Thom as Aquinas’
Sum m a Theologica
(Com plete Notes)
Pr oofs for the Existen ce of God
Sum m ar y
Question 1 of part 1 of the Summa considers the nature and extent of “sacred doctrine,”
or theology. Aquinas concludes that, although theology does not require philosophy to
promote knowledge of God, philosophy nevertheless can be of service to the aims of
theology.
Question 2 of part 1 concerns the existence of God and is subdivided into three Articles.
In the First Article, Aquinas maintains that the proposition “God exists” is self-evident
in itself, but not to us, and thus requires demonstration. The Second Article concludes
that such a demonstration is indeed possible, despite objections to the contrary. The
famous Third Article addresses the question of whether God exists, and in this Article,
Aquinas offers his Five Ways as proofs for the existence of God.
First, we observe that some things in the world are in motion. Whatever is in motion is
put into motion by another object that is in motion. This other object, in turn, was put
into motion by still another object preceding it, and so forth. This series cannot go on
backward to infinity, though, since there would otherwise be no first mover and thus no
subsequent movement. Therefore, we must conclude that there is a first unmoved
mover, which we understand to be God.
Second, we observe that everything has an efficient cause and that nothing is or can be
the cause of itself. It is impossible, though, that the series of causes should extend back
to infinity because every cause is dependent on a prior cause and the ultimate cause is
thus dependent on a previous cause. So if there is no first cause, there will be no
intermediate causes and no final cause. But the absence of such causes clearly does not
square with our observation, and so there must therefore be a first efficient cause, which
everyone calls God.
Third, we observe in nature things that are possible to be and not to be, as they come
into existence and pass out of existence. Such things could not always exist, though,
because something that could possibly not exist at some time actually does not exist at
some time. Thus, if it is possible for everything not to exist, then, at some time, nothing
, did exist. But if nothing ever did exist, then nothing would exist even now, since
everything that exists requires for its existence something that already existed. Yet it is
absurd to claim that nothing exists even now. Therefore, not all beings are merely
possible, but there must be something the existence of which is necessary. Now, every
necessary thing has its necessity caused by something else or it does not. Since it is
impossible for there to exist an infinite series of causes of necessary things, we must
conclude that there is something that is necessary in itself. People speak of this thing as
God.
Fourth, beings in the world have characteristics to varying degrees. Some are more or
less good, true, noble, and so forth. Such gradations are all measured in relation to a
maximum, however. Thus, there must be something best, truest, noblest, and so on.
Now, as Aristotle teaches, things that are greatest in truth are also greatest in being.
Therefore, there must be something that is the cause of being, goodness, and every other
perfection that we find in beings in the world. We call this maximum cause God.
Finally, we observe in nature that inanimate and nonintelligent objects act toward the
best possible purpose, even though these objects are not aware of doing so. It is clear
that these objects do not achieve their purpose by sheer chance but rather according to a
plan. Any inanimate or nonintelligent object that acts toward a purpose, though, must
be guided by a being that possesses knowledge and intelligence, just as an arrow is
directed by an archer. Therefore, there must be some intelligent being that directs all
natural things toward their purpose. We call this being God.
Having presented these proofs for the existence of God, Aquinas goes on to discuss God
in terms of his simplicity, perfection, goodness, infinity, knowledge, and other
attributes. This discussion leads into a protracted consideration of questions pertaining
to the Creation, the nature of angels, demons, and the work done on the individual six
days of the Creation, which culminated with the creation of man.
An alysis
The existence of God is the necessary foundation of any theology. Before discussing any
other topics, Aquinas needs to establish the crucial fact that God exists, since, without
certainty of God’s existence, the conclusions of the rest of the Summa would be in doubt
or even in vain. To this end, he advances five arguments intended to prove the existence
of God. Arguments 1, 2, and 5 are based on observation of the natural world, whereas
Arguments 3 and 4 are based on rational speculation. In Arguments 1, 2, 4, and 5,
Aquinas concludes that only the existence of God can provide a sufficient explanation for
the questions raised. In Argument 3, he concludes that God must necessarily exist for
his own sake. Thus, Arguments 1, 2, 4, and 5 conclude that God exists because the world
requires him as an explanation, and Argument 3 concludes that God could not not exist.
