St. Thomas Aquinas, a Catholic priest ordained a saint and held today as one of the developers of modern philosophy, argued that sequential logic proves the existence of God. His argument, sometimes referred to the Cosmological Argument, is an argument based on the principles of causality. If events occur in the natural world, and all events have a cause which precedes their effect, then there must be something outside of nature to initiate a first cause. This thing is God. It can be noted that this line of reasoning is very similar to Aristotle’s prime mover.
The first assumption is a reasonable one. Events do in fact occur in the natural world, and the proof of this is demonstrated easily enough in our day to day lives. The Sun rises, causing daytime and photosynthesis in plants. We then eat the plants for sustenance, which then allows us to procreate and send another generation deeper into the passage of time.
The next assumption follows from this. In the natural world, all events have an antecedent. As mentioned above, in order for plants to grow and provide us nourishment, there must be a cause for this. That cause would be the rising of the Sun, which will in turn cause photosynthesis, which will cause growth in the plant, which will cause it to look appealing for consumption. We understand this chain of cause and effect quite clearly. Time is linear, and only moves forward. There is no ambiguity in this idea. It is clear that if one fails to cause an event one cannot repeat that instance of time over again. As one only moves forward in time, one must repeat the attempt to cause the aforementioned event.
A following assumption, is that there are no infinite chains of cause and effect. The reasoning behind this is that if we live in a universe of cause and effect, with time moving forward, then there cannot be a chain of infinite events because the list of causes and effects would go on forever. This shows a paradox. If a cause must precede every effect, we must be able to find a first cause ( cause that sets this chain of events in motion). If the chain is infinite, then by definition of infinity, there is no first cause, and hence nothing that sets the principle of causality in motion.
Here in the argument, assumptions begin to turn to conclusions. Aquinas then argues that there must be a supernatural being who created the natural world. It is interesting to note that Aquinas assumes this being must be sentient. Of course, in his time there was no knowledge of the Big Bang Theory, but there is no reason to assume that this cause, which resides outside our natural world, must be a deity or something capable of thought. Even without knowledge of Darwinian Evolution, one could posit that thought came later in the chain of events and was not present at the initiation of the chain.
Aquinas’s final conclusion is that this first cause, or supernatural being, is God. This God is described as omnipotent, omniscient, omnibenevolent, and is also the Christian God. Considering the assumptions, there is no reason to reach this conclusion. A non-Christian could parallel this line of reasoning, and conclude that it is their preferred deity who exists.
Despite the Christian bias, none of the characteristics of this God are necessary to conclude from the premises. Why could the natural world have not been set in place by a being with limited power? Omnipotence is not necessary to create something, why suppose it is necessary to create a universe? The same could be said for omniscience and omnibenevolence. Although it is an uncomfortable idea, from the aforementioned assumptions, it could be concluded that an omnimalevolent being created the universe. Perhaps it was Satan who created the natural world, and that is why loving the things of this world is considered sinful.
Despite its biased conclusion, the argument demonstrates critical thinking quite well. Aristotle used this line of thought to posit that there must have been a first cause. Aquinas recognized the validity in such reasoning, and in a move some could consider lacking intellectual integrity, used the same logic to prove the existence of a specific God that he already believed in. It could also be noted that these assumptions could lead one to conclude that we live in a deterministic universe (due to the chain of cause and effect, all events are predetermined), one that lacks free will, and then by definition, removes the Christian God from the picture.