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On the Two Directions of Aquinas’ Proofs for the Existence of God
Theo Manahan
For Aquinas, philosophy serves the aims of theology, without becoming, itself, theology.
Further, theology is supported by philosophy, without becoming philosophy. If we fail to treat
this interplay delicately Aquinas appears to commit one of two mistakes. Either he is a
disingenuous philosopher who forces a place for God into the beginning of his thinking because
he cannot find one at the end; or, he is a bad theologian, who reduces God’s transcendence into
merely an important moment within an explanation of the knowable universe. I will argue that
Aquinas commits neither of these mistakes. More specifically, I will argue that it is precisely at
the moment of the Proofs1 that Aquinas is at once philosophical and theological. He never
merges the two into a simple identity, nor does he place a gulf between them. Aquinas can
accomplish this because the Proofs must be read in two ways. Read philosophically, the Proofs
explain the existence of the common world of things, by discovering God as their foundation.
This foundation is expressed by a series of paradoxes, including: unmoved mover, efficient cause
that has no cause, being in act that is not brought to act, etc. As paradoxes, these names do not
contradict Aristotelian metaphysics, but they do imply an intelligible relation to a cause that the
framework is at a loss to express. By offering such a paradoxical answer, the moment that ends
the highest philosophical science simultaneously shows the beginning of theology, which will
investigate this answer as its question.
I: Setting the Problematic
As is well known, Aquinas’ Proofs for the existence of God solve a series of infinite
regress problems. It is tempting to state the argument like this: ‘If there is no God, then there is
1
For ease of reference, whenever the proof under discussion is one of Aquinas’ proofs for the existence of God, the
first letter will be capitalized.
2
an infinite regress, which is contradictory. Since there cannot be such a contradiction, there must
be God.’ Of course, this line of argument is flawed, since infinite regresses never lapse into
contradiction. We can just as validly accept the regress as deny it. It is then tempting to argue
that any believer who still claims to prove the existence of God by a regress argument can only
argue unconvincingly. In order to understand how Aquinas claims to prove that God must exist
and claims to do so using infinite regress arguments, we must re-understand the way that
Aquinas sets up the problem. This can be done by breaking question 2 of the Summa Theologiae
into its smaller questions: 1) Is the existence of God self-evident? 2) Is the existence of God
demonstrable? The first two questions will give us a foothold on Aquinas’ style of proceeding to
the existence of God. Once we have that foothold, we can re-approach the Proofs and directly
consider 3) Does God Exist?
II: Against Self-Evidence of God’s Existence
At the beginning of question 2, Aquinas argues that the existence of God is not selfevident. The short way to argue this point begins with what, for Aristotle, was a fact: “No one
can think the opposite of what is self-evident” (ST I, Q2, A1, sed contra). Next, we notice that
“it is possible to think the opposite of God’s existing” (ST I, Q2, A1, sed contra). Not only is it
possible to think the opposite of God’s existing, but atheists show that some people actually do
think the opposite. As we know, Aquinas will argue that they are not correct; however, Aquinas
will need to counter the atheists’ affirmation with a demonstration.
It seems that Aquinas could demonstrate God’s existence to atheists by simply offering
them a definition of the word God, and then arguing to God’s existence by way of that definition.
This sort of argument, known as propter quid argumentation (literally: on account of what),
proceeds from knowledge of the essence of the thing under question. The definition provides the
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ground from which we can come to know whatever there is to know about the defined thing.
Although it would be convenient to use this method in order to demonstrate God’s existence, no
human being could begin his or her argumentation with a definition of God. To summarize the
argument, which comes ten questions later, God’s essence cannot be known by us since our
mode of existence is of a lower mode than God’s (ST I, Q12, A4). Therefore, we cannot define
God, and we cannot prove his existence with a propter quid argument,
III: Demonstration Quia
We can, however, engage in another sort of argumentation, called demonstration quia,
which means ‘demonstration that.’ The goal of demonstration quia argumentation is to
demonstrate that a cause exists, by beginning, not with a definition of the cause, but with the
existence of effects which must have come from a certain kind of cause. To take a simple
example: Because we hear a meow, we can also understand that some existing cause must have
made the sound. Further, based on our knowledge of meows, we can attribute something to that
cause based entirely on what we know about meows. We cannot know that the cause of the
meow is a carnivore, but we can know that the cause of the meow has vocal chords.
