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Because the chief aim of sacred doctrine is to teach the
knowledge of God, not only as He is in Himself, but also
as He is the beginning of things and their last end, and
especially of rational creatures, as is clear from what has
been already said, therefore, in our endeavor to expound
this science, we shall treat:
(1) Of God:
(2) Of the rational creature's advance towards God:
(3) Of Christ, Who as man, is our way to God.
In treating of God there will be a threefold division: —
For we shall consider
(1) whatever concerns the Divine Essence.
(2) Whatever concerns the distinctions of Persons.
(3) Whatever concerns the procession of creatures from Him.
Concerning the Divine Essence, we must consider: —
(1) Whether God exists ?
(2) The manner of His existence, or, rather, what is not the manner of His existence.
{3) Whatever concerns His operations — namely. His knowledge,
will, power.

Concerning the first, there are three points of inquiry: —

(1) Whether the proposition ' God exists ' is self-evident ?
(2) Whether it is demonstrable ?
(3) Whether God exists ?

First Article
whether the existence of god is self-evident ?

We proceed thus to the First Article : —

Objection 1. It seems that the existence of God is self-evident.
Now those things are said to be self-evident to us
the knowledge of which is naturally implanted in us, as
we can see in regard to first principles. But as Damascene
says {De Fid. Orth. i. i, 3). the knowledge of God is naturally
implanted in all. Therefore the existence of God is self-evident.

Obj, 2. Further, those things are said to be self-evident
which are known as soon as the terms are known, which the
Philosopher (i Poster, iii.) says is true of the first principles
of demonstration. Thus, when the nature of a whole and
of a part is known, it is at once recognized that every whole
is greater than its part. But as soon as the signification of
the word ' God ' is understood, it is at once seen that God
exists. For by this word is signified that thing than which
nothing greater can be conceived. But that which exists
actually and mentally is greater than that which exists
only mentally. Therefore, since as soon as the word
God ' is understood it exists mentally, it also follows
that it exists actually. Therefore the proposition ' God
exists ' is self-evident.

Obj. 3. Further, the existence of truth is self-evident.
For whoever denies the existence of truth grants that
truth does not exist : and, if truth does not exist, then the
proposition ' Truth does not exist ' is true : and if there is
anything true, there must be truth. But God is truth
itself: / am the way, the truth, and the life (John xiv. 6).
Therefore ' God exists' is self-evident.

On the contrary, No one can mentally admit the opposite
of what is self-evident; as the Philosopher {Metaph. iv.,lect. vi.)
states concerning the first principles of demonstration.
But the opposite of the proposition ' God is '
can be mentally admitted: The fool said in his heart,
There is no God (Ps. Iii. i). Therefore, that God exists is
not self-evident.

I answer that, A thing can be self-evident in either of
two ways; on the one hand, self-evident in itself, though
not to us; on the other, self-evident in itself, and to us.
A proposition is self-evident because the predicate is included
in the essence of the subject, as ' Man is an animal,'
for animal is contained in the essence of man. If, therefore
the essence of the predicate and subject be known to all,
the proposition will be self-evident to all; as is clear with
regard to the first principles of demonstration, the terms of
which are common things that no one is ignorant of, such
as being and non-being, whole and part, and suchlike.
If, however, there are some to whom the essence of the
predicate and subject is unknown, the proposition will be
self-evident in itself, but not to those who do not know the
meaning of the predicate and subject of the proposition.
Therefore, it happens, as Boethius says {Hebdom., the title of
which is : ' Whether all that is, is good '), ' that there are some
mental concepts self-e4dent only to the learned, as that
incorporeal substances are not in space.' Therefore I say
that this proposition, ' God exists,' of itself is self-evident,
for the predicate is the same as the subject; because God
is His own existence as will be hereafter shown (Q. III., A. 4)
Now because we do not know the essence of God, the pro-
position is not self-evident to us; but needs to be demon-
started by things that are more known to us, though less
known in their nature — namely, by effects.

Reply Ob). 1. To know that God exists in a general
and confused way is implanted in us by nature, inasmuch
as God is man's beatitude. For man naturally desires
happiness, and what is naturally desired by man must
be naturally known to him. This, however, is not to know
absolutely that God exists; just as to know that some-
one is approaching is not the same as to know that Peter
is approaching, even though it is Peter who is approaching;
for many there are who imagine that man's perfect good
which is happiness, consists in riches, and others in pleasures,
and others in something else.

