being supremely perfect and rational, created the best possible world. From an infinite number of possible universes in his mind, God freely chose to actualize the one with the greatest degree of perfection – the richest variety of beings governed by the simplest possible laws. As discussions of the Monadology explain, God’s plan for the universe necessarily produces the greatest amount of goodness because it reflects God’s absolute perfection.

This was not naïve optimism. Leibniz was well aware of the existence of evil and suffering. His point was metaphysical rather than experiential: a God of infinite reason and goodness would not settle for a lesser world when a better one was available. The Théodicée (1710) – Leibniz’s extended defense of God’s justice – grappled directly with the problem of evil within this framework.

The pre-established harmony that governs all monads is itself a product of God’s rational perfection. As the Monadology describes it, monads do not truly interact causally with one another – they merely appear to because God, at the foundation of creation, synchronized them perfectly, like clocks set to the same time by a single watchmaker.

Comparing Descartes and Leibniz 🔗

Both philosophers argued that God’s existence is accessible to reason without relying on empirical observation or faith alone. But their emphases differ. Descartes grounded his proofs primarily in the nature of ideas – the innate idea of a supremely perfect being, and the causal principle that an infinite idea requires an infinite cause. Leibniz, by contrast, anchored his argument in the structure of reality itself – the principle of sufficient reason demands a necessary being as the ultimate explanation for a world of contingent things.

Leibniz also went further in systematizing the relationship between God and the world. Where Descartes needed God mainly to escape the evil-demon scenario and validate clear perception, Leibniz built an entire metaphysics – the monadology – in which God is the logically indispensable foundation of all existence, unity, and rationality. God is not just a necessary conclusion at the end of an argument; God is the living source of every truth and every being.

Together, Descartes and Leibniz represent the high-water mark of rationalist theism in Western philosophy – the conviction that the deepest truths about God and existence are available to the careful exercise of reason. Their arguments shaped centuries of philosophical theology, provoking responses from Hume, Kant, and beyond, and they continue to be studied, challenged, and defended in academic philosophy today.

What do you think? If every contingent thing requires a sufficient reason for its existence, does that reasoning necessarily lead to a God – or could a necessary being take some other, non-personal form? And is Descartes right that the idea of a supremely perfect being is something every rational mind naturally possesses, or is it a concept we construct rather than discover?

References

1. https://plato.stanford.edu/entries/cosmological-argument/
2. https://plato.stanford.edu/entries/descartes-ontological/
3. https://study.com/academy/lesson/descartes-ontological-argument-premises-criticism.html
4. https://www.sparknotes.com/philosophy/principles/section4/
5. https://askaphilosopher.org/2014/05/22/descartes-cosmological-and-ontological-arguments/
6. https://www.britannica.com/topic/principle-of-sufficient-reason
7. https://plato.stanford.edu/entries/leibniz/
8. https://iep.utm.edu/leib-met/
9. https://spot.colorado.edu/~heathwoo/Phil100/leibniz.html
10. https://www.angelfire.com/md2/timewarp/leibniz.html
11. https://en.wikipedia.org/wiki/Monadology
12. https://www.philosophers.world/leibniz/monadology/Page-2.html
13. https://www.ebsco.com/research-starters/literature-and-writing/monadology-gottfried-wilhelm-leibniz