Thomas Aquinas And Giles Of Rome On The Existence Of God As Self-Evident (Gossiaux), E-book, do posegregowania
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American Catholic Philosophical Quarterly
Volume 77
Number 1, Winter 2003
Articles
Mark D. Gossiaux: Thomas Aquinas and Giles of Rome on the Existence of God as Self-Evident
Page Break 59
Sententiis
, bk. I, d. 3, q. 1 (1252–54), where he is concerned to show how one arrives at a
knowledge of God by means of his vestiges in creatures.
†5
In a. 1 he affirms that God can be
known by a created intellect, though not in such a way that his essence might be
comprehended. In a. 2 he asks whether God’s existence is self-evident. The opening
objections try to establish an affirmative answer. One argument, drawn from John
Damascene, asserts that God’s existence is self-evident because the knowledge of his
existence is naturally implanted in all men. Another argument reasons that his existence is
self-evident because he cannot be thought not to be. Two arguments are cited to support this
claim. The first comes from Anselm: God is that than which a greater cannot be thought; but
that which cannot be thought not to be is greater than that which can be thought not to be;
therefore, God cannot be thought not to be. A second argument is attributed by Thomas to
Avicenna: no thing can be thought without its quiddity (for example, we cannot think “man”
without thinking “mortal,” “rational,” “animal”); but God’s quiddity is his
esse
; thus he cannot
be thought not to be. The two contrary arguments, which attempt to show that God’s
existence is not self-evident, both make appeals to Aristotle. According to the first, what is
self-evident may be denied with one’s mouth, but never in one’s heart; however, as one
reads in Ps. 14:1, the fool has said in his heart that there is no God. According to the second
argument, the conclusion of a demonstration is not self-evident. Yet the philosophers have
demonstrated the existence of God.
Page 59
In his reply Thomas tells us that one can speak of the knowledge of something in two
ways: either in itself, or with regard to us. The existence of God is self-evident in itself;
however, with regard to us a further distinction is needed. One can speak of God according to
his likeness and participation, that is to say, according to his likeness and participation in
creatures, and then one may say that his existence is self-evident. For a thing is known by its
truth, which has its exemplar in God, and the existence of truth is self-evident. But we can
also speak of God as he subsists as something incorporeal, and then the existence of God is
not self-evident. The reason for this, Thomas explains, is that those things which are
self-evident to us are known immediately by means of the senses. Thus when one sees a
whole and a part, one immediately knows that every whole is greater than its part. However,
from the perception of sensible things we
Page Break 60
arrive at God only by a process of reasoning. Consequently, his existence is not self-evident
to us.
†6
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Since God is an immaterial being, we do not possess an immediate knowledge of his
existence. Replying to the objections, Thomas denies that the knowledge of God as he exists
in his own nature is naturally implanted in us.
†7
Nor is it the case that we cannot think that
God does not exist. According to Thomas, one must construe Anselm as asserting that after
one acquires a knowledge of God, one cannot understand that he exists and be able to think
him not to be. Thomas points out, however, that it does not follow from this that one could
not think that there is no God. For one could think that there is no such thing as that than
which nothing greater can be conceived. Anselm’s argument is ineffective as a proof for God’s
existence, since it assumes at the outset that there exists a being than which a greater
cannot be thought.
†8
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In
De veritate
q. 10, a. 12 (1257–58), Thomas again asks whether God’s existence is
self-evident. His analysis in this text is considerably more sophisticated than in the
Sentences
commentary. He begins his reply by delineating three possible positions. The first, which he
attributes to Maimonides, asserts that God’s existence is neither self-evident nor
demonstrable, but is held on faith alone. The second position, ascribed to Avicenna, maintains
Mark D. Gossiaux: Thomas Aquinas and Giles of Rome on the Existence of God as
Self-Evident
Page 57
Abstract
. Thomas Aquinas holds that the existence of God is self-evident in
itself (because God’s essence is his existence) but not to us (since we do not
know the divine essence). Giles of Rome agrees with the first part of
Thomas’s claim, but he parts company with Aquinas by maintaining that
God’s existence is self-evident to the wise. Since the wise can know that God
is his existence, they cannot think of him as not existing. This paper
reexamines Thomas’s teaching in the light of Giles’s criticisms. By examining
closely what is involved in the claim that God’s essence is his existence, and
how one’s knowledge of this claim is related to the knowledge that God
exists, it argues that Thomas’s position has the resources to withstand
Giles’s objections.