Sum m a Theologica
(Com plete Notes)
Pr oofs for the Existen ce of God
Sum m ar y
Question 1 of part 1 of the Summa considers the nature and extent of “sacred doctrine,”
or theology. Aquinas concludes that, although theology does not require philosophy to
promote knowledge of God, philosophy nevertheless can be of service to the aims of
theology.
Question 2 of part 1 concerns the existence of God and is subdivided into three Articles.
In the First Article, Aquinas maintains that the proposition “God exists” is self-evident
in itself, but not to us, and thus requires demonstration. The Second Article concludes
that such a demonstration is indeed possible, despite objections to the contrary. The
famous Third Article addresses the question of whether God exists, and in this Article,
Aquinas offers his Five Ways as proofs for the existence of God.
First, we observe that some things in the world are in motion. Whatever is in motion is
put into motion by another object that is in motion. This other object, in turn, was put
into motion by still another object preceding it, and so forth. This series cannot go on
backward to infinity, though, since there would otherwise be no first mover and thus no
subsequent movement. Therefore, we must conclude that there is a first unmoved
mover, which we understand to be God.
Second, we observe that everything has an efficient cause and that nothing is or can be
the cause of itself. It is impossible, though, that the series of causes should extend back
to infinity because every cause is dependent on a prior cause and the ultimate cause is
thus dependent on a previous cause. So if there is no first cause, there will be no
intermediate causes and no final cause. But the absence of such causes clearly does not
square with our observation, and so there must therefore be a first efficient cause, which
everyone calls God.
Third, we observe in nature things that are possible to be and not to be, as they come
into existence and pass out of existence. Such things could not always exist, though,
because something that could possibly not exist at some time actually does not exist at
some time. Thus, if it is possible for everything not to exist, then, at some time, nothing
, did exist. But if nothing ever did exist, then nothing would exist even now, since
everything that exists requires for its existence something that already existed. Yet it is
absurd to claim that nothing exists even now. Therefore, not all beings are merely
possible, but there must be something the existence of which is necessary. Now, every
necessary thing has its necessity caused by something else or it does not. Since it is
impossible for there to exist an infinite series of causes of necessary things, we must
conclude that there is something that is necessary in itself. People speak of this thing as
God.
Fourth, beings in the world have characteristics to varying degrees. Some are more or
less good, true, noble, and so forth. Such gradations are all measured in relation to a
maximum, however. Thus, there must be something best, truest, noblest, and so on.
Now, as Aristotle teaches, things that are greatest in truth are also greatest in being.
Therefore, there must be something that is the cause of being, goodness, and every other
perfection that we find in beings in the world. We call this maximum cause God.
Finally, we observe in nature that inanimate and nonintelligent objects act toward the
best possible purpose, even though these objects are not aware of doing so. It is clear
that these objects do not achieve their purpose by sheer chance but rather according to a
plan. Any inanimate or nonintelligent object that acts toward a purpose, though, must
be guided by a being that possesses knowledge and intelligence, just as an arrow is
directed by an archer. Therefore, there must be some intelligent being that directs all
natural things toward their purpose. We call this being God.
Having presented these proofs for the existence of God, Aquinas goes on to discuss God
in terms of his simplicity, perfection, goodness, infinity, knowledge, and other
attributes. This discussion leads into a protracted consideration of questions pertaining
to the Creation, the nature of angels, demons, and the work done on the individual six
days of the Creation, which culminated with the creation of man.
An alysis
The existence of God is the necessary foundation of any theology. Before discussing any
other topics, Aquinas needs to establish the crucial fact that God exists, since, without
certainty of God’s existence, the conclusions of the rest of the Summa would be in doubt
or even in vain. To this end, he advances five arguments intended to prove the existence
of God. Arguments 1, 2, and 5 are based on observation of the natural world, whereas
Arguments 3 and 4 are based on rational speculation. In Arguments 1, 2, 4, and 5,
Aquinas concludes that only the existence of God can provide a sufficient explanation for
the questions raised. In Argument 3, he concludes that God must necessarily exist for
his own sake. Thus, Arguments 1, 2, 4, and 5 conclude that God exists because the world
requires him as an explanation, and Argument 3 concludes that God could not not exist.
