Summary
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.
