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Five
Arguments for God's Existence
by St. Thomas Aquinas
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 moved is moved by another,
for nothing can be moved except it is in
potentiality to that towards which it is moved;
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 the 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 moved
must be moved by another. If that by which it is
moved be itself moved, then this also must needs be
moved 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 moved by the first mover;
as the staff moves only because it is moved by the
hand. Therefore it is necessary to arrive at a
first mover, moved by no other; and this everyone
understands to be God.
The second way is from the nature of efficient
cause. In the world of sensible things 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 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 be corrupted, and
consequently, it is possible for them to be and not
to be. But it is impossible for these always to
exist, for that which can not-be at some time is
not. Therefore, if everything can not-be, then at
one time there was nothing in existence. Now if
this were true, even now there would be nothing in
existence, because that which does not exist begins
to exist only through 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 admit 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 truest, something
best, something noblest, and, consequently,
something which is most being, for those things
that are greatest in truth are greatest in being,
as it is written 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, as is said in the same book.
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 knowledge,
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 they achieve their end, not
fortuitously, but designedly. Now whatever lacks
knowledge cannot move towards an end, unless it be
directed by some being endowed with knowledge and
intelligence; as the arrow is directed by the
archer. Therefore some intelligent being exists by
whom all natural things are directed to their end;
and this being we call God.
Excerpted from Summa
Theologica, by St. Thomas Aquinas
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