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A Posteriori Proofs of the Existence of God. (and objections)

Updated on May 10, 2012

St. Thomas Aquinas (born in 1225 to a family of Italian nobles (Halsall, 1996)) was a philosopher and theologian largely famous for his book, “Summa Theologica” in which he proposed five ways (Richards,2011) to prove the existence of god using a posteriori deduction. He believed that faith was not necessary to proving the existence of god and that it could be done philosophically through the use of reason. A posteriori refers to the reliance on experience to discover truth or knowledge (Berto, 2011). This contrasts to a priori logic which proposes that the only way to gain knowledge is through the use of reason. This is often called armchair philosophy because you need no experience to come to a priori truths. The only thing one requires is thought. Aquinas rejected the proposition that gods existence can be proven a priori as argued by theologians such as Anselm (Crane, 2004) due to the fact he believed that you must have experienced life as to be able to see gods effect in the world . He was one of the best known writers of the medieval scholasticism movement of the 1200s (Unknown, 2011). The fifth of the arguments included in “Summa Theologica” is well known as it gave birth to the Intelligent design movement prevalent in the bible belt of central America and the book is still perceived as some of the best philosophical arguments for the existence of a deity to date.

The “Five ways” proposed by Aquinas all share the same structure and form an argument. They all begin with a question . An example would be that the third way asks simply and very generally “Does god exist?”. The second section always begins with the phrase “It seems” followed by a short explanation of the arguments for and against the proposition (Berto, 2011). This proposes a point of view that Aquinas proceeds to attempt to refute. The argument against this statement takes the form of a theological quote, generally from the bible, which could be seen as a circular argument as the bible is only valid if god actually exists. After this comes Aquinas’ rebuttal of whatever initial argument is presented against the existence of god. This generally begins with “In reply” or “I answer that” (Berto, 2011) and comprises the majority of the actual argument. The way Aquinas proposes a point of view and then argues against it seems reminiscent of Aristotelian (or Platonic) writings which he was heavily influenced by (Berto, 2011).

He states that the first way seems to be the most obvious and is based on the concept of change and movement (Berto, 2011). He begins by pointing out that, “some things are in motion (Richards,2011)”. This is based on the a posteriori experience of motion in human perception. He believes this to be self evident. The argument continues with him proposing that all motion must have a previous cause and that it is illogical to assume something can cause itself to move. This is essentially an argument for cause and effect. From this he deduces that there must have been a stream of cause and effect going all the way back to the beginning of time (Richards,2011). An example would be that a pool ball would not be in motion unless another ball (that was already in motion) caused it too. This is in turn caused by your arms movement. Aquinas makes it clear that he believes the string of cause and effect must have an original cause and could not go back for infinity because without an initial cause nothing would be in motion (Richards, 2011). The conclusion of the argument is that there must have been a prime mover or “Prima Manifestior”. He believes the prime mover must be god although it is interesting to note that he proposes no proof that this would be the christian god (Richards, 2011), and Aristotle (the original architect of this argument) argued long before Aquinas that this Prima manifestior is a philosophers god and may not fit the descriptions of other gods (Berto, 2011). He sometimes refers to this entity as the unmoved mover as this beings itself, by definition, had no previous cause.