Page 57
Thomas Aquinas is well known for his efforts to demonstrate the existence of God.
†1
His
arguments would not be necessary, however, if the existence of God were self-evident (
per se
notum
), since a self-evident proposition is one that is known to be true as soon as its terms
are understood.
†2
Consequently, in order to establish the need (and the possibility) of proving
God’s existence, Thomas frequently takes pains to show that his existence is not self-evident
to us. Aquinas is also known for his claim that God’s essence is his existence, a view that I
Page Break 58
shall call the “identity thesis.”
†3
It is because of his acceptance of the identity thesis that
Thomas adds an important nuance to his teaching on our knowledge of God’s existence: that
God exists is self-evident
in itself
, but
not to us
. One might wonder whether Thomas is
justified in making this claim. Since we can know that God’s essence is his existence, and
hence that God’s existence is self-evident in itself, are we not also able to affirm that God’s
existence is self-evident even with respect to us? This line of criticism was directed at Thomas
near the end of his career by Giles of Rome. Like Thomas, Giles accepts the identity thesis, as
well as the claim that in all creatures there is a real distinction between essence and
existence. However, Giles parts company with Thomas by holding that God’s existence is
self-evident
to the learned
.
†4
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In this paper I wish to reexamine Thomas teaching on this issue in the light of Giles’s
criticisms. In particular, I shall consider Thomas’s views on the relationship between our
knowledge of the identity thesis and our knowledge of God’s existence. To establish a better
context for this examination, it will be useful to begin with a review of Thomas’s texts on the
self-evidential character of God’s existence.
I.
Page 58
Thomas Aquinas: God’s Existence is Self-Evident in itself, though not to us
. Thomas’s first
discussion of this theme is found in his
Scriptum super
that God’s existence is not self-evident, but is known by demonstration. The thirdposition,
attributed to Anselm, holds that God’s existence
exists in the mind.
†14
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Another argument to support the claim that God’s existence is self-evident is based upon
the notion that what cannot be thought not to be is of itself self-evident. But, following the
reasoning of
Proslogion
chap. 3, God cannot be thought not to be. To this Thomas replies that
God could be thought not to be, not on account of any imperfection of his being but from the
weakness of our intellect, which cannot grasp him in himself but only by way of his effects.
†15
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Still another argument is based upon the claim that those propositions are most evident
in which the same thing is said of itself (for example, man is man), or whose predicates are
contained within the subject terms (for example, man is an animal). But God’s essence is his
existence; therefore, when one says “God exists,” the predicate is either the same as the
subject, or at least is included within it. In response, Thomas concedes that God’s essence is
his existence, and that for those who see his essence his existence is therefore self-evident in
the greatest degree. However, because we do not see God’s essence, we arrive at a
knowledge of his existence only by way of his effects.
†16
Page 63
Finally, in
Summa theologiae
I, q. 2, a. 1 (1265–66), one finds a clear and very concise
presentation of Thomas’s teaching. Once again a distinction is drawn between two types of
self-evident propositions, and again the
Page Break 64
De hebdomadibus
is cited as an authority for this. Thomas argues that the proposition “God
exists” is in itself self-evident, because the predicate is identical with its subject, for God is his
own
esse
. However, because we do not know what God is, this proposition is not self-evident
to us, but requires demonstration.
