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The cosmological argument. Versions of this argument are defended by Alexander Pruss, Timothy O'Connor, Stephen Davis, Robert Koons, and Richard Swinburne, among others. A simple formulation of this argument is:

1. Everything that exists has an explanation of its existence, either in the necessity of its own nature or in an external cause.
2. If the universe has an explanation of its existence, that explanation is God.
3. The universe exists.
4. Therefore, the explanation of the universe's existence is God.

This argument is logically valid, so the only question is the truth of the premises. Premise (3) is undeniable for any sincere seeker of truth, so the question comes down to (1) and (2).

Premise (1) seems quite plausible. Imagine that you're walking through the woods and come upon a translucent ball lying on the forest floor. You would find quite bizarre the claim that the ball just exists inexplicably. And increasing the size of the ball, even until it becomes co-extensive with the cosmos, would do nothing to eliminate the need for an explanation of its existence.

Premise (2) might at first appear controversial, but it is in fact synonymous with the usual atheist claim that if God does not exist, then the universe has no explanation of its existence. Besides, (2) is quite plausible in its own right. For an external cause of the universe must be beyond space and time and therefore cannot be physical or material. Now there are only two kinds of things that fit that description: either abstract objects, like numbers, or else an intelligent mind. But abstract objects are causally impotent. The number 7, for example, can't cause anything. Therefore, it follows that the explanation of the universe is an external, transcendent, personal mind that created the universe—which is what most people have traditionally meant by "God."

The kalam cosmological argument. This version of the argument has a rich Islamic heritage. Stuart Hackett, David Oderberg, Mark Nowacki, and I have defended the kalam argument. Its formulation is simple:

1. Everything that begins to exist has a cause.
2. The universe began to exist.
3. Therefore, the universe has a cause.

Premise (1) certainly seems more plausibly true than its denial. The idea that things can pop into being without a cause is worse than magic. Nonetheless, it's remarkable how many nontheists, under the force of the evidence for premise (2), have denied (1) rather than acquiesce in the argument's conclusion.

Atheists have traditionally denied (2) in favor of an eternal universe. But there are good reasons, both philosophical and scientific, to doubt that the universe had no beginning. Philosophically, the idea of an infinite past seems absurd. If the universe never had a beginning, then the number of past events in the history of the universe is infinite. Not only is this a very paradoxical idea, but it also raises the problem: How could the present event ever arrive if an infinite number of prior events had to elapse first?

Moreover, a remarkable series of discoveries in astronomy and astrophysics over the last century has breathed new life into the kalam argument. We now have fairly strong evidence that the universe is not eternal in the past, but had an absolute beginning about 13.7 billion years ago in a cataclysmic event known as the Big Bang.

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July 2008, Vol. 52, No. 7