In order to prove that God exists, Aquinas must prove that there exists a being that has ALL of the following divine attributes:

• a person who is the creator of the universe
• an eternally bodiless person
• an eternally omnipotent person
• an eternally omniscient person
• an eternally perfectly morally good person

I don’t believe that Aquinas actually proves that there is a being with even just ONE of these key divine attributes, so I certainly don’t believe that Aquinas proves that there is a being that possesses ALL of these divine attributes.  I believe that Aquinas failed to prove that “God exists.”

But an argument for God need not PROVE that God exists.  An argument for God could simply provide evidence or support for the existence of God without proving that God exists.  So, if Aquinas had managed to prove only that there is a being who possesses one or two of the above divine attributes, that would fall short of proving that God exists, but it would still be a significant philosophical accomplishment because that would provide evidence or support for the claim that “God exists”.

My understanding of Aquinas’ book Summa Theologica is that in Part I of this book Aquinas attempts to prove that there exists a bodiless person who created the universe, who has full knowledge of everything that exists, who is perfectly loving and perfectly just, and who is eternal.  So, Aquinas does not exactly attempt to prove the existence of a being who has all of the divine attributes that I mention above, but the being that he attempts to prove is very similar to the sort of being that I think he needs to prove exists in order to prove that “God exists”.

So, if Aquinas was successful in his attempt to prove the existence of a bodiless person who created the universe, who has full knowledge of everything that exists, who is perfectly loving and perfectly just, and who is eternal, then Aquinas would have come close to proving that “God exists” and he certianly would have provided powerful evidence or support for the claim that “God exists.”

The traditional view of the Five Ways passage in Summa Theologica (Part I, Question 2, Article 3) is that Aquinas presents five proofs for the existence of God in that passage.  But this is a completely mistaken view of what Aquinas is doing in that passage.  There are actually ZERO proofs for the existence of God in that passage.

In the Five Ways passage Aquinas is attempting to prove the existence of five metaphysical beings:

1. an Unchanged Changer  (UC)
2. a First Efficient Cause  (FEC)
3. a First Necessary Being (FNB)
4. a Most Perfect Being (MPB)
5. an Intelligent Designer of Nature (IDN)

Proving the existence of one of these beings is NOT the same as proving the existence of God.  In order to use these metaphysical conclusions to prove the existence of God, one must add another premise to each of the Five Ways:

• IF an Unchanged Changer exists, THEN God exists.
• IF a First Efficient Cause exists, THEN God exists.
• IF a First Necessary Being exists, THEN God exists.
• IF a Most Perfect Being exists, THEN God exists.
• IF an Intelligent Designer of Nature exists, THEN God exists.

But Aquinas makes no effort to prove or support any of these additional claims, at least not in the Five Ways passage.  Therefore, there are ZERO proofs of the existence of God in the Five Ways passage in Summa Theologica.

Furthermore, NONE of the five beings for which Aquinas argues in the Five Ways passage is defined as possessing even ONE of the divine attributes.  None of these beings is (defined as) an eternally bodiless person, an eternally omnipotent person, an eternally omniscient person, or an eternally perfectly morally good person.

Proving the existence of an Intelligent Designer of Nature comes close to proving the existence of a person who is the creator of the universe, but further argumenation would be required to show that the existence of an Intelligent Designer logically implies the existence of a person who created the universe.

So, the existence of the five metaphysical beings would NOT immediately entail the existence of a being who possesses one of the primary divine attributes.  The Five Ways not only fall short of proving the existence of God, they also fall short of proving the existence of a being who possesses even just one of the main divine attributes.  Thus, the Five Ways are complete and utter failures as proofs of the existence of God.

However, I think it is a great mistake to view the Five Ways passage as containing five attempts to prove the existence of God.  This is unfair to Aquinas, an insult to one one of the greatest defenders of theism in the history of philosophy.  Aquinas uses the existence of the five different kinds of metaphysical beings as the foundation of an extensive and complex case for the existence of God, a case which extends for over one hundred pages (not just the Five Ways passage, which occupies less than two pages in the English translation that I’m using).

My view is that there is only ONE proof for the existence of God in Summa Theologica, and that proof relies heavily on the Second Way.  The complex proof for the existence of God presented by Aquinas requires the assumption that a First Efficient Cause (FEC) exists, that there is a being that is an FEC.  Without this assumption, Aquinas’ complex proof for the existence of God fails.

