From Aristotle's unmoved mover and Anselm's ontological proof to Spinoza's God-or-Nature, Pascal's wager, Hume's critique and Nietzsche's death of God.
Does God exist, can reason prove it, and what — if anything — could the word 'God' even mean?
No idea has drawn more argument than God — proved, defined, wagered on, redefined, and finally declared dead. The Greeks reasoned to a first mover or supreme good; the Abrahamic traditions fused this with a personal creator and asked whether reason or revelation grasps him. Anselm claimed the very concept entails existence; Aquinas offered five ways from the world to its cause. Spinoza identified God with Nature itself, Pascal appealed to prudence rather than proof, and Leibniz made God the reason there is something rather than nothing. Then came the great critiques: Hume dismantled the design argument, Kant showed the proofs overreach reason, Feuerbach called God a projection of humanity, and Nietzsche pronounced the death of God a cultural event. Read together, these positions trace the whole arc of belief, argument, and doubt.
Ancient Greek (Peripatetic)
Aristotle reasons to an Unmoved Mover: the eternal motion of the cosmos requires a first cause that moves without being moved, itself pure actuality. This divine being is 'thought thinking itself', drawing the world as an object of love and desire rather than by physical push — impersonal, self-contemplating, and utterly beyond change.
Metaphysics, Book XII (Lambda).
Scholasticism
Anselm's ontological argument: God is 'that than which nothing greater can be conceived'. Since a being that exists in reality is greater than one existing in the understanding alone, this greatest conceivable being must exist in reality — otherwise a greater could be conceived. Existence follows from the very concept of God.
Proslogion (1078).
Islamic Aristotelianism
Avicenna argues from contingency: everything whose existence is merely possible requires a cause, and the chain of contingent beings must terminate in a Necessary Existent whose essence is to exist. This God is one, simple, and the source from which all else necessarily emanates — proof of a creator from the very structure of being.
The Book of Healing (Metaphysics); Remarks and Admonitions.
Islamic theology (Ash'arite)
Against the philosophers' eternal, necessary emanation, Al-Ghazālī defends a personal God who freely created the world in time out of nothing. He argues that whatever begins to exist has a cause, and the universe began, so it has a creator endowed with will — a God who acts, wills and could have made things otherwise.
The Incoherence of the Philosophers.
Jewish philosophy
Maimonides insists on negative theology: God's essence is utterly beyond human concepts, so we can say only what God is not, never positively what he is. Attributing qualities to God compromises his absolute unity; even 'existence' and 'wisdom' apply to him equivocally. Reason clears away false images so faith may rest on a purified idea of the One.
The Guide for the Perplexed.
Scholasticism
Aquinas offers the Five Ways — arguments from motion, efficient causation, contingency, degrees of perfection, and the governance of things — each concluding to a first being all call God. Reason can demonstrate that God exists, though his nature is known only analogically; grace perfects but does not contradict this natural knowledge.
Summa Theologiae, I, q. 2 (the Five Ways).
Rationalism
God is not a personal creator apart from the world but the one infinite substance, identical with Nature itself (Deus sive Natura). This God does not act from will or purpose and does not intervene; everything follows from the divine nature by necessity. To love God is to understand this order — the 'intellectual love of God'.
Ethics (1677), Part I.
Christian apologetics
Reason cannot settle whether God exists, so Pascal frames a wager: if you believe and God exists you gain infinite happiness, while if he does not you lose little; the rational bet is therefore faith. Yet 'the heart has its reasons which reason knows nothing of' — living faith is a gift felt by the heart, not a conclusion of proof.
Pensées (published posthumously, 1670).
Rationalism
By the principle of sufficient reason, the existence of the contingent world needs an explanation outside itself: a necessary being, God, who is the reason why there is something rather than nothing. As supremely wise and good, God created the best of all possible worlds, so that the evils it contains are permitted for the sake of a greater overall perfection.
Theodicy (1710); Monadology (1714).
Empiricism / scepticism
Hume dismantles the argument from design: the analogy between the world and human artefacts is weak, the world might be more like an organism, and any inferred cause need not be infinite, single, or good. The presence of suffering makes it hard to infer a benevolent deity, and no testimony can establish a miracle against uniform experience.
Dialogues Concerning Natural Religion (1779); 'Of Miracles' (1748).
German idealism / critical philosophy
Kant argues that the theoretical proofs — ontological, cosmological, physico-theological — all fail, since existence is not a predicate and reason cannot reach beyond experience. Yet God remains a necessary postulate of practical reason: morality requires that virtue and happiness ultimately accord, which only God and immortality can guarantee.
Critique of Pure Reason (1781); Critique of Practical Reason (1788).
Existentialism / Christian thought
Faith is not the conclusion of an argument but a passionate 'leap' held fast against objective uncertainty. God is the absolute paradox that reason cannot comprehend; to believe is to venture everything on it, as Abraham did, in a subjective inwardness where 'truth is subjectivity' and the individual stands alone before God.
Fear and Trembling (1843); Concluding Unscientific Postscript (1846).
Anthropological critique of religion
Theology is really anthropology: God is the projection of humanity's own best qualities — love, wisdom, power — abstracted, magnified, and worshipped as an alien being. In deifying this projection humans impoverish themselves; the task is to reclaim these perfections as human, turning love of God into love of the human.
The Essence of Christianity (1841).
Genealogical critique
'God is dead, and we have killed him.' Nietzsche means not a proof but a cultural fact: belief in the Christian God has become unbelievable, and with it the ground of Western values collapses, opening onto nihilism. The challenge is to create new values without divine sanction, lest the 'shadow' of God linger in secular morality.
The Gay Science (1882); Thus Spoke Zarathustra (1883–85).