Aquinas is often thought of as a rigourously logical and systematic thinker.  This is only half-true.  There is a good deal of vaguness, ambiguity, and illogical thinking in his book Summa Theologica, as far as I can see.

Here is a cautionary note from a philosopher who is an expert on Aquinas:

From the concept of God as ipsum esse subsistens, Thomas deduces certain other properties which must belong to God [i.e. in order to prove that “God”, in the ordinary sense of the word, exists].  The precise logical structure of the series of deductions undertaken by Thomas is very difficult to ascertain.  It is a very complicated structure for one thing; and although it resembles a series of proofs for theorems in a calculus, this comparison is probably not fair.  Thomas does nowhere systematically and exhaustively set out his equivalents of the definitions, axioms, and rules of inference of which he makes use.  The order in which he proves his “theorems” is no order of strict logical dependence. …Nor is it certain that he was absolutely clear in his own mind about the precise nature of his undertaking.  Thus, when we say that Thomas tries to deduce the other properties of God [i.e to prove that God exists] from the notion of ipsum esse subsistens, this must be taken as a kind of reconstruction of his intentions.  He nowhere says in so many words that this is what he is about to do.

(Knut Tranoy, “Thomas Aquinas”, in A Critical History of Western Philosophy;hereafter: CHOWP, p.111, emphasis added)

The first hint that Aquinas is something less than a rigorously logical thinker is his misuse of the word “God”.  Many people mistakenly think that Aquinas produced five arguments for the existence of God in the famous Five Ways passage from Summa Theologica (the Christian philosopher and apologist Peter Kreeft makes this mistake, for example).  That is probably because Aquinas claims to be proving the existence of God in that passage, but Aquinas is using the word “God” in an odd and non-standard way, and thus his arguments in the Five Ways are actually just a tiny piece of his long and complicated argument for the existence of God (in the ordinary sense of the word):

…it may appear that Aquinas is unjustified in describing the first efficient cause as God, at least if by “God” one has in mind a person possessing the characteristics Christian theologians and philosophers attribute to him (for example, omniscience, omnipotence, omnipresence, love, goodness, and so forth.).  Yet Aquinas does not attempt to show through the previous argument that the demonstrated cause has any of the qualities traditionally predicated of the divine essence.  He says:  “When the existence of a cause is demonstrated from an effect, this effect takes the place of the definition of the cause in proof of the cause’s existence” (ST Ia 2.2 ad 2).  In other words, the term God—at least as it appears in ST Ia 2.2—refers only to that which produces the observed effect.  In the case of the second way, God is synonymous with the first efficient cause;  it does not denote anything of theological substance.  

(Shawn Floyd in “Aquinas: Philosophical Theology“, section 2b, from the Internet Encyclopedia of Philosophy)

Aquinas fails to grasp this basic principle of philosophical reasoning:

Before we can try to prove anything at all we must, of course, have some idea of the nature or properties of the being whose existence we want to prove. (Knut Tranoy, CHOWP, p.110)

Aquinas works ass-backwards by first proving the existence of “God” and THEN proving that “God” has various divine attributes.  So, in order to understand his argument for the existence of God, we must constantly replace the word “God” in his arguments, with the appropriate metaphysical concept for that particular phase of his argument.  For example, in the Five Ways passage, the conclusion of the 2nd Way is NOT that “God” exists but that “a first efficient cause” (an FEC) exists.

My attempt to begin to reconstruct the “very difficult to ascertain” structure of Aquinas’ argument for the existence of God starts in the middle of Aquinas’ argument, when he infers the existence of a being that has PERFECT KNOWLEDGE from the existence of a being that is IMMATERIAL:

Since therefore God is in the highest degree of immateriality, as stated above (Q. VII, A. 1), it follows that He occupies the hightest place in knowldege.  (Summa Theologica, Part I, Question 14, Article 1)

When I take a look at the section where Aquinas claims to have previously shown that “God is in the highest degree of immateriality” it turns out that the section is arguing for God being INFINITE (Q 7, A 1), not for God being IMMATERIAL.  There is mention of materiality in the argument, but it is difficult to take an argument for God being infinite and to try to revise the argument to be about God being “in the highest degree of immateriality”.

So, this is another bit of confusion and unclarity from Aquinas.  I’m NOT impressed by this sloppy presentation of a supposed “proof”, where the reader is expected to take a proof given for one thing and reformulate it so that it works as a proof for something else.  Also, the argument about infinity is a rambling one and its logic is difficult to discern.  If the argument for God’s infinity was more clear, I might be able to figure out how to reshape it into an argument for God being “in the highest degree of immateriality”.  But the argument is not very clear, and I’m not going to spend lots of time (at this point) trying to make sense out of it.