Page 64
In all of these texts Thomas holds that the proposition “God exists,” viewed simply in
itself, is self-evident. However, this tells only part of the story. In order to determine whether
a proposition is self-evident, it is not sufficient to examine only its objective condition
(namely, the relation between subject and predicate); one must also take into account its
subjective condition, that is to say, whether the individual understands the meaning of its
terms. Since in the present life we do not have a quidditative knowledge of God, the existence
of God is not self-evident to us. It is possible, therefore, for someone to think that God does
not exist.
Page 64
Thomas is untroubled by the fact that someone could affirm that God’s existence is
self-evident in itself even though he does not know what God is. We could establish that God
is the subsistent act of existence (
ipsum esse subsistens
), and still it is not self-evident to us
that God exists. On these points Thomas will be challenged by Giles of Rome.
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is self-evident, such that no one may think internally that God does not exist, although one
might assert this externally. Of these three positions, Thomas rejects the first as false, since
there are irrefutable arguments to prove that God exists. Each of the other two positions,
however, is true in a certain respect. To explain this Thomas introduces a distinction between
two ways in which something may be self-evident, namely, in itself and to us. He then notes
that God’s existence is self-evident in itself though not to us; therefore, in order to know his
existence we require a demonstration drawn from his effects.
†9
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Thomas supports his position by identifying the conditions required for a self-evident
proposition. A proposition is self-evident in itself when the predicate is included in the
definition (
ratio
) of its subject, for then the subject cannot be thought without the predicate
appearing in it. However, in order for it to be self-evident to us, it is necessary that the
definition of the subject be known to us. Some propositions are self-evident to all: this occurs
when the subject-terms are known to everyone; others are self-evident only to wise people,
who alone know the meanings of these terms.
†10
Thomas finds support for this position in
Boethius’s
De hebdomadibus
, where a distinction is drawn between two types of “common
conceptions of the mind.” One is common to all (for example, if equals are subtracted from
equals the remainders are equal), while the other is understood only by the learned (for
example, incorporeal things do not exist in place).
†11
Thomas
Page Break 62
goes on to note that since existence (
esse
) is not included in the essence of a creature, one
cannot say that the existence of a creature is in itself self-evident. However, existence is
included within the quiddity of God, since God is his very existence. Therefore, the existence
of God is in itself self-evident. But since the quiddity of God is not known to us, it is not
self-evident to us that God exists; rather, this requires demonstration. Thomas adds that in
heaven, where we shall see God’s essence, his existence will be much more self-evident to us
than the principle of non-contradiction is at present.
†12
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Thomas revisits this topic in
Summa contra gentiles
I, chaps. 10–11 (1259–64). There he
considers and rejects a series of arguments to support the claim according to which the
existence of God is self-evident. He also makes some interesting remarks concerning the
origin of this opinion. It arises partly from the fact that we have been accustomed from our
earliest days to hear about and call upon God. Such a long-standing habit makes us think
that his existence is practically self-evident. This opinion also arises from a failure to
distinguish between what is absolutely self-evident and what is self-evident with regard to us.
That God exists is self-evident absolutely, since God is his existence. However, because our
minds cannot conceive what God is, his existence is unknown to us.
†13
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Among the arguments to support the self-evident character of God’s existence, one is
based on the notion that a self-evident proposition is known immediately when its terms are
known. But since God is understood to be that than which a greater cannot be conceived, one
immediately grasps that he exists. To this Thomas replies that it is not known to all, not even
to those who concede that God exists, that the Anselmian description of God is a proper one.
Even if one were to agree that this is what the term “God”
II.
Page 64
Giles of Rome: God’s Existence is Self-Evident even with regard to us
. Giles of Rome
studied under Thomas Aquinas during the latter’s second Parisian regency. On many points
his personal thought shows the influence of Aquinas, though he is by no means a Thomist.
Giles’s academic career was interrupted by the stormy events of March 1277. Under the
direction of Stephen Tempier, the commission of theologians who had assembled the syllabus
of 219 articles condemned on March 7 turned their attention to Giles’s
Commentarius in
Primum Sententiarum
(1271–73). They identified fifty-one propositions and asked for a
retraction. Rather than do this, Giles attempted a formal defense of his views, and he was
subsequently “exiled” from the University. His readmission to the Faculty of Theology
occurred only upon the intervention of Pope Honorius IV in 1285.
