Arguments for the existence of God go back at least to Aristotle, who argued that there must be a first mover, itself unmoved. All the great medieval philosophers (Arabic and Jewish as well as Christian) proposed and developed theistic arguments – for example, Augustine, al-Ghazali, Anselm, Moses Maimonides, Thomas Aquinas and Duns Scotus. Most of the great modern philosophers – in particular René Descartes, Gottfried Leibniz and Immanuel Kant – have also offered theistic arguments. They remain a subject of considerable contemporary concern; the twentieth century has seen important work on all the main varieties of these arguments.
These arguments come in several varieties. Since Kant, the traditional Big Three have been the cosmological, ontological and teleological arguments. The cosmological argument goes back to Aristotle, but gets its classic statement (at least for European philosophy) in the famous ‘five ways’ of Aquinas, in particular his arguments for a first uncaused cause, a first unmoved mover, and a necessary being. According to the first-mover argument (which is a special case of the first-cause argument), whatever is moved (that is, caused to move) is moved by something else. It is impossible, however, that there should be an infinite series of moved and moving beings; hence there must be a first unmoved mover. Aquinas goes on to argue that a first mover would have to be both a first cause and a necessary being; he then goes on in the next parts (Ia, qq.3–11) of the Summa theologiae to argue that such a being must have the attributes of God.
The perennially fascinating ontological argument, in Anselm’s version, goes as follows: God is by definition the being than which none greater can be conceived. Now suppose God did not exist. It is greater to exist than not to exist; so if God did not exist, a being greater than God could be conceived. Since God is by definition the being than which none greater can be conceived, that is absurd. Therefore the supposition that God does not exist implies an absurdity and must be false. This argument has had many illustrious defenders and equally illustrious attackers from Anselm’s time to ours; the twentieth century has seen the development of a new (modal) version of the argument.
Aquinas’ fifth way is a version of the third kind of theistic argument, the teleological argument; but it was left to modern and contemporary philosophy to propose fuller and better-developed versions of it. Its basic idea is simple: the universe and many of its parts look as if they have been designed, and the only real candidate for the post of designer of the universe is God. Many take evolutionary theory to undercut this sort of argument by showing how all of this apparent design could have been the result of blind, mechanical forces. Supporters of the argument dispute this claim and retort that the enormously delicate ‘fine tuning’ of the cosmological constants required for the existence of life strongly suggests design.
In addition to the traditional Big Three, there are in fact many more theistic arguments. There are arguments from the nature of morality, from the nature of propositions, numbers and sets, from intentionality, from reference, simplicity, intuition and love, from colours and flavours, miracles, play and enjoyment, from beauty, and from the meaning of life; and there is even an argument from the existence of evil.
Necessary Being: The Cosmological Argument