The Modern Philosophical Landscape: From Descartes to Hume on God’s Existence

Does God exist? Few questions have occupied philosophers more persistently – or more fiercely – than this one. In the modern philosophical era, stretching roughly from the 17th to the 18th century, thinkers stopped relying solely on scripture or tradition to answer it. Instead, they turned to reason, logic, and systematic argument. What emerged was one of the richest and most consequential debates in intellectual history – a debate that fundamentally reshaped how we think about God, knowledge, and the limits of human understanding. From Descartes’ rationalist proofs to Hume’s devastating skepticism and Kant’s critical philosophy, each thinker left a distinct mark on this conversation.

Descartes: the rationalist foundation 🔗

René Descartes (1596-1650) is the natural starting point. Famous for his declaration cogito ergo sum – “I think, therefore I am” – Descartes made the existence of God central to his entire philosophical project. Without God, he could not escape the possibility of a deceiving demon corrupting all his knowledge. Proving God’s existence wasn’t just a theological exercise for Descartes; it was the linchpin of his epistemology.

Descartes offered two distinct arguments, both found in his Meditations on First Philosophy. The first, in Meditation III, is a causal argument: he possessed within his mind an idea of a supremely perfect, infinite being. Since no finite being could produce an idea with that level of “objective reality,” the cause of this idea must itself be infinite and perfect. Only God could have placed that idea in him – like, as Descartes put it, a craftsman’s mark stamped on his work.

The second argument, in Meditation V, is the famous ontological argument. As the Stanford Encyclopedia of Philosophy explains, Descartes’ version is grounded in his theory of innate ideas and the doctrine of clear and distinct perception – God’s existence is inferred directly from the fact that necessary existence is contained in the clear and distinct idea of a supremely perfect being. If God is defined as a being possessing all perfections, and existence is itself a perfection, then God must necessarily exist. To deny God’s existence, on this view, is as contradictory as claiming a triangle has four sides.

This argument differed from the earlier version by St. Anselm of Canterbury in an important way: where Anselm proceeded from the meaning of the word “God,” Descartes proceeded from an innate idea whose content was, he claimed, simply given to the mind – not invented, but discovered.

Spinoza: God as nature itself 🔗

Baruch Spinoza (1632-1677) radically reworked the concept of God. He accepted the rationalist framework but arrived at a conclusion that scandalized his contemporaries: God and Nature are one and the same thing. His monumental work Ethics, written in the geometric style of Euclid, argued that there is only one infinite substance in the universe – and that substance is God, or Nature (Deus sive Natura).

Spinoza’s argument for God’s existence begins with substance. He defined substance as that which exists independently and is conceived through itself alone. Given this definition, as the Stanford Encyclopedia details, the very concept of God involves the concept of existence: God’s essence necessarily involves existence because a being that cannot be conceived except as existing is self-causing. This was Spinoza’s version of the ontological argument, and it differed from Descartes’ in that it avoided claiming existence is a “perfection” – a move that would later attract Kantian criticism.

Crucially, as the Internet Encyclopedia of Philosophy notes, Spinoza’s God was not the transcendent, personal God of Biblical tradition. Thought and matter were both attributes of God, and human souls were part of God as well. The highest good of the human mind, Spinoza wrote in the Ethics, was the knowledge of God – not worship or obedience, but intellectual understanding of the whole of nature. This pantheistic conception made Spinoza one of the most controversial thinkers of his era, frequently labeled an “atheist” by critics who found his God indistinguishable from the universe itself.

Leibniz: refining the argument and defending God’s perfection 🔗

Gottfried Wilhelm Leibniz (1646-1716) engaged directly with both Descartes and Spinoza – correcting the former and vigorously opposing the latter. His contributions to the debate on God’s existence were systematic and multifaceted, drawing on rationalist logic while insisting on a personal, transcendent, and morally perfect God.