†17
Page Break 63
signifies, it is not necessary that there should be something in reality that corresponds to this
description. For the thing and the meaning of the name must be posited in the same manner.
From the fact that one understands the meaning of the term “God,” it follows only that God
Page Break 65
Page 65
In his
Commentarius in Primum Sententiarum
Giles raises three questions concerning our
knowledge of God that are pertinent to our topic. The first concerns whether we can know
God in this life.
†18
Drawing a distinction between knowing that something is and knowing
what it is, Giles adopts a position not unlike the one defended by Aquinas: in the present life
we cannot know what God is, although we can know that he is.
Page 65
In support of his claim that we cannot know the
quod quid est
of God, Giles reasons that
our knowledge of simple things is analogous to our knowledge of composites. The latter may
be known in three ways: (1) immediately, in the manner of first principles; (2) by a process
of discovery, for example, when from one proposition we deduce another; and (3) by means
of teaching. However, in none of these three ways can we know the quiddity of God. We
cannot know it immediately, since in this life our knowledge is never free from phantasms.
Thus we cannot have quidditative knowledge of a thing that lacks phantasms or that is not
proportionate to something having phantasms. Yet God neither has phantasms nor is
proportionate to something having phantasms.
†19
For the same reason, Giles rejects the
claim that we can arrive at a knowledge of God’s quiddity by a process of discovery: creatures
cannot lead us to knowledge of God’s essence.
†20
Nor is it possible for someone to teach us
the nature of God, since we can be taught only those things that we can attain by the light of
our mind; however, in the present life our limited understanding cannot grasp the quiddity of
God.
†21
Gilesisusing, althoughhiscitationof the
De hebdomadibus
suggests that he was awareof
Thomas’s other texts on this topic. Be this as it may, Giles is quick to reject Thomas’s
position:
But this position is not good. For since in this life we can see that God is his
esse
, and that his
esse
is included in his quiddity, in this life we can know
that no one conceives of God unless he thinks of him as existing. For in all
things other than God
esse
is outside the definition of their quiddity, and so
they can be understood without understanding them to exist. But only God
has a quiddity not distinct from his
esse
; he alone cannot be thought not to
exist. Moreover, because what is self-evident cannot be thought not to be, it
is necessary that we say that God’s existence is in some way self-evident
even in regard to us.
†24
Page 67
Since we are able to know that God’s essence is identical with his existence, we should
say not only that the existence of God is self-evident in itself, but also that it is self-evident
to
us
. Granted that God’s existence may not be self-evident to everyone, it is at least
self-evident to those who have a correct metaphysical understanding of the way in which God
possesses being. Giles does not see this view as entailing that we have a quidditative
knowledge of God. For one should distinguish between the knowledge of what a thing is (
quid
rei
), and the knowledge of the meaning of a name (
quid nominis
). Although we cannot know
what God is, we can know what the term “God” means. Thus when we understand “God” to
mean
ipsum esse
, and “something than which a greater cannot be thought,” we immediately
understand him to exist. Therefore it is necessary to concede that God’s existence is
self-evident even with regard to us.
†25
Page 67
To support his position Giles makes an interesting observation, which again seems aimed
at Aquinas. That incorporeal beings do not exist in place
Page Break 68
is said to be self-evident to the wise, yet even the wise cannot see in a positive manner the
quiddities of incorporeal things, though in some manner they can know what is meant by this
name. It would therefore seem inconsistent to hold (as Aquinas does) that this statement is
self-evident to the wise, while God’s existence is not, since we have quidditative knowledge
neither of angels nor of God.
