William Lane Craig offers the following argument for the claim that the universe has a cause.
Finally, Craig offers a brief argument in favor of the conclusion that the cause of the universe must be a person.
Below I concentrate on the two philosophical arguments for premise 2, and on Craig's argument for thinking that the universe is a person.
First philosophical argument against an infinite temporal regress of events.
Craig first distinguishes between a "potential" and an "actual" infinite. He claims that an infinite temporal regress would be an actual infinite, and that it is impossible for there to be an actual infinite.
Potential vs. Actual Infinite
A series if actually infinite if all of its members are actual. It isn't growing toward infinity - the number of its members is infinite.
Now suppose that the series of past events is infinite. Since is consists entirely of events that have already happened, it must be an actual infinite, and not merely a potential one.
Craig offers the following illustration. Imagine a hotel in which there actually are infinitely many rooms. Each room is filled. Now a new guest arrives and asks for a room. The hotel proprietor then "makes room" for the new guest in room number #1 by simply moving the guest in room #1 to room #2, the guest in room #2 to room #3, and so on, ad infinitum. Surely this is absurd? But where did we go wrong? We went wrong, Craig thinks, when we supposed that there could - even in principle - be a hotel that actually has infinitely many rooms.
Now suppose that the universe has no beginning. Then it seems that the past must consist in a series of events that is actually, and not merely potentially, infinite. But this is absurd for the same reason that a library with infinitely many books, or a hotel with infinitely many rooms, is absurd. So the universe must have a beginning.
But even if the idea of an actual infinite were not absurd in general, Craig thinks that the idea of an actually infinite series of past events would still be absurd. His argument goes like this:
Now Craig thinks that if the past consisted in an infinite series of events, then the series of events that terminates in the present would have been formed by successive addition in this same sense. One event after another would have been "added" to the sum of those that have already passed by, up until the present event.
But Craig thinks this is absurd. Since the series stretches infinitely into the past, it seems that we have arrived at the "end" of an actually infinite series of events, each of which was successively added to the previous ones.
A lot depends on precisely what is meant by "forming a series by successive addition." If what is meant is that we must begin with the first member of the series and continue adding others until we reach the end, then of course it is not possible to form an infinite series by successive addition. But the hypothesis that the past consists in an infinite series of events does not imply that this is possible. If, on the other hand, what is meant is only that each member of the series succeeds the previous one, then this is an implication of the hypothesis Craig is trying to refute. Unfortunately, it's hard to see why an infinite series could not be formed by successive addition in this sense.
How, exactly, is this supposed to show that the series of past events could not have been formed by successive addition? Perhaps Craig means to be arguing along the following lines:
If this is Craig's argument, it isn't a very good argument. If the past is infinite, then prior to any given day, there are infinitely many days. But if this were so, there would be no reason at all why Jones could not be "finishing" an infinite count of days on every day. But of course the "count" that he finished yesterday is not the same as the "count" he finishes today. Today's "count" includes today, whereas yesterday's "count" does not. What would be absurd would be the idea that Jones had already finished today's count prior to today. But that is not an implication of the claim that there has been an infinite series of days prior to today.
What can we say about the nature of the first cause of the universe? The first thing to see is that, prior to creating the universe, the cause of the universe must be an eternal being. It cannot have begun to exist. Otherwise it would be just as much in need of a cause as the universe. So Craig thinks it is best to assume that the cause of the universe enters time only when it creates the universe. The first moment of time then coincides with the beginning of the universe.
But is there a divine person who decides to create the universe? Or is the cause of the universe only a "mechanical", non-personal, cause that is sufficient to produce it? Craig thinks it can't be the latter, since if it were, the universe would have to be just as eternal as its cause, contrary to what we have proved. Let me explain.
By a "mechanical" cause, Craig means a cause that automatically produces its result as soon as it comes into existence. For example, if water falls below a certain temperature, it automatically freezes. If the universe had this sort of cause, it would have to be just as eternal as its cause. Like its cause, it could not have begun to exist.
The only other possibility, Craig thinks, is that the universe was created by a person who freely chose to exercise its power to create. That way, we don't get the unwanted implication that the universe is beginningless, since a person does not automatically exercise its creative power.
A problem with Craig's defense of premise 1
There is one important difficulty that I want to mention briefly. If there is no time prior to the beginning of the universe, then Craig's main argument for saying that the universe must have a cause is considerably weakened. Recall that he thinks it would be absurd to suppose that the universe just popped into existence out of nothing. That's why he thinks that if the universe has a beginning, it must have a cause. But whether or not the universe has a cause, the universe doesn't pop into existence out of nothing. If there is no time prior to the beginning of the universe, then even if the universe has no cause, it is still not the case that once upon a time there was nothing, and then - pop! - the universe sprang into existence.