The Cosmological argument begins with the fact that the universe exists, and seeks to show that the best explanation of this fact is that it was created by God. There are many slightly different formulations of the argument - we will consider two of them.
The word 'universe' is used in two slightly different ways - it can mean the space-time that we live in, which began at the big bang (I will call this 'our universe'), or all space-time, matter and energy that has existed, which may be more than our universe (I will call this 'the universe'). It is possible that the universe consists of a series of universes leading up to our universe, or many universes existing in parallel; equally it is possible that 'our universe' is all there is and it and 'the universe' are one and the same.
God, if he exists, is not physical, and would be outside the universe, outside of space and time.
This is probably the most popular and easily understood version of the cosmological argument.
The argument is logically valid, so we need to look at the justification of the two premises.
Premise 1 seems obvious at first glance. Everything we experience and know in the universe has a cause (or many causes), and a thing that doesn't exist cannot cause itself to exist. Even in quantum physics, where quantum events can occur unpredictably, there are still antecedent conditions which cause the events. So it isn't much of an extrapolation to say that everything in the universe has a cause, and therefore the universe as a whole has a cause.
Premise 2 is a little more complicated. There are several ways we might look at it:
It seems therefore that the conclusion 3 is justified for the whole universe. (Certainly it is known to be scientifically true for our universe, but science cannot tell us if there was anything before the big bang.) Thus the conclusion that the universe had a cause also seems reasonable.
But what can we know of this cause? Does the argument get us very far?
If the conclusion of the Kalam argument is accepted as true, then some properties of the first cause can be gleaned from the argument. For example, the cause is outside the universe and therefore, by definition, outside of space and time; it must be powerful, beginningless and necessary.
This cause may even be seen to be personal, on the following grounds. We currently only know two types of explanation - scientific (in terms of physical processes and laws, e.g. the plate fell because of gravity) and personal (in terms of a person's motivations and action, e.g. the plate fell because I wasn't looking when I put it down). But the cause is not physical, spatial or temporal, so cannot be scientific, so it seems it must be personal.
The disagreements about the possibility of an infinite or causeless universe have meant this form of the cosmological argument is also popular.
Philosophers have identified two types of beings or things: necessary beings whose nature it is to exist (they couldn't not exist), and have no external cause, and contingent beings, whose existence is explained by factors outside themselves, and who could possibly have not existed, or they could have had different qualities.
Premise 2, sometimes called the Principle of Sufficient reason, tries to capture what we know about reality. Most things, perhaps everything we know about, are contingent and have an external cause which explains their existence. It is not a large extrapolation to conclude that everything has such an explanation. While this may not be certain, it surely is more likely , based on our experience of reality, than that things may have no such explanation.
At first sight, premise 4 seems hard to justify - why assume that God is the only possible explanation? There are two reasons to accept this premise:
These two versions of the cosmological argument have been contested on several counts - some objections apply to only one or other version, some apply to both.
1. Who made God? If God can be uncaused, why can't the universe?
This rather trivial riposte is surprisingly made by some intelligent critics. It is clearly stated in the argument that God is a necessary being which is eternal and uncaused. However it is clear that the universe is contingent and has a cause (unless we can show that it is the one contingent thing that doesn't have a cause.
2. How do we know everything has a cause?
This is a much more substantial counter-argument.
3. There is so much we don't know, especially about time and infinity, and this argument assumes we know the answers.
4. The concept of a timeless, necessary God is meaningless. If God is outside time, how can he make anything happen? It seems like God is defined in this way to just to satisfy this argument.
These concepts are difficult to grasp, but not meaningless, and less difficult than the alternatives, such as the universe occurring without a cause. We have plenty of experience of the universe, enough to know how it behaves, but we have less experience of God, so we shouldn't be surprised if he is beyond our understanding.
5. Causal explanations make a phenomenon more intelligible, but concluding God is the cause doesn't help us understand anything any better. If the universe is contingent, why did God create this particular universe and not another?
Perhaps it doesn't help us understand the mechanism by which the universe was created (i.e. a scientific explanation), but it certainly helps us understand why the universe exists (i.e. a personal explanation) - as a personal being, God chose to create this universe for reasons we can only partly guess.
6. Perhaps the universe caused itself.
This has been suggested, but is contrary to logic. Surely a thing must exist before it can do anything, include cause something?
7. Even if we have a first cause, why must it be God? A force might be a more believable option.
It is true that the cosmological argument, if successful, doesn't establish everything about the first cause that we'd like to know. But it surely is an enormous step if we can conclude that something outside the universe, something non-material and eternal, caused it to happen - we can then look for more evidence about the cause. Whether an immaterial force can be a necessary cause is arguable. Further, there is an argument that the first cause must be capable of making a choice (i.e. is personal) otherwise we have no explanation for the universe to appear when it did.
8. Perhaps there could be an everlasting contingent being - in this case, the universe - which simply exists without explanation.
But it seems this is contrary to the definition of a contingent being, and such a being could only exist if a necessary being caused it.
The most, perhaps only, substantial counter argument to the cosmological argument is the reference to the lack of cause in quantum physics. However, a quantum event requires a quantum vacuum and certain conditions before it can occur, and these are part of the universe. Therefore it remains true that they cannot explain the physical universe or be its cause.
The objections show that the cosmological argument cannot be considered to be a "proof" of God's existence. But the rather simple logic of the Kalam argument, in particular, and the speculative nature of the objections, together show that it is a powerful and persuasive argument. I can't help concluding that the objections don't really dent the basic logic of the argument.
Whether we accept that the argument shows that God probably exists as a first cause, or not, is likely to depend on the presuppositions we bring. If we will only accept a conclusion that can be absolutely proven in a scientific manner, we are less likely to be convinced, though we will probably accept that the objections are weaker than the argument. However if we are willing to choose what accords most with 'common sense', we are more likely to conclude that the argument is at least suggestive of God being the first cause.
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