†26
Page 68
In the final part of his reply Giles identifies three characteristics of self-evident
propositions, all of which he finds present in the proposition “God exists.” The first is that the
predicate belongs to the intelligible content (
ratio
) of the subject. The second is that such
propositions cannot be
directly
denied. Those who deny God’s existence do so only indirectly,
because they do not grasp what is meant by the term “God.” The third characteristic of
self-evident propositions is that they are proven immediately upon being heard. Again,
whoever understands what is meant by the term “God” immediately understands that God
exists.
Page 68
If one concedes to Giles that the existence of God is self-evident even with respect to us,
one might wonder whether it can also be the object of a demonstration. Can we have “proofs”
for the existence of God? Giles addresses this question in the next article.
†27
The text begins
with a series of objections to show that God’s existence cannot be demonstrated. One
argument states that the proposition “God exists” could not be the conclusion of a
demonstration, since a conclusion consists of a subject and a property (
passio
); however,
there are no accidents in God. Other arguments are grounded on the claim that there is no
demonstration of substance, that the same thing cannot be demonstrated of itself, and that
Page Break 66
Page 66
Although Giles rejects the claim according to which we can attain a quidditative
knowledge of God in the present life, he concedes that we can know that God exists, since
every effect leads to a knowledge of its cause. In the next question of his
Sentences
commentary Giles asks whether God’s existence is self-evident.
†22
This text begins with a
series of arguments to show that it is
not
self-evident. That God’s existence is not easily
known, that it is denied by some, and that some have proven that God exists, are all reasons
to suggest that his existence is not self-evident. Then two counter-arguments are given: one,
drawn from Anselm, reasons that God cannot be thought not to be; the other, citing
Augustine’s
De Trinitate
V. 2, argues that God’s existence is self-evident because of the
identity in him of essence and existence.
Page 66
Giles begins his reply by stating the view of Thomas Aquinas. Some thinkers, he tells us,
distinguish two ways in which the existence of God might be self-evident, namely, in itself or
in relation to us. Considered in itself, the existence of God is self-evident, because existence
belongs most of all to the notion (
ratio
) of God, since he
ipsum esse
. In relation to us,
however, a further distinction is needed, for one may consider God in his likeness or in
himself. According to the former, God’s existence is self-evident, since whatever we know is a
certain likeness and participation of the divine truth; according to the latter, however, God’s
existence is not self-evident. The reason for this, Giles explains, is that a self-evident
proposition is one whose predicate is contained within its subject. Thus, when one knows the
quiddity of the subject, one immediately assents to the proposition. However, as Boethius
distinguished in the
De hebdomadibus
, some conceptions of the soul are common to all,
others only to the wise. Therefore, if God’s existence were self-evident to us, either everyone
would know what God is, or at least the wise would know this. But since no one in this life
knows the quiddity of God, that he exists is self-evident to no one.
†23
Page Break 67
Page 67
Although Giles does not name the author of this opinion, he clearly has in mind Thomas
Aquinas. Thomas’s own
Sentences
commentary would appear to be the main source that
what is self-evident is not demonstrable. Of the two arguments
in contrarium
,oneappealsto
the authority of Avicenna, who held that God’s existence could be demonstrated; the other
reasons that, since the existence of a cause is made known through its effects, one can prove
that God exists through creatures, since the latter are effects of God.
Page 68
In his reply Giles notes that there are two sources of difficulty concerning this question.
The first is that God is his
esse
; the second is that God’s existence is self-evident. Each claim
is repugnant to a demonstration
exists” (though clearly they imply such a statement), but rather “God is x.” By “x” is meant a
being that is most perfect, best, uncreated cause of all things, immovable, etc. It would
Page Break 71
seem to be the case, therefore, that Giles regards the existence of God as indubitable, at
least for those who have a correct metaphysical understanding of God. If this is so, then his
thinking is in harmony with an earlier theological tradition represented by Anselm and
Bonaventure.
†32
Page 71
Nevertheless, Giles’s claim that God’s existence is self-evident to the wise seems to have
been regarded as theologically suspect by his contemporaries. One of the propositions for
which he was censured in 1277 reads as follows: “That God exists not only is known
absolutely, but even with respect to us, because we know what God is, because [we know
that he is] that than which a greater cannot be thought.”