There might be one or two other metaphysical beings from the Five Ways that Aquinas’ proof also requires to be successful, but I have not identified such a logical dependency yet.   Since the success of the 2nd Way is essential to the success of Aquinas’ complex proof of the existence of God, there are NOT five different ways of proving the existence of God in the Summa Theologica; there is just ONE attempted proof of the existence of God.

In Part I, Question 14, Article 1, Aquinas argues that “there is knowledge in God”.  This would be better translated as “there is knowledge in the first principle” or “there is knowledge in the god of Aristotle”.  This is a very important claim in Aquinas’ attempted proof of the existence of God.  It is the first step towards establishing that there is a being who has full knowledge of everything that exists (which comes close to the divine attribute of omniscience), and it is also an important step towards establishing that this being, this god of Aristotle, has will and is perfectly loving (which comes close to the divine attribute of a person who is perfectly morally good).  So, if Aquinas fails to prove this point about there being knowledge in the first principle, then his proof for the existence of God fails.  Aquinas gives only ONE argument for this claim and that argument, so far as I can tell, requires the assumption that there exists a First Efficient Cause (the conclusion of the 2nd Way).

Aquinas argues that “there is knowledge in God” (better: “there is knowledge in the first principle”) on the grounds that “God is in the highest degree of immateriality, as stated above (Q. VII, A. I)…”  (better: “the first principle is in the highest degree of immateriality…”).  If we then take a look at the argument for that premise, we find it is based on the assumption that “the divine being…is His own subsistent being as was shown above (Q. III, A. 4)…”  If we then take a look at the arguments (three of them) for that assumption, they are based on the existence of three kinds of beings (respectively):

1. First Efficient Cause
2. A being that is pure act
3. The First Being

The argument for the existence of a being that is pure act is found in Question 3, Article 1:

…the first being must of necessity be in act, and in no way in potency.

Therefore, both the second and third kinds of being mentioned above refer to the existence of  “The First Being”.  This is an ambiguous technical expression used by Thomas to refer to some being that is discussed in the Five Ways passage.  But there are three different “first” beings discussed in the Five Ways:  [the first] Unchanged Changer,  the First Efficient Cause, and the First Necessary Being.  Based on various details in the text, I have concluded that the expression “The First Being” refers to the “First Efficient Cause”.  Thus, all three arguments for the claim that “the divine being…is His own subsistent being…” require the assumption that there exists a First Efficient Cause.  Therefore, the key claim by Aquinas that “there is knowledge in God” (better: “there is knowledge in the first principle”) requires the assumption that there exists a First Efficient Cause.  Aquinas’ attempt at a complex proof of the existence of God rests upon the success or failure of the 2nd Way of the Five Ways.

Why have so many people wrongly believed that the Five Ways passage contains five proofs for the existence of God?  The main problem is that the word “God” is a mistaken or very misleading translation of the Latin text of the Five Ways passage.  Here are some key points to keep in mind:

• Aquinas wrote in Latin, not in English.  
• Aquinas was heavily influenced by Aristotle.
• Aristotle wrote in Greek before the English language existed, and before Christianity existed.
• The English language has been heavily influenced by Christianity.

The word “God” is a word in the English language, and it’s meaning is based in Christian thought, because the English language developed in countries that were predominantly Christian.  The word “God” is a proper noun, a name, not a category.  But since we cannot see “God” or meet “God”, the meaning of this name is based on a description of this (alleged) person in terms of various divine attributes (see the list of key divine attributes at the beginning of this post).

But when Aquinas used the Latin word that is translated as “God” in the Five Ways passage, it is clear that Aquinas was referring to the god of Aristotle, who spoke Greek, and who was not a Christian (because Christianity did not yet exist).  The god of Aristotle is NOT the same as the God of the Christian religion, and was NOT the same as the being referred to by the ordinary use of the word “God” in the English language.  Aristotle’s god was something like a “first principle”, something like a First Efficient Cause.  The word “God” in the English language does NOT mean “first principle” or “First Efficient Cause”.  But that is what Aristotle and Aquinas have in mind in their arguments that are presented as “arguments for the existence of God”.

Thus, people take the word “God” in English translations of Summa Theologica to mean what this word means in the ordinary use of the word, but that is clearly a mistake.  The Latin word that is translated as “God” in English, does NOT mean “God” in the ordinary sense of the word.  In the Five Ways passage (at least), it only means something like a “first principle”; it only refers to the god of Aristotle, which is different than the God of the Christian religion.

Aquinas first attempts to prove the existence of the god of Aristotle in the Five Ways passage, and then on the basis of that assumption he goes on to attempt to prove the existence of the God of the Christian faith in a complex proof that extends over one hundred pages in Part I of Summa Theologica.