In any case, the argument for God’s infinity rests upon the concept of an IES (ipsum esse subsistens) being:

Since therefore the divine being is not a being received in anything, but He is His own subsistent being as was shown above (Q. III, A. 4), it is clear that God Himself is infinite and perfect.  (Summa Theologica, Part I, Question 7, Article 1)

Presumably since the derivation of God’s infinity is based on the concept of an IES being, the derivation of the conclusion that “God is in the highest degree of immateriality” is also based on the concept of an IES being.  In that case, we can at least summarize the flow of Aquinas’ logic:

(MC6) There exists an IES being.

(CC2) If there exists an IES being, then there exists an IES being that is in the highest degree of immateriality.

Therefore:

(MC8) There exists an IES being that is in the highest degree of immateriality.

(CC3) If there exists an IES being that is in the highest degree of immateriality, then there exists an IES being that is in the highest degree of immateriality and that has perfect knowledge.

Therefore:

(MC9) There exists an IES being that is in the highest degree of immateriality and that has perfect knowledge.

Now we can work forward from the five metaphysical claims (that Aquinas argues for in the Five Ways passage) to get to the metaphysical claim of the existence of an IES being, (MC6), and thus complete the line of reasoning from the Five Ways to the existence of an IES being that has perfect knowledge.

The key passage about the existence of an IES being appears to be Question 3 , Article 4 (which Aquinas references in the above quotation).  In that passage, there are three different arguments to establish the existence of an IES being, which can be summarized in terms of three conditional claims:

(CC4) If there exists an FEC (first efficient cause) being, then there exists and FEC being that is also an IES being.

(CC5) If there exists an AP (actus purus, i.e. purely actual) being, then there exists an AP being that is also an IES being.

(CC6) If there exists a First being that is its own essence, then there exists a being that is its own essence and that is both a First Being and an IES being. 

The First Line of Reasoning Supporting (MC6)

The existence of an FEC being was argued for in the Five Ways passage, so that particular line of reasoning has been traced back to its starting point: the 2nd Way, which is an argument for this metaphysical claim.  We can combine that metaphysical claim with one of the above conditional claims to form a modus ponens:

(MC2)  There exists an FEC being.

(CC4) If there exists an FEC (first efficient cause) being, then there exists an FEC being that is also an IES being.

Therefore:

(MC10) There exists an FEC being that is also an IES being.

Therefore:

(MC6) There exists an IES being.

Here is a simple diagram showing the flow of this first line of reasoning:

FEC–>FEC & IES–>IES

The Second Line of Reasoning Supporting (MC6)

The second argument for the existence of an IES being is based on the following key metaphysical claim:

(MC7) There exists an AP being.

This metaphysical claim is argued for in Question 3:

Secondly, beause the first being must of necessity be in act, and in no way in potency. …Now it has already been proved that God is the First Being.  It is therefore impossible that in God there should be anything in potency.  (Summa Theologica, Question 3, Article 1, in the second argument)

The reasoning can again be put into the form of a modus ponens:

(MC11) There exists a First Being.

(CC7) If there exists a First Being, then there exists a First Being that is also an AP being.

Therefore:

(MC12) There exists a First Being that is also an AP being.

Therefore:

(MC7) There exists an AP being.

However, because Aquinas is somewhat careless in his reasoning, it is unclear what he means by a “First Being”.  This is a technical term, and Aquinas introduces it without providing a definition, and without providing any explanation or clarification of what this term means.

Since the passage I quoted that refers to “the First Being” occurs immediately after the famous Five Ways passage, this new technical term presumably refers to one of the following “first” beings discussed in the Five Ways passage:

(MC1) There exists a UFC being.  

(“a UFC being” = a being that is an unchanged first changer – often misleadingly translated “unmoved first mover” )

(MC2) There exists an FEC being.  

(“an FEC being” =  a being that is a first efficient cause)

(MC3) There exists an FN being.

(“an FN being” = a being that is a first necessary being, a being that is necessary but does not get its necessity from another being)

The 4th and 5th Ways do not speak of a being that is “first”, and unlike the other three Ways, they do not make use of the rejection of an infinite regress (to establish the existence of a being as the “first” in a chain of dependency), so the COP (cause of all perfections) being, and the IDN (intelligent designer of nature) being, do NOT seem to be good candiates for the referent of the expression “the First Being”.