†33
In his
Apologia
, Giles protests
that this is a misreading of his text. He had maintained that we possess a nominal knowledge
(
quid nominis
) of God, not a real knowledge of the divine essence (
quid rei
), and that God’s
existence is
per se notum
, because it is
per se notum
to the wise. Giles admits that this
position is not one commonly held; nevertheless, it presents no danger to faith.
†34
Page 71
Interestingly enough, the claim that God’s existence is self-evident to the wise was also
defended by Siger of Brabant in his
Impossibilia
, which dates from 1271–73, exactly
contemporaneous with Giles’s
Commentarius in Primum Sententiarum
.
†35
Siger argued that,
since God is the first cause and does not depend upon anything prior to himself for his being,
he must have being by his own nature (
ex ratione sua
). Thus the very notion of God (
de sui
ratione
) includes
esse
. But self-evident propositions are those which are known by a
knowledge of their terms, because the predicate belongs to the
ratio
of the subject.
Therefore, the proposition “God exists” is self-evident to the wise, since they understand the
ratio
of the subject of this proposition, and for them the existence of God is not even a matter
for demonstration.
†36
Page Break 69
of God’s existence. Giles then makes an interesting claim. When one demonstrates that God
exists one does not demonstrate that
esse
belongs to God, since in this case the subject and
predicate are identical and there would be a demonstration of substance. Instead, to
demonstrate that God exists is to make evident what he is, that is to say, what is meant by
the term “God.’ Thus the more popular arguments show that the term “God” means
something completely immovable, something wholly perfect, that than which a greater cannot
be thought, etc. Since these cannot not exist, the intellect immediately affirms that God
exists.
†28
Page 69
Giles adds that this manner of demonstration is not repugnant to the claim that God is
his
esse
, since the latter is not demonstrated; rather, the meaning of the term “God” is
explained. This kind of demonstration is also compatible with the claim that God’s existence is
self-evident to the wise, since what is self-evident in this way is not self-evident to just
anyone—hence the meaning of the terms can be made evident to the less learned.
†29
Page 69
In the final part of his text Giles distinguishes three ways in which the meaning of the
term “God” may be made known to us, in accordance with the threefold Dionysian approach
to God, namely, the way of excellence, the way of causality, and the way of negation. With
respect to the way of excellence, Giles observes that God excels creatures in four ways: (1)
by the eminence of his power; (2) by the plenitude of his perfection; (3) by the excellence of
his goodness; and (4) by the plenitude of his beauty. God is that being which is most
eminent, most perfect, best, and most beautiful. Since such a being must exist, one may
speak of these as four demonstrations for God’s existence.
†30
Page Break 72
III.
Page 72
Thomas’s Position Reconsidered
. As we have seen, Thomas and Giles agree that in the
present life we can have no quidditative knowledge of God. They also agree that God’s
essence is his existence (the identity thesis), and that the existence of God is self-evident in
itself. However, Giles rejects Thomas’s claim, according to which God’s existence is not
self-evident to us. Since the wise are able to know that God’s essence is his existence, they
cannot think of God without thinking of him as existing. Hence the existence of God is
self-evident to the wise, that is to say, to those who understand and accept the identity
thesis.
Page 72
One might wonder how Thomas himself would have responded to Giles’s criticisms. As we
have seen, Aquinas rejected the claim that one could infer the existence of God from a mere
concept. Whether one thinks of God as that than which nothing greater can be conceived, or
as that which cannot be thought not to be, or even as that whose very essence is to exist,
one may not conclude without additional evidence that God actually exists. A nominal
definition of God does not contain his real existence; hence one must resort to a
demonstration.
Page 72
Following Aristotle, Thomas understands a demonstration to be a syllogism productive of
scientific knowledge. Science is causal knowledge: when one knows scientifically one
understands why something is the way that it is.