The ontological argument, completed 🔗

Leibniz identified a weakness in Descartes’ ontological argument: Descartes had never proved that the concept of a “supremely perfect being” was coherent in the first place. According to the Wikipedia entry on the ontological argument, Leibniz proposed that unless the coherence of a supremely perfect being could be demonstrated, the entire argument fails. He then attempted to complete what Descartes had left unfinished, arguing that all perfections are compatible and can coexist in a single entity – making the concept of God logically consistent and the argument valid.

The principle of sufficient reason and God 🔗

Beyond the ontological argument, as summarized at Logos, Leibniz built his case for God on four distinct proofs. The most philosophically original was his cosmological argument from contingency: the universe is contingent – it didn’t have to exist, and logic alone cannot explain why it does. Since the universe contains no reason for its own existence within itself, there must be a sufficient reason beyond it. That reason is God. This use of the principle of sufficient reason – that everything has a reason or cause – became one of the most influential moves in early modern philosophy of religion.

Leibniz also offered a teleological argument: the world is filled with things that cannot be explained by blind natural forces alone, pointing to an intelligent creator. And he argued that eternal truths – mathematical and logical facts that are true in all possible worlds – must be grounded in an eternal mind. God, for Leibniz, was the ultimate source of all necessary truth.

Spinoza vs. Leibniz: immanence vs. transcendence 🔗

The contrast between Spinoza and Leibniz on God is sharp and philosophically significant. As one comparative analysis puts it, Spinoza believed in an immanent God – a God who is the world, creating it the way an essence creates its properties, inseparable from nature. Leibniz argued for a transcendent God – a creator who stands outside the world, choosing freely among all possible worlds to bring the best one into existence. For Spinoza, God had no choice; for Leibniz, God always had the option not to create. This distinction – immanence versus transcendence – continues to shape debates in philosophy of religion and cosmology to this day.

Hume: the empiricist challenge 🔗

David Hume (1711-1776) brought the rationalist edifice crashing down – or at least, dealt it a blow from which it never fully recovered. Hume was a thoroughgoing empiricist: knowledge comes from experience and sense impression, not from pure reason. This directly undermined the entire rationalist project of proving God’s existence a priori.

As the Stanford Encyclopedia’s entry on Hume and religion explains, the central debate shaping Hume’s views was not between empiricism and rationalism as such, but between philosophical defenders of theism and their critics. Hume took a position of methodical skepticism throughout. In his Dialogues Concerning Natural Religion, he attacked each major proof in turn.

Against the ontological argument, Hume argued that we have no abstract idea of existence apart from ideas of particular objects. Any conception of God we have, we can equally conceive either as existing or as not existing. Existence, Hume insisted, is not a quality or perfection – so denying God’s existence is not a logical contradiction. Against the cosmological argument, he challenged the assumption that an infinite series of causes requires a first cause outside the series: why should the whole require a cause if each part within it is explained? Against the design argument, he argued in the Dialogues that the analogy between the universe and a human artifact (like a machine designed by a craftsman) is too weak to carry the weight placed on it.

Importantly, Hume never claimed to prove that God does not exist. His position was closer to a radical skepticism about the knowability of God’s nature. He described God as a being so remote and incomprehensible that we have no legitimate authority to ascribe to him any specific attributes or perfections. As the Stanford Encyclopedia notes, this position was effectively indistinguishable from the atheism his contemporaries feared.

Kant: the critical turn 🔗

Immanuel Kant (1724-1804) is the pivotal figure in this whole story. He described Hume as having awoken him from his “dogmatic slumbers,” and his response was to transform philosophy itself. Kant’s Critique of Pure Reason (1781) set out to determine what human reason can and cannot legitimately know – and its verdict on the traditional proofs of God’s existence was devastating.

Dismantling the traditional proofs 🔗

Kant’s most famous objection targeted the ontological argument directly. As Wikipedia’s entry on the ontological argument summarizes, Kant argued that the argument fails because it treats existence as a predicate – as a property that something can either have or lack. But existence, Kant insisted, is not a real predicate at all. To say “God exists” does not add any property to our concept of God; it only asserts that the concept has an instance in reality. You cannot define something into existence. His criticism, shaped by his central distinction between analytic and synthetic propositions, effectively closed the door on purely conceptual proofs of God.