Reply Obj. 2. Perhaps not everyone who hears this word
' God ' understands it to signify something than which
nothing greater can be thought, seeing that some have
believed God to be a body. Yet, granted that everyone
understands that by this word ' God ' is signified something
than which nothing greater can be thought, nevertheless,
it does not therefore follow that he understands that what
the word signifies exists actually, but only that it exists
mentally. Nor can it be argued that it actually exists,
unless it be admitted that there actually exists something
than which nothing greater can be thought ; and this precisely
is not admitted by those who hold that God does not exist.
Reply Oh]. 3. The existence of truth in general is self-evident
but the existence of a Primal Truth is not self-evident to us.

Second Article,
whether it can be demonstrated that god exists ?

We proceed thus to the Second Article : —

Objection 1. It seems that the existence of God cannot
be demonstrated. For it is an article of faith that God
exists. But what is of faith cannot be demonstrated,
because a demonstration produces scientific knowledge;
whereas faith is of the unseen (Heb. xi. i). Therefore it
cannot be demonstrated that God exists.

Obj. 2. Further, the essence is the middle term of demonstration.
But we cannot know in what God's essence consists,
but solely in what it does not consist ; as Damascene says
{De Fid. Orth. i. 4.). Therefore we cannot demonstrate that God exists.

Obj. 3. Further, if the existence of God were demonstrated,
this could only be from His effects. But His effects are not
proportionate to Him, since He is infinite
and His effects are finite ; and between the finite and infinite
there is no proportion. Therefore, since a cause cannot be
demonstrated by an effect not proportionate to it, it seems
that the existence of God cannot be demonstrated.

On the contrary, The Apostle says: The invisible things
of Him are clearly seen, being understood by the things that
are made (Rom. i. 20). But this would not be unless the
existence of God could be demonstrated through the things
that are made ; for the first thing we must know of anything
is, whether it exists.

I answer that, Demonstration can be made in two ways:
One is through the cause, and is called a prior, and this
is to argue from what is prior absolutely. The other is
through the effect, and is called a demonstration a posteriori ;
this is to argue from what is prior relatively only
to us. When an effect is better known to us than its cause,
from the effect we proceed to the knowledge of the cause.
And from every effect the existence of its proper cause
can be demonstrated, so long as its effects are better
known to us; because since every effect depends upon
its cause, if the effect exists, the cause must pre-exist.
Hence the existence of God, in so far as it is not self-evident
to us, can be demonstrated from those of His effects which
are known to us.

Reply Obj. 1. The existence of God and other like truths
about God, which can be known by natural reason, are
not articles of faith, but are preambles to the articles; for
faith presupposes natural knowledge, even as grace pre-supposes
nature, and perfection supposes something that
can be perfected. Nevertheless, there is nothing to prevent
a man, who cannot grasp a proof, accepting, as a matter
of faith, something which in itself is capable of being
scientifically known and demonstrated.

Reply Obj. 2. 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.
This is especially the case in regard to God, because, in
order to prove the existence of anything, it is necessary
to accept as a middle term the meaning of the word, and
not its essence, for the question of its essence follows on
the question of its existence. Now the names given to God
are derived from His effects; consequently, in demonstrating
the existence of God from His effects, we may take
for the middle term the meaning of the word ' God .'

Reply Obj. 3. From effects not proportionate to the
cause no perfect knowledge of that cause can be obtained.
Yet from every effect the existence of the cause can be
clearly demonstrated, and so we can demonstrate the
existence of God from His effects; though from them we
cannot perfectly know God as He is in His essence.

Third Article,
whether god exists ?

We proceed thus to the Third Article : —

Objection 1. It seems that God does not exist; because
if one of two contraries be infinite, the other would be
altogether destroyed. But the word ' God ' means that
He is infinite goodness. If, therefore, God existed, there
would be no evil discoverable; but there is evil in the world.
Therefore God does not exist.

Obj. 2. Further, it is superfluous to suppose that what
can be accounted for by a few principles has been produced
by many. But it seems that everything we see
in the world can be accounted for by other principles,
supposing God did not exist. For all natural things can
be reduced to one principle, which is nature; and all
voluntary things can be reduced to one principle, which
is human reason, or will. Therefore there is no need to
suppose God's existence.

On the contrary, It is said in the person of God : I am Who
am (Exod. iii. 14).

I answer that. The existence of God can be proved in five
ways.

