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The arguments about God's existence in Western philosophy of religion are mostly aimed at one particular idea of God, the God of classical theism, which comes from the Jewish, Christian and Islamic traditions as developed by medieval philosophers such as Augustine, Anselm, Maimonides, Avicenna and Aquinas. Knowing what that God is meant to be explains both what the arguments set out to prove and why atheists attack the idea itself.

The God of classical theism

Classical theism describes God by a set of attributes, usually presented as belonging together.

Not every theist holds every item. Whether God is timeless or everlasting, for instance, is disputed among believers, and the tension between an unchanging God and a God who answers prayer is one of the pressure points that atheist philosophers use.

Kinds of argument

Arguments for God are sorted by where their premises come from and by how the conclusion is meant to follow.

An a priori argument starts from reason alone, with premises that can be known without observing the world. Anselm's ontological argument is the standard case. An a posteriori argument starts from something observed or experienced, such as the existence of the universe, its order or the fact that things move and change. The cosmological and teleological arguments are both a posteriori, since each begins from a feature of the world.

A deductive argument claims that if the premises are true, the conclusion must be true. A deductive argument is valid when the conclusion really does follow, and sound when it is valid and its premises are in fact true. An inductive argument claims only that its premises make the conclusion probable, so it can be strong or weak but never certain. Aquinas' ways and the kalam argument are often put in deductive form, while Paley's design argument and the modern fine-tuning arguments are inductive. A deductive argument can be attacked by denying a premise or by showing that the conclusion does not follow. An inductive argument can be attacked by offering a better explanation of the same evidence, or by showing that the analogy or probability it relies on is weak.

What the arguments aim to prove

Even a successful cosmological or design argument establishes less than the full God of classical theism. A first cause need not be good, a designer need not be one being, and a designer need not be all-powerful or still alive. Theists reply that the arguments work together as a cumulative case, and that further reasoning (as Aquinas gives in the Summa Theologiae after his five ways) shows that a first cause must have the attributes of God. Critics reply that each step adds assumptions.

Natural and revealed theology

Natural theology is the attempt to establish truths about God by reason and observation of nature, without relying on scripture or revelation. The arguments in this area belong to it. Revealed theology starts from what God is held to have made known in scripture and in religious experience. Some theologians, including Karl Barth in the twentieth century, reject natural theology as unnecessary or even idolatrous, since in their view knowledge of God comes only from God's self-revelation. Others, including the Catholic tradition following Aquinas, treat natural theology as a legitimate preparation for faith, and the First Vatican Council (1869–70) taught that God can be known with certainty by natural reason.

Faith and reason

Whether belief in God needs evidence at all is a separate question from whether the arguments succeed.

Evidentialism, the view that belief should be proportioned to the evidence (W. K. Clifford's 1877 essay "The Ethics of Belief" is the classic statement), makes the arguments central: if they fail, belief is unjustified. Fideism holds that faith does not rest on rational proof and may even go beyond reason; Kierkegaard and, in some readings, Wittgenstein are linked to it. Reformed epistemology, associated with Alvin Plantinga, holds that belief in God can be properly basic, rational without being supported by argument, in the way that belief in other minds or in the past is rational. These positions matter when judging how much damage a failed argument does to theism.

Terms used throughout

A necessary being exists in all possible worlds, so its non-existence is impossible; a contingent being might not have existed. An explanation can be scientific, giving earlier causes and laws, or personal, giving the intentions of an agent. The principle of sufficient reason says that everything has an explanation. A brute fact is something that has no explanation and simply is the case. A posteriori and a priori are as above. Theism is belief in a personal God who created and sustains the world; deism holds that God created the world but does not intervene in it; atheism is the denial of God or the lack of belief in God; agnosticism is the view that we cannot know whether God exists.