In question 2, article 2, Aquinas expands our meow example. Instead of attempting to
find a cause proper to the meow, Aquinas is attempting to find the cause proper to the existence
of any effect, qua effect. From this beginning, Aquinas’ Proofs cannot prove anything about
God without proving something about things. Specifically, the Proofs show that effects can only
exist because only a certain kind of being, which we call God, suffices as their cause. In proving
the existence of God, then, Aquinas does utilize an infinite regress, but his concern is not about
absurdity in the order of logic, but absurdity in the order of existence: If God does not exist,
things do not exist either. Now, just as there is more to know about the cat that can be proven by
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considering him as cause of the meow, there is more to ask about God than can be asked simply
by considering him as cause of effects, but this takes us beyond our current point.
What is important to note is that Aquinas’ Proofs for the existence of God are not pious
suppositions added, gratuitously, to his scientific framework. Rather, his Proofs are a necessary
endpoint to the Aristotelian framework within which he is working. Without the Proofs, there
would not be an adequate explanation of the existence of moving, changing, and contingent
being(s). Therefore, it is evident that the Proofs serve two functions. They prove God’s
existence, and they prove how it is that things are able to exist.
IV: Two Ways to Use the Proofs: Metaphysically or Theologically
When we use the Proofs for the existence of God as Proofs for the cause of the existence
of things and of the properties underlying them, we do not use them as part of sacred theology,
but as part of metaphysics. To understand how the Proofs end metaphysics, we must understand
the Aristotelian relation between the subject of a science and the object of the science. Then, we
must apply the relation to metaphysics. Aquinas does both in his introduction to his commentary
on Aristotle’s Metaphysics, by explaining Aristotle’s three names for metaphysics: ‘first
philosophy,’ ‘metaphysics,’ and ‘theology.’ The three names correspond to three ways of
understanding ‘most intelligible beings’ (those beings which are treated in metaphysics.)
The first way of understanding the phrase ‘most intelligible beings’ comes from
understanding the phrase as meaning “that from which the intellect derives its certainty” (CM
PR2). In Aristotelian science, the intellect derives its certainty from the knowledge of causes.
The highest causes are also called the first causes, and the “science treating of first causes seems
to be the supreme” (CM PR), and this science can be called first philosophy.
2
CM PR stands for ‘Commentary on Aristotle’s Metaphysics, Prooemium. Quotations come from Appendix 2 of
Aquinas’ commentary on Boethius’ De Trinitate, translated by Armand Maurer.
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The second way that Aquinas understands most intelligible beings comes from
“comparing the intellect with the senses” (CM PR). By analogy, this is a comparison of the
apprehension of universals with the apprehension of particulars. The most intellectual science,
then, will be the one “that treats of the most universal principles. These are being and the
properties that accompany being as such, e.g., one and many, potency and act” (CM PR). These
properties cannot be apprehended in any particular thing, qua particular thing, nor can they be
apprehended solely by the senses. They can be apprehended by the intellect, however, in
everything that is, insofar as it is. The science is, in this sense, “called metaphysics because […]
these objects that go beyond physics are discovered by the process of analysis as the more
universal is discovered after the less universal” (CM PR).
Aquinas draws his third sense of the ‘most intelligible beings’ from comparing the
immaterial with the material. “Because anything has intellectual capacity owing to its freedom
from matter,” states Aquinas, “those things must be supremely intelligible that are most
disengaged from matter” (CM PR). Those things most separated from matter in thought and in
existence are “God and the Intelligences” (CM PR). Therefore, the science “is called divine
science or theology inasmuch as it treats of the substances referred to above” (CM PR).
Now we have three names for the science which treats of the most intelligible objects.
However, these three names are “not to be attributed to different sciences but to one” (CM PR).
The unity of the three names under one science follows from the Aristotelian notion of the
relation of subject of a science to its object. Succinctly stated: “[T]he subject in a science is that
whose causes and properties we investigate, but [it is] not the causes themselves of any genus
under inquiry. For the knowledge of the causes of any genus is the end attained by the inquiry of
the science” (CM PR).
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With the formula in hand, we can now apply it, specifically, to the science which treats of
the most intelligible objects. Of this science of the most intelligible beings, Aquinas writes that
“although this science is concerned with the three objects mentioned, nevertheless it does not
concern just any one of them as its subject, but only being-in-general” (CM PR).
‘Being-in-
general’ corresponds to the sense of most intelligible objects wherein what is studied is “being
and the properties that accompany being” (CM PR); therefore, the science which treats of the
most intelligible objects is properly called metaphysics.
