I answer that, The <A href="http://www.newadvent.org/cathen/06608b.htm">
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 understands 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 only one. 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 intermediate efficient
causes; all of which is plainly
false. Therefore 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 greatest 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 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, but 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 by the archer. Therefore some
intelligent being
exists by whom all
natural things are directed to their end; and this being we call
God.
got this from:
http://www.newadvent.org/summa/1002.htm