There are many issues with Aquinas’ first way of proving gods existence in Summa theologica. An obvious counterargument is that the proposed prime mover could be a physical or scientific phenomenon unrelated to god as yet undiscovered. It could have no connection to the divine whatsoever. It is also clear that the concept of the Prima Manifestior does not necessitate the christian god which is what is implied in “Summa Theologica” (Richards, 2011) as Aquinas himself was a christian. The only conclusion of the argument is that there must be an initial un-caused cause which started the process of cause and effect which continued until the present moment. This in no way necessitates a conscious being. The general definition of god, as well as the christian definition, requires that he is omnibenevalent (all good), Omnipotent (all powerful) and omniscient (all knowing). While it is possible to argue that Aquinas’ unmoved mover is all powerful in a sense there is no reason that this being would satisfy the other two requirements. As well as this it could theoretically be any number of gods theoretically including Allah or Zeus. Despite this the biggest problem with the argument put forward by Aquinas Is the circular logic inherent in the problem of infinite regress. One of the propositions used to prove gods existence is, “Nothing can be moved from a state of potentiality to actuality, except by something in a state of actuality (Richards,1999)”. The essential argument here is that nothing can be moved except by something that is already itself in motion. He then states that because this line of cause and effect cannot go back for infinity, “it is necessary to arrive at the first mover” who must himself, by definition, has no cause. The proposition that god exists and is an “unmoved mover (Berto, 2011)” is contradictory to the previous proposition which argues that there cannot be an action or any type of motion without a previous cause for it. This is circular because gods (the unmoved movers) existence relies on the proposition that there cannot be an unmoved mover. In other words the initial proposition necessitates that there cannot be an initial cause and that the stream of cause and effect must be in fact be infinite. His proposition that the series of cause and effect cannot go back for infinity is also questionable. The universe could theoretically be cyclic and have existed forever. It is also plausible that there is a rational naturalistic cause for the universes existence. The big bang theory is one such proposition. In this theory time itself didn’t exist until the cosmic event occurred. We know nowadays of more evidence and date about the workings of the universe than ever before and we still have little or no solid facts concerning the big bang event. Although these theories have not been categorically proven they do seem more plausible than a fully formed conscious and powerful being existing before the universe began. In terms of proving the initial cause is the christian god the burden of proof is certainly on the believer as there is certainly more evidence to support scientific explanations.

Although Aquinas does propose interesting philosophical questions in a very novel manner there are a series of problems with his arguments. Apart from the use of almost unanswerable philosophical questions (The question of what happened at the beginning of the universe) to prove his conclusion the circular logic used in his first proof destroys the validity of the argument and his conclusion that there must have been a prime mover does not seem to inherently prove gods existence as he assumes. As a result his conclusion seems flawed and provides little evidence for the christian gods existence.

Works Cited

Berto, F. "Metaphysics." PH2019. Aberdeen University, Aberdeen. 2011. Lecture.

Crane, Tim, and Katalin Farkas. Metaphysics: a Guide and Anthology. Oxford: Oxford UP, 2004. Print.

Halsall, Paul. "Internet History Sourcebooks Project." FORDHAM.EDU. Web. 01 Dec. 2011. <>.

Richards, Stephen A. "Thomas Aquinas' Five Ways (Part 1): Introduction, Motion, Causation | That Religious Studies Website." That Religious Studies Website. Web. 01 Dec. 2011. <>.

Unknown, Unknown. "First Cause Argument." QHST Home. Web. 01 Dec. 2011. <>.


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    • profile image

      John F. Feury 

      4 years ago

      Re : the seat belt, because its a common sense thing to do. Mankind is evolving to higher states of consciousness, it's the moulding of a species, experiences, lessons of life in millions of different ways. Nothing in this life is perfect, be it physical, mental, emotional etc, for we are evolving to perfection. When my time comes I will cross over to a higher vibration of life which is off the radar of most peoples five senses,until then I too will were a seat belt. peace and out.

    • profile image

      Sebastian Tyrrell 

      7 years ago

      Nice one Paddy (voted up). The prime mover argument seems circular: Aquinas asserted there must be a prime mover and then uses this assertion to "prove" itself.

    • Civil War Bob profile image

      Civil War Bob 

      7 years ago from Glenside, Pennsylvania

      Well written hub, Paddy...voted up, useful, interesting. I've often said that the only way to "prove" God exists is to send a person into the next life...everything on this side of the grave, be it Anselm or Aquinas (you might find Cornelius Van Til's presuppositional apologetics interesting...or read 1 Corinthians 2:14) is going to end up faith.

      My answer to the seatbelt question: Because it's the law and Romans 13:1 points out I should follow it...and I actually like the snug fit! Enjoy your here Hopping.


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