Medieval Sourcebook:
Thomas Aquinas:
Reasons in Proof of the Existence of God, 1270
Summa Theologia
Article II. Whether the existence of God is demonstrable:
Let us proceed to the second point. It is objected (1) that the
existence of God is not demonstratable: that God's existence is
an article of faith, and that articles of faith are not demonstratable,
because the office of demonstration is to prove, but faith pertains
(only) to things that are not to be proven, as is evident from
the Epistle to the Hebrews, 11. Hence that God's existence is
not demonstratable. Again, (2) that the subject matter of demonstration
is that something exists, but in the case of God we cannot know
what exists, but only what does not, as Damascenus says (Of the
Orthodox Faith, I., 4.) Hence that we cannot demonstrate God's
existence. Again, (3) that if God's existence is to be proved
it must be from what He causes, and that what He effects is not
sufficient for His supposed nature, since He is infinite, but
the effects finite, and the finite is not proportional to the
infinite. Since, therefore, a cause cannot be proved through an
effect not proportional to itself, it is said that God's exisence
cannot be proved.
But against this argument the apostle says (Rom. I., 20), "The
unseen things of God are visible through His manifest works."
But this would not be so unless it were possible to demonstrate
God's existence through His works. What ought to be understood
concerning anything, is first of all, whether it exists. Conclusion.
It is possible to demonstrate God's existence, atthough not a
priori (by pure reason), yet a posteriori from some work of His
more surely known to us.
In answer I must say that the proof is double. One is through
the nature of a cause and is called propter quid: this is through
the nature of preceding events sirnply. The other is through the
nature of the effect, and is called quia, and is through the nature
of preceding things as respects us. Since the effect is better
known to us than the cause, we proceed from the effect to the
knowledge of the cause. From any effect whatsoever it can be proved
that a corresponding cause exists, if only the effects of it are
sufficiently known to us, for since effects depend on causes,
the effect being given, it is necessary that a preceding cause
exists. Whence, that God exists, although this is not itself known
to us, is provable through effects that are known to us.
To the first objection above, I reply, therefore, that God's existence,
and those other things of this nature that can be known through
natural reason concerning God, as is said in Rom. I., are not
articles of faith, but preambles to these articles. So faith presupposes
natural knowledge, so grace nature, and perfection a perfectible
thing. Nothing prevents a thing that is in itself demonstratable
and knowable, from being accepted as an article of faith by someone
that does not accept the proof of it.
To the second objection, I reply that, since the cause is proven
from the effect, one must use the effect in the place of a definition
of the cause in demonstrating that the cause exists; and that
this applies especially in the case of God, because for proving
that anything exists, it is necessary to accept in this method
what the name signifies, not however that anything exists, because
the question what it is is secondary to the question whether it
exists at all. The characteristics of God are drawn from His works
as shall be shown hereafter, (Question XIII). Whence by proving
that God exists through His works as shall be shown hereafter,
(Question XIII). Whence by proving that God exists through His
works, we are able by this very method to see what the name God
signifies.
To the third objection, I reply that, although a perfect knowledge
of the cause cannot be had from inadequate effects, yet that from
any effect manifest to us it can be shown that a cause does exist,
as has been said. And thus from the works of God His existence
can be proved, although we cannot in this way know Him perfectly
in accordance with His own essence.
Article III. Whether God exists.
Let us proceed to the third article. It is objected (1) that God
does not exist, because if one of two contradictory things is
infinite, the other will be totally destroyed; that it is implied
in the name God that there is a certain infinite goodness: if
then God existed, no evil would be found. But evil is found in
the world; therefore it is objected that God does not exist. Again,
that what can be accomplished through a less number of principles
will not be accomplished through more. It is objected that all
things that appear on the earth can be accounted for through other
principles, without supposing that God exists, since what is natural
can be traced to a natural principle, and what proceeds from a
proposition can be traced to the human reason or will. Therefore
that there is no necessity to suppose that God exists. But as
against this note what is said of the person of God (Exod. III.,
14) I am that I am. Conclusion. There must be found in the nature
of things one first immovable Being, a primary cause, necessarily
existing, not created; existing the most widely, good, even the
best possible; the first ruler through the intellect, and the
ultimate end of all things, which is God.
I answer that it can be proved in five ways that God exists.
The first and plainest is the method that proceeds from
the point of view of motion. It is certain and in accord with
experience, that things on earth undergo change. Now, everything
that is moved is moved by something; nothing, indeed, is changed,
except it is changed to something which it is in potentiality.
