We proceed to the third article thus:

1. It seems that God does not exist. If one of two contraries were to be infinite, the other would be wholly excluded. Now the name "God" means that he is infinite good. There would therefore be no evil if God were to exist. But there is evil in the world. It follows that God does not exist.

2. Again, what can be explained by comparatively few principles is not the consequence of a greater number of principles. Now if we suppose that God does not exist, it appears that we can still account for all that we see in the world by other principles, attributing all natural things to nature as their principle, and all that is purposive to human reason or will. There is therefore no need to suppose that God exists.

On the other hand: in Ex.3:14 God says in person: "I AM THAT I AM."

I answer: God's existence can be proved in five ways. The first and clearest proof is the argument from motion. [10] It is certain, and in accordance with sense experience, that some things in this world are moved. Now everything that is moved is moved by something else, since nothing is moved unless it is potentially that to which it is moved, whereas that which moves is actual. To move is nothing other than to bring something from potentiality to actuality, and a thing can be brought from potentiality to actuality only by something which is actual. Thus a fire, which is actually hot, makes wood, which is potentially hot, to be actually hot, so moving and altering it. Now it is impossible for the same thing to be both actual and potential in the same respect, although it may be so in different respects. What is actually hot cannot at the same time be potentially hot, although it is potentially cold. It is therefore impossible that, in the same respect and in the same way, anything should be both mover and moved, or that it should move itself. Whatever is moved must therefore be moved by something else. If, then, that by which it is moved is itself moved, this also must be moved by something else, and this in turn by something else again. But this cannot go on for ever, since there would then be no first mover, and consequently no other mover, because secondary movers cannot move unless moved by a first mover, as a staff cannot move unless it is moved by the hand. We are therefore bound to arrive at a first mover which is not moved by anything, and all men understand that this is God.

The second way is from the nature of an efficient cause. We find that there is a sequence of efficient causes in sensible things. But we do not find that anything is the efficient cause of itself. Nor is this possible, for the thing would then be prior to itself, which is impossible. But neither can the sequence of efficient causes be infinite, for in every sequence the first efficient cause is the cause of an intermediate cause, and an intermediate cause is the cause of the ultimate cause, whether the intermediate causes be many, or only one. Now if a cause is removed, its effect is removed. Hence if there were no first efficient cause, there would be no ultimate cause, and no intermediate cause. But if the regress of efficient causes were infinite, there would be no first efficient cause. There would consequently be no ultimate effect, and no intermediate causes. But this is plainly false. We are therefore bound to suppose that there is a first efficient cause. And all men call this God.

The third way is from the nature of possibility and necessity. There are some things which may either exist or not exist, since some things come to be and pass away, and may therefore be or not be. Now it is impossible that all of these should exist at all times, because there is at least some time when that which may possibly not exist does not exist. Hence if all things were such that they might not exist, at some time or other there would be nothing. But if this were true there would be nothing existing now, since what does not exist cannot begin to exist, unless through something which does exist. If there had been nothing existing, it would have been impossible for anything to begin to exist, and there would now be nothing at all. But this is plainly false, and hence not all existence is merely possible. Something in things must be necessary. Now everything which is necessary either derives its necessity from elsewhere, or does not. But we cannot go on to infinity with necessary things which have a cause of their necessity, any more than with efficient causes, as we proved. We are therefore bound to suppose something necessary in itself, which does not owe its necessity to anything else, but which is the cause of the necessity of other things. And all men call this God.

The fourth way is from the degrees that occur in things, which are found to be more and less good, true, noble, and so on. Things are said to be more and less because they approximate in different degrees to that which is greatest. A thing is the more hot the more it approximates to that which is hottest. There is therefore something which is the truest, the best, and the noblest, and which is consequently the greatest in being, since that which has the greatest truth is also greatest in being, as is said in 2 Metaph., text 4. Now that which most thoroughly possesses the nature of any genus is the cause of all that the genus contains. Thus fire, which is most perfectly hot, is the cause of all hot things, as is said in the same passage. There is therefore something which is the cause of the being of all things that are, as well as of their goodness and their every perfection. This we call God.

The fifth way is from the governance of things. We see how some things, like natural bodies, work for an end even though they have no knowledge. The fact that they nearly always operate in the same way, and so as to achieve the maximum good, makes this obvious, and shows that they attain their end by design, not by chance. Now things which have no knowledge tend towards an end only through the agency of something which knows and also understands, as an arrow through an archer. There is therefore an intelligent being by whom all natural things are directed to their end. This we call God.

On the first point: as Augustine says (Enchirid.11): "since God is supremely good, he would not allow any evil thing to exist in his works, were he not able by his omnipotence and goodness to bring good out of evil." God's infinite goodness is such that he permits evil things to exist, and brings good out of them.

On the second point: everything that can be attributed to nature must depend on God as its first cause, since nature works for a predetermined end through the direction of a higher agent. Similarly, whatever is due to purpose must depend on a cause higher than the reason or will of man, since these are subject to change and defect. Anything which is changeable and subject to defect must depend on some first principle which is immovable and necessary in itself, as we have shown.

[10] This paragraph may be compared with Aristotle's Physics, bk. 7, ch. 1, 242a; bk. 8, ch. 4, 254b, ch. 5, 256a. Cf. also S. Contra Gentiles I, ch. 13, which contains all except the third way. The third way is contained with slight variations in ibid. I, ch. 15, II, ch. 15.