The first and more manifest way is the argument from
motion. It is certain, and evident to our senses, that in the
world some things are in motion. Now whatever is in
motion is put in motion by another, for nothing can be in
motion except it is in potentiality to that towards which it is
in motion; whereas a thing moves inasmuch as it is in
act. For motion is nothing else than the reduction of
something from potentiality to actuality. But nothing
can be reduced from potentiality to actuality, except
by something in a state of actuality. Thus that which
is actually hot, as fire, makes wood, which is potentially
hot, to be actually hot, and thereby moves and changes
it. Now it is not possible that the same thing should
be at once in actuality and potentiality in the same
respect, but only in different respects. For what is
actually hot cannot simultaneously be potentially hot;
but it is simultaneously potentially cold. It is therefore
impossible that in the same respect and in the same
way a thing should be both mover and moved, i.e., that
it should move itself. Therefore, whatever is in motion
must be put in motion by another. If that by which
it is put in motion be itself put in motion, then this
also must needs be put in motion by another, and that
by another again. But this cannot go on to infinity,
because then there would be no first mover, and, consequently,
no other mover; seeing that subsequent movers
move only inasmuch as they are put in motion by the first
mover; as the staff moves only because it is put in motion
by the hand. Therefore it is necessary to arrive at a first
mover, put in motion by no other; and this everyone under-
stands to be God.

The second way is from the nature of the efficient cause.
In the world of sense we find there is an order of efficient
causes. There is no case known (neither is it, indeed,
possible) in which a thing is found to be the efficient cause
of itself; for so it would be prior to itself, which is impossible.
Now in efficient causes it is not possible to go on to infinity,
because in all efficient causes following in order, the first is
the cause of the intermediate cause, and the intermediate
is the cause of the ultimate cause, whether the intermediate
cause be several, or one only. Now to take away the cause
is to take away the effect. Therefore, if there be no first
cause among efficient causes, there will be no ultimate, nor
any intermediate cause. But if in efficient causes it is
possible to go on to infinity, there will be no first efficient
cause, neither will there be an ultimate effect, nor any inter-
mediate efficient causes; all of which is plainly false. There-
fore it is necessary to admit a first efficient cause, to which
everyone gives the name of God.

The third way is taken from possibility and necessity,
and runs thus. We find in nature things that are possible to
be and not to be, since they are found to be generated, and
to corrupt, and consequently, they are possible to be and
not to be. But it is impossible for these always to exist,
for that which is possible not to be at some time is not.
Therefore, if everything is possible not to be, then at
one time there could have been nothing in existence.
Now if this were true, even now there would be nothing
in existence, because that which does not exist only
begins to exist by something already existing. Therefore,
if at one time nothing was in existence, it would have
been impossible for anything to have begun to exist; and
thus even now nothing would be in existence — which is
absurd. Therefore, not all beings are merely possible, but
there must exist something the existence of which is necessary.
But every necessary thing either has its necessity
caused by another, or not. Now it is impossible to go on
to infinity in necessary things which have their necessity
caused by another, as has been already proved in regard
to efficient causes. Therefore we cannot but postulate
the existence of some being having of itself its own necessity,
and not receiving it from another, but rather causing
in others their necessity. This all men speak of as God.

The fourth way is taken from the gradation to be found
in things. Among beings there are some more and some
less good, true, noble, and the like. But ' more ' and
' less ' are predicated of different things, according as they
resemble in their different ways something which is the
maximum, as a thing is said to be hotter according as it
more nearly resembles that which is hottest ; so that there is
something which is truest, something best, something
noblest, and, consequently, something which is uttermost
being; for those things that are greatest in truth are great-
est in being, as it is written in Metaph. ii. Now the
maximum in any genus is the cause of all in that genus ; as
fire, which is the maximum of heat, is the cause of all hot
things. Therefore there must also be something which is
to all beings the cause of their being, goodness, and every
other perfection ; and this we call God.

The fifth way is taken from the governance of the world.

We see that things which lack intelligence, such as natural
bodies, act for an end, and this is evident from their acting
always, or nearly always, in the same way, so as to obtain
the best result. Hence it is plain that not fortuitously,
hue designedly, do they achieve their end. Now whatever
lacks intelligence cannot move towards an end, unless it
be directed by some being endowed with knowledge and
intelligence ; as the arrow is shot to its mark b}^ the archer.
Therefore some intelligent being exists by whom all natural
things are directed to their end ; and this being we call God.
Reply Obj. 1. As Augustine says {Enchir. xi.) : Since God is
the highest good, He would not allow any evil to exist in His
works, unless His omnipotence and goodness were such as to
bring good even out of evil. This is part of the infinite goodness
of God, that He should allow evil to exist, and out of it
produce good.

. Reply Obj. 2. Since nature works for a determinate end
under the direction of a higher agent, whatever is done
by nature must needs be traced back to God, as to its first
cause. So also whatever is done voluntarily must also be
traced back to some higher cause other than human reason
or will, since these can change and fail ; for all things that are
changeable and capable of defect must be traced back to an
immovable and self-necessary first principle, as was shown
in the body of the Article.