My best guess is that when Aquinas speaks of “the First Being” in Question 3, he is referring back to the being that he tries to prove exists in his 2nd Way:  an FEC being.  The expression “the First Being” suggests the idea of “the First thing that exists”, and it is the 2nd Way that focuses in on the cause of the existence of beings in general.  The 1st Way is focused on the cause of changes, and the 3rd Way is focused on the cause of necessary beings, which is a special category of beings.  So, only the 2nd Way relates to the cause of the existence of beings in general.  I’m not certain of this interpretation of “the First Being”, but it seems to be the best of the three main alternatives.

Assuming my interpretation of “First Being” is correct, we can trace this line of reasoning back to the 2nd Way:

(MC2) There exists an FEC being.  

(CC8) If there exists an FEC being, then there exists an FEC being that is also an AP being.

Therefore:

(MC13) There exists an FEC being that is also an AP being.

Therefore:

(MC7) There exists an AP being.

(CC5) If there exists an AP (actus purus, i.e. purely actual) being, then there exists an AP being that is also an IES being.

Therefore:

(MC14) There exists an AP being that is also an IES being.

Therefore:

(MC6) There exists an IES being.

Here is a simple diagram showing the flow of this second line of reasoning:

FEC–>FEC & AP–>AP–>AP & IES–>IES

The Third Line of Reasoning Supporting (MC6)

The third line of reasoning involves another reference to the “First Being”:

(CC6) If there exists a First being that is its own essence, then there exists a being that is its own essence and that is both a First Being and an IES being. 

If we interpret the expression “First Being” as a reference to an FEC being, then this conditional claim (CC6) can be stated more clearly:

(CC9) If there exists an FEC being that is its own essence, then there exists a being that is its own essence and that is both an FEC being and an IES being. 

We have part of what is needed to show the truth of the antecedent of this conditional claim:

(MC2) There exists an FEC being.  

But it needs to be shown that such a being “is its own essence” in order for us to be able to affirm the truth of the antecedent of (CC9).  Aquinas argues for the existence of a being that “is its own essence” in Question 3, Article 3: Whether God Is the Same As His Essence or Nature?  The argument in that section is based on another metaphysical concept:

Since God then is not composed of matter and form, He must be His own Godhead, His own Life, and whatever else is thus predicated of Him.  (Summa Theologica, Part I, Question 3, Article 3)

This suggests the following conditional claim:

(CC10) If there exists a being that is not composed of matter and form, then there exists a being that is its own essence.

Since Aquinas needs to prove the existence of a being that is both an FEC being and that is its own essence, he needs to prove the following modified version of the above conditional claim:

(CC11) If there exists an FEC being that is not composed of matter and form, then there exists an FEC being that is its own essence.

Aquinas argues that God is “not composed of matter and form” in Question 3, Article 2: Whether God is Composed of Matter and Form?  He gives three different arguments to support this claim, and the third argument is based on the concept of an FEC being:

Now God is the first agent, since He is the first effecient cause as we have shown (Q. II, A. 3).  He is therefore of His essence a form, and not composed of matter and form.  (Summa Theologica, Part I, Question 3, Article 2).

So we can now trace this line of reasoning back to the 2nd Way, because Aquinas has argued for this conditional claim:

(CC12) If there exists an FEC being, then there exists an FEC being that is not composed of matter and form.

Add to this the metaphysical claim from the 2nd Way for a modus ponens:

(MC2) There exists an FEC being.

Therefore:

(MC15) There exists an FEC being that is not composed of matter and form.

(CC11) If there exists an FEC being that is not composed of matter and form, then there exists an FEC being that is its own essence.

Therefore:

(MC16) There exists an FEC being that is its own essence.

(CC9) If there exists an FEC being that is its own essence, then there exists a being that is its own essence and that is both an FEC being and an IES being. 

Therefore:

(MC17) There exists a being that is its own essence and that is both an FEC being and an IES being. 

Therefore:

(MC6) There exists an IES being.

Here is a simple diagram showing the flow of this third line of reasoning:

FEC–> FEC & not composed of matter and form–>FEC & is its own essence–>is its own essence & FEC & IES–>IES

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If we ignore the conditional claims and focus on just the metaphysical claims in the above arguments, we can depict the flow of Aquinas’ reasoning from the conclusion of the 2nd Way (MC2), to the conclusion that there is an IES being with PERFECT KNOWLEDGE (MC9).  Click on the image below for a clearer view of the diagram: