St. Thomas Aquinas (1225-1274)
Summa Theologica
First Part
Question 2
Article 3

Whether God exists?

Objection 1. It seems that God does not exist; because if one of two  contraries be infinite, the other would be altogether destroyed. But the  word "God" means that He is infinite goodness. If, therefore, God  existed, there would be no evil discoverable; but there is evil in the  world. Therefore God does not exist.

Objection 2. Further, it is superfluous to suppose that what can be accounted  for by a few principles has been produced by many. But it seems that  everything we see in the world can be accounted for by other principles,  supposing God did not exist. For all natural things can be reduced to one  principle which is nature; and all voluntary things can be reduced to one  principle which is human reason, or will. Therefore there is no need to  suppose God's existence.

On the contrary, It is said in the person of God: "I am Who am." (Ex.  3:14)

I answer that, 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 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.

Reply to Objection 1. As Augustine says (Enchiridion xi): "Since God is the  highest good, He would not allow any evil to exist in His works, unless  His omnipotence and goodness were such as to bring good even out of  evil." This is part of the infinite goodness of God, that He should allow  evil to exist, and out of it produce good.

Reply to Objection 2. Since nature works for a determinate end under the  direction of a higher agent, whatever is done by nature must needs be  traced back to God, as to its first cause. So also whatever is done  voluntarily must also be traced back to some higher cause other than  human reason or will, since these can change or fail; for all things that  are changeable and capable of defect must be traced back to an immovable  and self-necessary first principle, as was shown in the body of the  Article.

Translated by the Fathers of the English Dominican Province. Copyright © 1947 Benzinger Brothers Inc., Hypertext Version Copyright © 1995, 1996 www.newadvent.org.