Philosophy › Faith and reason › Thomas Aquinas
Does God exist?
argued since antiquity; classic proofs 1078–1274· Big question· Reality, mind & God
Mostly agreedGood evidence; some details uncertain.
One of philosophy's oldest questions: is there a God, and can reason, evidence or experience settle it?
See it on the Philosophy map →The idea
Three classic families of argument. Ontological: the idea of a perfect being implies it exists (Anselm). Cosmological: everything contingent needs a cause, ending in a necessary being (Avicenna, Aquinas). Design: the order of nature points to a designer (Paley's watchmaker, 1802). Against them: Hume questioned the design analogy, Kant rejected the ontological argument, Darwin explained apparent design naturally, and the problem of evil challenges God's goodness. Others answer by faith or a wager (Kierkegaard, Pascal).
Why it matters
The answer shapes ethics, ideas about life's meaning, and politics, and the arguments sharpened logic, cosmology and the philosophy of science.
Where it comes from
The classic arguments survive in Anselm's Proslogion, Aquinas' Summa and Hume's Dialogues; in the 2020 PhilPapers survey, about 67% of philosophers leaned atheist and 19% theist.
Still debated
Whether the question can be settled by argument at all, or only by faith or experience, is itself disputed.
Connections (8)
▸Avicenna (Ibn Sina) gave the contingency argument to Does God exist?Mostly agreed
The connection
Avicenna argued that anything whose existence is merely possible needs a cause outside itself. A chain of possible things, even an infinite one, is as a whole still merely possible, so there must be a Necessary Existent, whose essence is to exist. That is God.
When and how
Persia, early 11th century, in The Book of Healing and The Book of Salvation (al-Najat).
How sure are we?
The argument is set out in his surviving works; Aquinas' third way and Leibniz's contingency argument closely follow its pattern.
Debates and open questions
Whether a necessary existent is the personal God of religion, as al-Ghazali doubted, and whether a whole chain needs an outside cause, are disputed.
▸Does God exist? was answered a priori by Anselm's ontological argumentWell documented
The connection
Most arguments for God start from the world: motion, causes, design. The ontological argument answers the question a priori, from reason alone, before any observation. If it worked, denying God would be a contradiction, like a square circle.
When and how
Bec, Normandy, 1077–78; restated by Descartes (1641) and Leibniz, rejected by Kant (1781).
How we know
Anselm's text survives; its purely a priori structure is why philosophers class it apart from cosmological and design arguments.
Debates and open questions
Whether any a priori argument could prove that something exists is the deep dispute; Kant said existence is not a property.
▸Does God exist? was answered from experience by Aquinas' Five WaysWell documented
The connection
The Five Ways are the classic answer to whether God exists that starts from experience rather than from definitions. They set the template for later a posteriori (experience-based) cosmological and design arguments, and framed the debate in which Hume and Kant argued that reasoning from the world cannot reach a perfect God.
When and how
Summa Theologiae, about 1266–1268, written in contrast to Anselm's ontological argument of 1077–1078 and drawing on Aristotle, Avicenna and Maimonides.
How we know
Aquinas explicitly poses the question 'whether God exists' and answers that 'it can be proved in five ways'; modern textbooks treat them as standard.
Debates and open questions
Whether they prove only a first cause or designer, rather than the God of any religion, has been disputed since Hume and is still argued.
▸Baruch Spinoza answered "God is Nature" to Does God exist?Well documented
The connection
Spinoza answered that God exists necessarily but is not a person above the world: God is the one infinite substance, identical with nature itself, 'God or Nature' (Deus sive Natura). God has no plans or purposes; everything follows from God's nature as theorems follow from axioms.
When and how
Rijnsburg, Voorburg and The Hague, written c. 1661–1675; published as the Ethics in 1677, after his death.
How we know
The Ethics, Part 1, argues it in geometric form, and contemporaries condemned the book as atheism.
Debates and open questions
Whether Spinoza was a pantheist, an atheist in disguise, or a deeply religious thinker is still debated.
▸Søren Kierkegaard chose faith over proof in answering Does God exist?Mostly agreed
The connection
Kierkegaard held that God cannot be proved: 'if God does not exist it would be impossible to prove it; and if he does exist it would be folly to attempt it.' Faith is passionate commitment despite 'objective uncertainty,' and Christianity's claim that God became a man is an 'absolute paradox' reason cannot digest.
When and how
Copenhagen, 1844–46, in Philosophical Fragments and Concluding Unscientific Postscript, written under the pseudonym Johannes Climacus.
How sure are we?
Both texts survive; scholars widely read them as founding statements of Christian existentialism and fideism, faith independent of rational proof.
Debates and open questions
Whether he was an irrationalist or put faith above, not against, reason; the phrase 'leap of faith' itself is not exactly his.
▸Darwin's theory of evolution undercut the design argument in Does God exist?Mostly agreed
The connection
The design argument held that living things fit their lives so well that a designer must exist, as a watch implies a watchmaker (Paley). Natural selection explained that fit through variation and differential survival over long ages, undercutting what many saw as the strongest evidence-based argument for God.
When and how
England, On the Origin of Species (1859); Darwin had studied Paley's Natural Theology at Cambridge.
How sure are we?
Darwin's Autobiography says natural selection made Paley's argument fail; reviews and debates of the 1860s show the impact.
Debates and open questions
Whether evolution refutes design or only moves it to the laws of nature, as theistic evolutionists argue, is still disputed.
▸Blaise Pascal and the wager answered with a bet, not a proof, Does God exist?Well documented
The connection
Pascal said reason cannot prove or disprove God, so treat the question as a bet. If you believe and God exists, you gain infinitely; if God does not exist, you lose little. Disbelief risks infinite loss, so a prudent person wagers on God.
When and how
Paris, c. 1657–1662; his notes were published after his death as the Pensées (1670).
How we know
The wager passage survives in his own manuscript notes, now kept at the Bibliothèque nationale de France.
Debates and open questions
Critics ask which God to bet on (the many-gods objection) and whether belief adopted for gain can be genuine.
▸David Hume challenged the design-argument answer to Does God exist?Well documented
The connection
In Hume's Dialogues, the skeptic Philo argues that the universe is not much like a machine, that with only one universe we have nothing to compare it to, and that a flawed world suggests a flawed or indifferent maker. Design reasoning cannot reach the all-good God of theology.
When and how
Edinburgh, written c. 1751–1776; Dialogues Concerning Natural Religion, published in 1779, after his death.
How we know
The Dialogues survive; Hume delayed publication for fear of backlash, and Kant and later critics built on them.
Debates and open questions
Which character speaks for Hume, Philo or Cleanthes, is debated, as is whether he was an atheist or a skeptical deist.