Kant also rejected the cosmological and teleological arguments as inconclusive. His broader argument, developed in the Critique of Pure Reason, was that human understanding is structured to apply only within the bounds of possible experience. God, by definition, transcends all possible experience. Therefore, as the Internet Encyclopedia of Philosophy explains, God’s existence is not something that reason can demonstrate – nor, crucially, can reason demonstrate that God does not exist. The question simply falls outside the boundaries of theoretical knowledge.

The moral argument: God as a practical postulate 🔗

Yet Kant did not abandon God. In his Critique of Practical Reason, he offered a radically different kind of argument – not a proof, but a practical postulate. Kant argued that morality requires us to pursue the summum bonum: the highest good, combining perfect moral virtue with proportional happiness. But humans cannot guarantee that virtue leads to happiness in this life. As Wikipedia’s entry on the argument from morality explains, Kant concluded that we must therefore assume the existence of God as a being with the power to ensure that virtue is ultimately rewarded – not because we can prove it, but because moral reason demands it.

This was a landmark shift. Kant was not claiming that God exists in the sense that chairs and planets exist. He was claiming that believing in God is a rational and morally necessary commitment. As philosopher Peter Sjöstedt-Hughes explains, for Kant, God is a “necessary hypothesis” – not knowledge, but a postulate that moral reason cannot do without. God moved from the domain of theoretical proof into the domain of practical faith.

The philosophical shift: from proof to critique 🔗

Taken together, the arc from Descartes to Kant traces a profound transformation in Western philosophy. Descartes, Spinoza, and Leibniz were confident that God’s existence could be established through reason alone – through logic, innate ideas, and the principle of sufficient reason. Their arguments were ambitious, systematic, and mutually reinforcing, even as they disagreed sharply on what kind of God they were proving.

Hume introduced a disruptive empiricist challenge: if all legitimate knowledge comes from experience, then metaphysical claims about God are suspect from the start. He didn’t destroy belief in God, but he demolished the philosophical confidence behind rational proofs. And then Kant completed the critical turn, drawing a firm boundary between what reason can know and what it must merely assume. His conclusion – that God cannot be theoretically proved but is morally indispensable – defined the terms of the debate for generations that followed.

The diversity within modern philosophy on this question is itself significant. These were not thinkers casually disagreeing; they were building careful, competing systems, each responding to and building on the last. As the Internet Encyclopedia of Philosophy notes on Kant’s philosophy of religion, Descartes and his followers favored a priori proofs of God, while the British empiricists like Locke and Hume turned to cosmological and design arguments grounded in experience – and found those wanting too. The result was a philosophical landscape that moved steadily toward skepticism and criticism, not because thinkers abandoned God, but because they held the arguments for God to an increasingly rigorous standard.

What do you think? If Kant is right that God’s existence cannot be theoretically proven but is a necessary postulate of moral reason, does that kind of “moral faith” satisfy what most people mean when they ask whether God exists? And does Spinoza’s identification of God with Nature represent a genuinely religious position – or does it dissolve the concept of God entirely?

References

1. https://en.wikipedia.org/wiki/Trademark_argument
2. https://plato.stanford.edu/entries/descartes-ontological/
3. https://plato.stanford.edu/entries/spinoza-modal/
4. https://iep.utm.edu/spinoz-m/
5. https://en.wikipedia.org/wiki/Ontological_argument
6. https://www.logos.com/grow/rationalism-descartes-spinoza-leibniz/
7. http://philogb.github.io/notes/spinoza-leibniz-god
8. https://plato.stanford.edu/entries/hume-religion/
9. https://iep.utm.edu/kant-rel/
10. https://en.wikipedia.org/wiki/Argument_from_morality
11. https://www.philosopher.eu/texts/kants-moral-argument-for-god/