For centuries philosophers have offered arguments that God exists. Here “God” usually means the God of classical theism: a single, all-powerful, all-knowing, perfectly good creator. This level presents five main arguments as their defenders would, each followed by the main replies. Believers differ on how much such arguments matter; many say their faith rests on other grounds.

1. The cosmological argument

Things around us have causes, and those causes have causes. Thomas Aquinas (thirteenth century), in his “Five Ways,” argued that such a chain cannot go back forever without explanation, so there must be a first cause or necessary being, which people call God. A version called the kalam argument, developed by medieval Islamic thinkers such as al-Ghazali, reasons that whatever begins to exist has a cause, and the universe began to exist, so the universe has a cause. Leibniz asked why there is something rather than nothing, and answered that only a necessary being can explain it.

Replies: Why must the first cause be God, let alone a personal and good God? If everything needs a cause, what caused God? (Defenders answer that the argument concerns things that begin or depend on others, not a necessary being.) Hume and Kant questioned whether ideas of cause that work within the universe apply to the universe as a whole. Perhaps the universe itself is simply a brute fact.

2. The design argument

In Natural Theology (1802), William Paley imagined finding a watch on a heath. Its parts are so precisely arranged for a purpose that we would conclude it had a maker: intricate arrangement for a purpose points to a designer. Living things, such as the eye, show far more intricate design. A modern version, the fine-tuning argument, notes that certain physical constants seem set within the narrow range that allows life.

Replies: Hume's Dialogues Concerning Natural Religion (1779) had argued that the analogy is weak, that the world contains flaws and suffering, and that it cannot prove a single, perfect designer. Darwin's theory of natural selection (1859) explained adaptation without a designer. Against fine-tuning, critics suggest there may be many universes, or note that we could only ever observe a universe that permits observers.

3. The ontological argument

Anselm of Canterbury, in his Proslogion (around 1078), defined God as, in an older translation, “a being than which nothing greater can be conceived.” Even someone who denies God understands this idea, so it exists in the mind. But a being existing in reality would be greater than one existing only in the mind. So if God existed only in the mind, we could conceive something greater, which is a contradiction. Therefore God exists in reality.

Replies: The monk Gaunilo replied that the same reasoning could prove a perfect island exists, which is absurd. Kant argued that existence is not a property that makes a thing greater. Defenders, from Descartes to modern logicians, have offered revised versions.

4. The moral argument

Many people sense that some things, such as torturing innocents for fun, are really wrong, not merely disliked. Defenders argue that such objective moral truths are best explained by a perfectly good God. Kant gave a related argument that morality requires us to hope for a God who unites virtue and happiness.

Replies: Secular philosophers offer other foundations for morality (Level 5), and evolutionary accounts explain why we have moral feelings.

5. Religious experience

Many people across cultures report experiences of God's presence. Defenders argue that, just as we trust our senses unless we have reason to doubt them, it is reasonable to trust such experiences unless there is reason not to.

Replies: Experiences in different traditions support conflicting beliefs; they may have psychological explanations; and unlike sight, they cannot easily be checked by others.

Worked example: stating the fine-tuning argument fairly

Two students, Amira, a Muslim, and Ben, an agnostic, are asked to present the fine-tuning argument fairly and then assess it together.

Stating it well. Amira lays out the premises. (1) Physicists report that several constants of nature, such as the strength of gravity, fall within narrow ranges that allow stable stars, chemistry and life. (2) This fine-tuning is due to physical necessity, to chance or to design. (3) It is not plausibly due to necessity or chance. (4) Therefore it is due to design. Ben agrees this is a fair statement and that the first premise reflects real scientific discussion.

Finding the pressure points. Ben questions premise 3. Perhaps a deeper physical theory will show the constants could not have been otherwise. Or perhaps there are many universes with different constants, and we naturally find ourselves in one that permits observers. Amira replies that a multiverse is itself unobserved and may need fine-tuning of its own, and that design is the simpler explanation.

Weighing honestly. Ben admits the argument has more force than he expected. Amira admits that even if it succeeds, it points to a designer, not necessarily to the God of any particular religion, so her own faith rests on more than this argument.

Notice the method. Each student stated the other side's view in a way its defenders would accept, identified exactly which premise was in dispute, and conceded what was fair. Neither changed their mind entirely, but both understand the question far better than before. This is the approach the course recommends for every argument in this level and the next.