Moreover, anything moves in accordance with something actually
existing; change itself, is nothing else than to bring forth something
from potentiality into actuality. Now, nothing can be brought
from potentiality to actual existence except through something
actually existing: thus heat in action, as fire, makes fire-wood,
which is hot in potentiality, to be hot actually, and through
this process, changes itself. The same thing cannot at the same
time be actually and potentially the same thing, but only in regard
to different things. What is actually hot cannot be at the same
time potentially hot, but it is possible for it at the same time
to be potentially cold. It is impossible, then, that anything
should be both mover and the thing moved, in regard to the same
thing and in the same way, or that it should move itself. Everything,
therefore, is moved by something else. If, then, that by which
it is moved, is also moved, this must be moved by something still
different, and this, again, by something else. But this process
cannot go on to infinity because there would not be any first
mover, nor, because of this fact, anything else in motion, as
the succeeding things would not move except because of what is
moved by the first mover, just as a stick is not moved except
through what is moved from the hand. Therefore it is necessary
to go back to some first mover, which is itself moved by nothing---and
this all men know as God.
The second proof is from the nature of the efficient cause.
We find in our experience that there is a chain of causes: nor
is it found possible for anything to be the efficient cause of
itself, since it would have to exist before itself, which is impossible.
Nor in the case of efficient causes can the chain go back indefinitely,
because in all chains of efficient causes, the first is the cause
of the middle, and these of the last, whether they be one or many.
If the cause is removed, the effect is removed. Hence if there
is not a first cause, there will not be a last, nor a middle.
But if the chain were to go back infinitely, there would be no
first cause, and thus no ultimate effect, nor middle causes, which
is admittedly false. Hence we must presuppose some first efficient
cause---which all call God.
The third proof is taken from the natures of the merely
possible and necessary. We find that certain things either may
or may not exist, since they are found to come into being and
be destroyed, and in consequence potentially, either existent
or non-existent. But it is impossible for all things that are
of this character to exist eternally, because what may not exist,
at length will not. If, then, all things were merely possible
(mere accidents), eventually nothing among things would exist.
If this is true, even now there would be nothing, because what
does not exist, does not take its beginning except through something
that does exist. If then nothing existed, it would be impossible
for anything to begin, and there would now be nothing existing,
which is admittedly false. Hence not all things are mere accidents,
but there must be one necessarily existing being. Now every necessary
thing either has a cause of its necessary existence, or has not.
In the case of necessary things that have a cause for their necessary
existence, the chain of causes cannot go back infinitely, just
as not in the case of efficient causes, as proved. Hence there
must be presupposed something necessarily existing through its
own nature, not having a cause elsewhere but being itself the
cause of the necessary existence of other things---which all call
God.
The fourth proof arises from the degrees that are found
in things. For there is found a greater and a less degree of goodness,
truth, nobility, and the like. But more or less are terms spoken
of various things as they approach in diverse ways toward something
that is the greatest, just as in the case of hotter (more hot)
which approaches nearer the greatest heat. There exists therefore
something that is the truest, and best, and most noble, and in
consequence, the greatest being. For what are the greatest truths
are the greatest beings, as is said in the Metaphysics Bk. II.
2. What moreover is the greatest in its way, in another way is
the cause of all things of its own kind (or genus); thus fire,
which is the greatest heat, is the cause of all heat, as is said
in the same book (cf. Plato and Aristotle). Therefore there exists
something that is the cause of the existence of all things and
of the goodness and of every perfection whatsoever---and this
we call God.
The fifth proof arises from the ordering of things for
we see that some things which lack reason, such as natural bodies,
are operated in accordance with a plan. It appears from this that
they are operated always or the more frequently in this same way
the closer they follow what is the Highest; whence it is clear
that they do not arrive at the result by chance but because of
a purpose. The things, moreover, that do not have intelligence
do not tend toward a result unless directed by some one knowing
and intelligent; just as an arrow is sent by an archer. Therefore
there is something intelligent by which all natural things are
arranged in accordance with a plan---and this we call God.
In response to the first objection, then, I reply what Augustine
says; that since God is entirely good, He would permit evil to
exist in His works only if He were so good and omnipotent that
He might bring forth good even from the evil. It therefore pertains
to the infinite goodness of God that he permits evil to exist
and from this brings forth good.
My reply to the second objection is that since nature is ordered
in accordance with some defined purpose by the direction of some
superior agent, those things that spring from nature must be dependent
upon God, just as upon a first cause. Likewise, what springs from
a proposition must be traceable to some higher cause which is
not the human reason or will, because this is changeable and defective
and everything changeable and liable to non-existence is dependent
upon some unchangeable first principle that is necessarily self-existent
as has been shown.
Source:
From: Oliver J. Thatcher, ed., The Library of Original Sources (Milwaukee: University Research Extension Co., 1907), Vol. V:
The Early Medieval World, pp. 359-363.
Scanned by Jerome S. Arkenberg, Dept. of History, Cal. State Fullerton
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