Next we must turn our attention to the object of metaphysics. At this point the names
first philosophy and theology reappear, for the reason that “the above-mentioned separated
substances are the universal and primary causes of being” (CM PR). Since it:
belongs to the same science to investigate the proper causes of any genus [ie: subject] and
the genus itself […] So it must belong to the same science to investigate the separated
substances [God and the Intelligences] and being-in-general (ens commune), which is the
genus of which the above mentioned substances are common and universal causes. (CM
PR)
Therefore, metaphysics can properly be called, also, first philosophy and theology since, at the
end of its study, it treats of God/first cause as the cause of its proper subject. When we spoke of
the Proofs as proof for the properties necessary for the existence of things, we spoke of God’s
existence as something discovered as the end of that analysis. As the science progressed, it
progressed to God.
But there is also a second way of understanding the Proofs, neatly summarized in
question 5, article 4 of Aquinas’ commentary on Boethius’ On the Trinity. In addition to
studying those beings “only to the extent that their effects reveal them to us” (CdT3, Q5, A4) we
can also study them “insofar as they are beings in their own right” (CdT, Q5, A4). This second
way to understand the Proofs is part of a second theology which “investigates divine things for
3
‘CdT’ stands for ‘Commentary on Boethius’ De Trinitate.’
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their own sakes as the subject of the science” (CdT, Q5, A4) and not merely as its object and end.
Therefore, it is now our task, to understand how the question which ends metaphysics can come
to begin theology. To find our answer we turn, briefly, to the questions immediately following
the Proofs.
V: Transcendence, Closeness, and the Flash of Infinity
Question 2 of the Summa Theologiae tells us that God is. Questions 3-11 tell us what
God must not be, if God is to be the explanatory principle and first cause underlying that which
we 1) observe and 2) investigate as the subject of metaphysics. Specifically, God must be:
unbodied, without composition of form and matter (ie: not a composite being), identical in his
subject to his essence, identical in his essence to his existence, outside of any genus, without any
accidental properties, absolutely simple, and unable to enter into composition with other things.
Although each of these questions elaborates on the conclusion of the previous questions, all of
them are, essentially, saying the same thing: God is not like any other things; if he were, those
other things could not exist.
In answering the question of what God must not be, we have shown, then, what a strange
being God is. Question 1, article 2 of Aquinas’ commentary on Boethius’ On the Trinity allows
us to pause and consider the significance of God’s strangeness. It also illustrates the way in
which the recognition of God’s strangeness allows the Proofs to become a part of theology,
distinct from their role as the end of metaphysics. In this article, Aquinas asks “Can the Human
Mind Arrive at a Knowledge of God?” (CdT, Q1, A2). As we learned above, Aquinas holds that
we cannot know what God is, but, through demonstration quia, we can know that God is. As we
have just seen in our brief treatment of the questions after the Proofs, demonstration quia also
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allows us to extrapolate some understanding of what God cannot be. We can know all of this
because finite effects must rely, in the last analysis, on an infinite cause.
In the question currently under discussion, however, Aquinas discusses three degrees of
knowledge of God which can be had through his effects. These three degrees of knowledge
correspond to the three ways in which effects can relate to a cause which is disproportionate to
those effects. The three respects in which such a cause can relate to its effect are “[1] with
respect to the coming forth of the effect from the cause, [2] with respect to the effect acquiring a
likeness to its cause [and, 3] with respect to its falling short of perfectly acquiring it” (CdT, Q1,
A2).
Regarding the first relation of cause to effect, the human mind advances to a knowledge
of God “by knowing more perfectly his power in producing things” (CdT, Q1, A2). The Proofs
gave us a beginning of an understanding of God’s power in producing by showing that God must
be the unmoved mover and, by extension, the uncaused cause of all that is created. The second
relation of effects to causes allows us to expand on our glimpse into God’s power of producing
by showing him as “the cause of lofty effects which, because they bear some resemblance to
him, give more praise to his greatness” (CdT, Q1, A2). The most evident way to proceed in this
knowledge of God’s loftiness would be to investigate the relation of God to the Intelligences,
which we did not do; however, in noting the implications of God being an uncaused cause, we
also noted that God must be unlike creatures in every way. God’s unlikeness to creatures, then,
shows us how God would be greater than any of his effects, no matter how lofty that effect.
