Many people, when they first learn about the cosmological argument in its basic form raise an immediate objection: Who caused God? While this is an entirely valid criticism, we need to understand this objection in more detail.
Let’s focus on Aquinas’ second way which argues that everything has a cause and since nothing can cause itself, there must be an uncaused first cause.
So, exactly how is the question What caused God? an objection to this?
This question suggests that there is a contradiction in the argument. Notice how at the beginning of the argument, Aquinas states that everything has a cause, but by the end of the argument, he claims that God is an uncaused first cause. Here we have what seems to be a contradiction.
The two statements ‘Everything has a cause’ and ‘God is the uncaused first cause’ are saying two contradictory things. If everything has a cause, then nothing can be uncaused. If God is uncaused, then we can’t claim that everything has a cause. Everything has a cause, or not everything has a cause. They can’t both be true. And in philosophy, contradictions are very bad. If you are trying to write a convincing argument, then using two contradictory ideas within that argument will severely weaken it.
So there seems to be a contradiction in the second way. But if we look at all three arguments together, we might seem to be another contradiction.
One of the key parts of the argument involves the rejection of the infinite regress, arguing that it is not possible to have an infinite number of causes or movers. But notice that in the third way, Aquinas argues that God is a necessary being, that is, God cannot not exist. Well, if God cannot not exist, if God must exist, there is no time in which God does not exist. This surely means that God is infinite, since God always exists and at no time can God not exist. So God exists at all times and must surely therefore be infinite.
Why can God be infinite but a sequence of causes can’t be infinite. Surely if the idea of God being infinite is possible, then so the idea of a series of causes being infinite must also be possible. Surely there is a contradiction in saying you can’t have an infinite series of causes but you can have an infinite God? Either you can have infinity or you can’t. Both ideas can’t be true.
And it is this that forms our first objection. When Aquinas rejects the possibility of infinite regress, thus forming the most important plank of his argument, the objection arising simply argues the opposite, that is, accepting the possibility of infinite regress. If we are allowing that God is infinite, thus accepting that infinity is a real possibility, then we should accept the possibility that there is an infinite regress.
William of Ockham gives us an important philosophical principle called Ockham’s Razor which says that causes should not be multiplied unnecessarily. In other words, when we have competing explanations, we should always go with the one that has the fewest causes, the simpler of the competing thesis.
Here we have two competing explanations for the existence of the universe. One says that there cannot be an infinite regress of causes, so there must be an uncaused first cause. The other says that there could be an infinite regress of causes.
Since the second of these theories does not require us to have an uncaused first cause, it is the simpler and therefore the more likely explanation. Rather than overcomplicating the answer by including God, this objection claims that the simpler, more likely explanation is that there is an infinite regressive sequence of causes.