The only adequate explanation...

The only adequate explanation, the arguments suggest, is that it was created by God. What distinguishes the kalam cosmological argument from other forms of cosmological argument is that it rests on the idea that the universe has a beginning in time. Modal forms of the cosmological argument are consistent with the universe having an infinite past.

With the kalam cosmological argument, however, it is precisely because the universe is thought to have a beginning in time that the existence of the universe is thought to stand in need of explanation. The argument has the following structure: (1) Everything that has a beginning of its existence has a cause of its existence. (2) The universe has a beginning of its existence. Therefore: (3) The universe has a cause of its existence.

(4) If the universe has a cause of its existence then that cause is God. Therefore: (5) God exists. The first premise of the argument is the claim that everything that begins to exist has a cause of its existence. In order to infer from this that the universe has a cause of its existence the proponent of the kalam cosmological argument must prove that the past is finite, that the universe began to exist at a certain point in time.

The crucial premise of the kalam cosmological argument, then, is the second: “The universe has a beginning of its existence”. How do we know that the universe has a beginning of its existence? Might not the universe stretch back in time into infinity, always having existed? The proponent of the kalam cosmological argument must show that this cannot be the case if his argument is to be successful.

Advocates of the kalam cosmological argument claim that it is impossible that the universe has an infinite past. The existence of an infinite past, they say, entails all manner of logical absurdities. If there exists an infinite past, then if we were to assign a number to each past moment then every real number (i.e. every postive integer) would be assigned to some moment. There would therefore be no unassigned number to be assigned to the present moment as it passes into the past.

However, by reassigning the numbers such that moment number one becomes moment number two, and moment number two becomes moment number three, and so on, we could free up moment number one to be assigned to the present. If the past is infinite, therefore, then there both is and is not a free number to be assigned to the present.