Knowledge of God’s transcendence over all of his effects brings us to the third way of
coming to know something of God, which proceeds “by an ever-growing knowledge of him as
distant from everything that appears in his effects” (CdT, Q1, A2). For example, the First Proof
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shows us that God must be an unmoved mover. By tracing out the implications of the First
Proof, we see that God must be an uncaused cause, a being that is pure act, and a being that is his
own existence (ST I, Q 3, A 4). Further, we said that all of these elaborations state, in different
ways, that God is not a creature. Finally, near the beginning of this paper, we said that God’s
unlikeness to any creature means that God is not the sort of thing which can be imagined by a
creature (ST I, Q 12, A 4). When someone realizes that such a being that is implied by those
simple demonstration quia Proofs is unlike the creatures which imply its existence, she makes
her “greatest advance in knowledge [because she] knows that [God] essence transcends
everything that can be known in the present life” (CdT, Q1, A2). Yet, the sort of transcendence
which the Proofs reveal is a very strange transcendence.
The Proofs, through their use of demonstration quia and its basis on the relation of
dependency between effect and cause, illustrate that this supremely transcendent God must, at
the same time, be supremely close to all things. If God were not close to things, the Proofs
would never have been able to prove God’s existence from beginning with the observed world.
Even stranger, the Proofs show that the transcendent God is not near the created order despite his
transcendence; rather, the reasons that we have for saying that God is transcendent, are the same
ones which allow him to be intimately near to things, since they are the reasons that God is able
to be the cause of creation.
In God’s nearness to things, not only does God become strange, things do as well. Prior
to the recognition of God’s nearness, God seemed transcendent, and things seemed utterly below
him, disconnected from him by an infinite distance. After the recognition of God’s nearness,
however, one realizes that God is not only now, but always has been, close; one realizes that
creation has always been attesting, through its act of existence, to an intelligible relation to a
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Being of beings which utterly transcends it. 4 The transcendence of God does not become entirely
clear until one realizes how close that God must be, and this, because he must also be
transcendent.
The human person’s full grasp of this transcendence is no moment of omniscience.
Rather, it is the opposite. To describe the moment, Aquinas borrows a phrase from Gregory the
Great: “When the eye of the soul turns to God, it recoils at the flash of infinity” (CdT, Q1, A2). 5
Before the flash, God served only as a cause; the Proofs of his existence provided only a neat
answer with which to finish out the metaphysician’s account of his subject; and, the transcendent
being shown in questions 3-11 gave merely a crisp extrapolation of what God must not be, more
‘word and logic’ than ‘infinite and existing.’ On the recoil, however, these tame words become
unsettled, alive, and new. The sober metaphysician is struck by a startling strangeness. At the
flash, under full impact of the realization of his strange transcendence, God becomes a question,
the question, for another, higher, science. Among the first few questions asked as part of this
new science is, once again, ‘Does God exist?’ The answer does not change, nor do the
arguments; however, the science, and the scientist, asks the question again, under a new light.
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Here, I am in agreement with Thomas A.F. Kelly, whose work I discovered after I came to this conclusion. He
makes the same point with reference to the created order as an icon for God, in his essay ‘Heidegger on Aquinas on
God: Gesamtausgabe 23, Erster Abschnitt’ found in the anthology, Thomas Aquinas: Teacher and Scholar: The
Aquinas Lectures at Maynooth, Volume 2: 2002-2010.
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There is much that could, and should, be said by way of an explicit psychological analysis of this ‘flash of
infinity,’ but this would embark us on a long discussion of Aquinas’ psychology of faith which, for a variety of
reasons, cannot be undertaken in the present paper.
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Works Cited
Aquinas, Thomas. The Division and Methods of the Sciences: Questions V and VI of his
Commentary on the De Trinitate of Boethius. Trans. Armand Maurer. 4th ed. Toronto:
Pontifical Institute of Medieval Studies, 1986. Print.
---. Faith, Reason and Theology: Questions I-IV of his Commentary on the De Trinitate of
Boethius. Trans. Armand Maurer. Toronto: Pontifical Institute of Medieval Studies, 1987.
Print.
---. Treatise on the Divine Nature: Summa Theologiae I 1-13. Trans. Brian Shanley, O.P.
Indianapolis: Hackett, 2006. Print.
Kelly, Thomas A.F. “Heidegger on Aquinas on God: Gesamtausgabe 23, Erster Abschnitt.”
Thomas Aquinas: Teacher and Scholar: The Aquinas Lectures at Maynooth, volume 2:
2002-2010. Ed. Michael W. Dunne, Julia Hynes, James McEvoy. Dublin: Four Courts
Press, 2012. 231-251. Print.