Without the cause, the effect does not follow. Thus, if the first cause did not exist, neither would the middle and last causes in the sequence. If, however, there were an infinite regression of efficient causes, there would be no first efficient cause and, therefore, no middle causes or final effects, which is obviously not the case. Thus, it is necessary to posit some first efficient cause, which everyone calls “God”. (Aquinas).
Here we can see that Aquinas derives the principle of causality upon which his argument is based from his experience of the world around him. When Aquinas looks at the world, he realizes there is always an order of cause followed by effect, but no event is ever its own cause or something without a cause. He also rules out the explanation that there could be an infinite regress of causes because, when he looks at the world around him, there is always a first cause in the sequence. Therefore, there must be a first cause for the creation of the Universe, and that cause is God.
There are a couple of problems with Aquinas’s argument that are immediately apparent. The first is that he makes a false and unjustified assumption that it is impossible for the causal chain to regress to infinity. There is no reason to assume this, and it is perfectly plausible that the Universe has always existed. In fact, this account would work better with Aquinas’s principle that every cause has another cause prior to it because if there was a first cause that cause would have to have been caused itself, which according to Aquinas is impossible. The second is that, even if it is true that there was a first cause that does not necessarily entail another cause prior to it, the first cause was God. That cause could have been something else, such as the Big Bang.