St ThomasAquinas'
5 Proofs of God

SparkNotes Editors.  "SparkNotes on Thomas Aquinas (c. 1225-1274)."  SparkNotes LLC.
2005.  (accessed March 31, 2015). 

The first of Thomas Aquinas's 5 proofs of 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.

The second of Thomas Aquinas's 5 proofs of the Existence of 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.

The third of Thomas Aquinas's 5 proofs of the Existence of 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

The fourth of Thomas Aquinas's 5 proofs of the Existence of 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.

The fifth of Thomas Aquinas's 5 proofs of the Existence of God

Finally, we observe in nature that inanimate and non-intelligent 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 non-intelligent 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.

For those wanting to read more of this topic and Aquinas's "Summa Theologica" go to the