†37
In a demonstrative syllogism it is the
middle cause that explains why one should conclude that a given predicate belongs to a
Page Break 70
Page 70
With respect to the way of causality, Giles distinguishes four things in the universe that
are caused by God, namely, existence, motion, harmony, and order. Again, this gives rise to
four demonstrations for God’s existence. (1) Some being is changeable (
vertibile
); but the
changeable presupposes the unchangeable (
invertibile
), just as what is able to be created
presupposes the uncreated. Therefore, there is an uncreated being that gives existence to all
things, and this is God. (2) Since everything which is moved is moved by another, one must
arrive at something completely immovable, and this is God. (3) Since we observe that the
parts of the universe are contrary and diverse, there must be some being that connects these
parts together to form an ordered cosmos, and this being is God. (4) Since we see in nature
things acting to attain a good, there must be something which directs these things to their
proper ends, and this is God.
†31
Page 70
Finally, with respect to the way of negation, Giles observes that this way proves the same
thing as the previous ways. For example, from the fact that one must affirm that there is a
most perfect being, one must grant that there is something in which there is no imperfection,
and this is God. One follows a similar reasoning with the other descriptions of God.
Page 70
According to Giles, therefore, to demonstrate God’s existence is to explicate the meaning
of the term “God.” Strictly speaking, such arguments do not arrive at the conclusion “God
certain subject. Like Aristotle, Thomas distinguishes two kinds of demonstration. A
propter
quid
demonstration is through the cause, that is to say, through what is prior
simpliciter
. This
is a demonstration in the strict sense; the middle term expresses the
quod quid est
of the
subject, and it explains why the predicate belongs to the subject. A
quia
demonstration, on
the other hand, is through the effect, that is to say, through what is prior with regard to us.
Here the middle term is not a definition of the subject but rather an effect: it allows us to
know that a conclusion is true, although it does not explain why it is true. As Thomas
observes, when an effect is more evident to us
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than its cause, we arrive at a knowledge of the cause through its effect. The reason for this,
of course, is that an effect depends upon its cause; therefore, from an effect one can
demonstrate that its proper cause exists.
†38
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According to Thomas, the proposition “God exists” cannot be the conclusion of a
propter
quid
demonstration. Since we do not know the
quod quid est
of God, it follows that we must
resort to a
quia
demonstration to prove God’s existence. In such a proof the middle term is a
nominal definition of God, which is imposed from his effects.
†39
One sees this exemplified in
Thomas’s Five Ways. There he shows that there must be a first unmoved mover, a first
efficient cause, a
per se
necessary being, a being which is the cause of perfection in all
things, and a being which orders all things to their end. In each case these are names which
we call God. It is clear that in these
quia
demonstrations Thomas actually intends to conclude
that God exists. Unlike Giles, he does not intend simply to manifest the nominal definition of
God; rather, he wishes to use a nominal definition to conclude to God’s real existence.
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Thomas would also reject the claim that in demonstrating God’s existence one violates
the Aristotelian dictum that there can be no demonstration of substance. He makes this
evident in considering an objection that one might make against the identity thesis. One
might argue, he notes, that since we can know that God is (
an sit
), but not what he is (
quid
sit
), God’s
esse
and essence are not the same. To this Thomas responds that
esse
is said in
two ways: in one way it signifies the act of being; in another way the truth of a proposition.
Understood in the former sense, God’s
esse
is not known to us, just as we do not know his
essence; however, in the latter sense God’s
esse
is known to us, since we know that the
proposition “God exists” is true. And we know that this proposition is true only by reasoning
from effects to causes.
†40
developing this “negative theology” Thomas takes as a starting point what has already been
revealed about God in proving his existence,
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namely, that God is completely immovable.
†43
From this he argues in chap. 15 that God is
eternal, in chap. 16 that he lacks matter, and in chap. 18 that there is no composition within
him. In chap. 22 Thomas undertakes to show that
esse
and essence are the same in God. The
arguments that he employs all clearly presuppose the argumentation of the preceding
chapters. Thus Thomas reasons that, if God were not his
esse
, he would not be a
per se
necessary being, his being would be caused, he would not be pure act, he would possess
composition, and there would be something prior to him.
