Aquinas’ Second Way begins with the observation that:
‘In the world of sense we find there is an order of efficient causes.’
When Aquinas uses the term ‘efficient causes’ he is talking about the thing that made something happen. For example, the efficient cause of a statue is the sculptor, the person who made the statue.
Having observed that things are made by other things, that is, that effects have causes, Aquinas thinks about what this cause might be.
Just as in the first way, Aquinas considers whether something can be the cause of itself. He argues that nothing can be the cause of itself because it would have to exist before itself. A cause exists before an effect, so something can’t cause itself because it would have to exist prior to itself, which is impossible. Fir example, you could not have caused your own existence, because you would have had to exist before you were born, which is obviously impossible.
So, if things cannot use themselves, they must be caused by something else that exists prior to that thing. Again, think of yourself as an example. You were caused by your parents who obviously existed before you did.
Once again we have a regressive sequence, a sequence of causes that you can trace backwards.
But how far back can this sequence of causes go? Can it go back infinitely? Can this sequence go back ad infinitum?
Aquinas argues that this sequence cannot be infinite. He rejects the possibility of an infinite regress of causes and argues that there must have been a first cause. If there were no first case,then there would be no subsequent causes, no second, third of fourth causes.
However, as the argument clearly states at the beginning, there are causes. If there were no first case, there would still be no causes now. Since there are causes now, we should conclude that there is a first cause which is not caused by anything else.
Aquinas concludes by saying that this first uncaused cause is God.
P1 Every event has an efficient cause.
P2 Nothing can be the efficient cause of itself.
P3 Events are caused by things other than themselves.
P4 This regressive sequence of causes cannot continue ad infinitum.
C1 There is an uncased first cause.
C2 This is God.