It seems that God does not exist; because if one of two contraries be infinite, the other would altogether be 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.
Ob]. 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 that appears 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 things that happen intentionally 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 (Exod. iii. 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 some things are in motion. Whatever is in motion is moved by another, for nothing can be in motion except it have a potentiality for that towards which it is being moved; whereas a thing moves inasmuch as it is in act. By "motion we mean nothing else than the reduction of something from a state of potentiality into a state of actuality unless by something already 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. It is not possible that the same thing should be at once in a state of actuality and potentiality from the same point of view, but only from different points of view. What is actually hot cannot simultaneously be only potentially hot; still, it is simultaneously potentially cold. It is therefore impossible that from the same point of view and in the same way anything should be both moved and mover, or 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. This cannot go on to infinity, because then there would he no first mover, and, consequently, no other mover-seeing that subsequent movers only move inasmuch as they are put in motion by the first mover; as the staff only moves 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 formality of efficient causation. In the world of sense we find there is an order of efficient causation. 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 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. 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 cause, nor any intermediate. 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 put forward 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 could either exist or not exist, since they are found to be generated, and then to corrupt; and, consequently, they can exist, and then not exist. It is impossible for these always to exist, for that which can one day cease td exist must at some time have not existed. Therefore, if everything could cease to exist, then at one time there could have been nothing in existence. 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 - Every necessary thing either has its necessity caused by another, or not. 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 predicted of different things, according as they resemble in their different ways something which is in the degree of "most," 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 the truer things are, the more truly they exist. What is most complete in any genus is the cause of all in that genus; as fire, which is the most complete form of heat, is the cause whereby all things are made hot. 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; for we see that things which lack intelligence, such as natural bodies, act for some purpose, which fact 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 purpose. Whatever lacks intelligence cannot fulfill some purpose, unless it be directed by some being endowed with intelligence and knowledge; as the arrow is shot to its mark by the archer. Therefore some intelligent being exists by whom all natural things are ordained towards a definite purpose; and this being we call God.
Reply Obj 1. As Augustine says: Since God is wholly 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 Obj. 2. Since nature works out its determinate end under the direction of a higher agent, whatever is done by nature must needs be tracedback to God, as to its first cause. So also whatever is done designedly must also be traced back to some higher cause other than human reason or will, for these can suffer change and are defective; whereas things capable of motion and of defect must be traced back to an immovable and self-necessary first principle.