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In the
Summa theologiae
and the
Compendium theologiae
Thomas follows a similar
procedure. After proving that God exists in ST I, q. 2, a. 3, Thomas begins an inquiry into the
divine nature. However, since we cannot know what God is, but only what he is not, we
cannot consider how God is, but rather how he is not. In q. 3 Thomas undertakes to show
that God is simple; he considers and rejects various forms of metaphysical composition, such
as form and matter (a. 2), nature and supposit (a. 3), essence and existence (a. 4), genus
and difference (a. 5), and subject and accidents (a. 6). In arguing for the identity thesis
Thomas again reasons that if essence and
esse
were not the same in God, his
esse
would be
caused, he would not be pure act, and he would not be the first being (
primum ens
). In his
Compendium theologiae
(1265–67), Thomas develops a proof of God’s existence in chap. 3.
He then shows that God is immobile (chap. 4), eternal (chap. 5), and a necessary being
(chap. 6). In chap. 9 he argues that God is simple; from the fact of the divine simplicity he
then infers that God is his essence (chap. 10) and that his essence is his existence (chap.
11).
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In all three works Thomas establishes the identity thesis only
after
having proven that
God exists. In each text he arrives at the truth of the identity thesis by means of a
metaphysical reflection on what he has discovered about God in proving his existence. When
one turns to the
De potentia
(1265–66), one finds Thomas attempting to establish the
identity thesis without having offered prior argumentation that God exists. Nevertheless, even
there his discussion clearly presupposes God’s existence. In q. 7 Thomas is concerned with
the simplicity of the divine essence. In a. 1 he offers three arguments to show that God is
absolutely simple. His first argument begins
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Moreover, Thomas denies that God has a definition. Strictly speaking, a definition is
formed from a genus and a specific difference. Yet God is not a member of a genus, nor does
he have any differentiating characteristics.
†41
Even if one could define God, his definition
would not be being, since being is not a genus and thus cannot enter the definition of
anything.
†42
Thus the identity thesis does not provide us with a definition of God.
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But if the identity thesis by itself neither shows that God exists nor reveals what he is, it
is reasonable to ask what in fact it
does
tell us about God. One might also wonder about the
relation between the identity thesis and our knowledge of God’s existence, and further, about
how one comes to know that the identity thesis is indeed true. To examine these questions it
will be useful to turn to those texts in which Thomas explicitly attempts to establish the
identity thesis. His first text is found in the
Summa contra gentiles
. After setting forth a very
involved and detailed argument for God’s existence in bk. I, chap. 13, Thomas begins an
inquiry into God’s nature. In chap. 14 he tells us that we must use the way of negation (
via
remotionis
) when considering the divine substance. God excels by his immensity every form
that our intellect grasps. Hence we cannot know what he is, only what he is not. In
Page Break 76
with the claim that all beings proceed from a first being (God), who has within himself no
admixture of potentiality. But in any kind of composition there is a mixture of act and
potency. For in a composite either one part is in potency to another (for example, matter to
form, subject to accident, genus to difference), or all the parts are in potency to the whole.
Therefore, since God is pure act, he must be wholly simple.
†44
In a second argument Thomas
reasons that in any composite the parts need to be united to form a whole. It follows that
every composite depends upon a prior cause. Therefore, the first being (God), from whom all
things exist, cannot be something composite.
†45
In a third argument Thomas reasons that
God must be most perfect and “best.” But the best is that in which there is nothing that lacks
goodness. Yet this cannot be something composite, since in that thing whose goodness
results from a composition of parts, the goodness of the whole is not present in some of the
parts. Therefore, that which is best must be most simple and lack all manner of
composition.
†46
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Having shown that God lacks all form of composition, Thomas turns in a. 2 to show that
God’s
esse
is the same as his essence. There he points out that any cause has its own proper
effect, in accordance with its essential
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