I answer that it can be proved in five ways that God exists.
The first and plainest is the method that proceeds from the point of view of motion. It is certain and in accord with experience, that things on earth undergo change. Now, everything that is moved is moved by something; nothing, indeed, is changed, except it is changed to something which it is in potentiality. Moreover, anything moves in accordance with something actually existing; change itself, is nothing else than to bring forth something from potentiality into actuality. Now, nothing can be brought from potentiality to actual existence except through something actually existing: thus heat in action, as fire, makes fire-wood, which is hot in potentiality, to be hot actually, and through this process, changes itself. The same thing cannot at the same time be actually and potentially the same thing, but only in regard to different things. What is actually hot cannot be at the same time potentially hot, but it is possible for it at the same time to be potentially cold. It is impossible, then, that anything should be both mover and the thing moved, in regard to the same thing and in the same way, or that it should move itself. Everything, therefore, is moved by something else. If, then, that by which it is moved, is also moved, this must be moved by something still different, and this, again, by something else. But this process cannot go on to infinity because there would not be any first mover, nor, because of this fact, anything else in motion, as the succeeding things would not move except because of what is moved by the first mover, just as a stick is not moved except through what is moved from the hand. Therefore it is necessary to go back to some first mover, which is itself moved by nothing---and this all men know as God.
The second proof is from the nature of the efficient cause. We find in our experience that there is a chain of causes: nor is it found possible for anything to be the efficient cause of itself, since it would have to exist before itself, which is impossible. Nor in the case of efficient causes can the chain go back indefinitely, because in all chains of efficient causes, the first is the cause of the middle, and these of the last, whether they be one or many. If the cause is removed, the effect is removed. Hence if there is not a first cause, there will not be a last, nor a middle. But if the chain were to go back infinitely, there would be no first cause, and thus no ultimate effect, nor middle causes, which is admittedly false. Hence we must presuppose some first efficient cause---which all call God.
The third proof is taken from the natures of the merely possible and necessary. We find that certain things either may or may not exist, since they are found to come into being and be destroyed, and in consequence potentially, either existent or non-existent. But it is impossible for all things that are of this character to exist eternally, because what may not exist, at length will not. If, then, all things were merely possible (mere accidents), eventually nothing among things would exist. If this is true, even now there would be nothing, because what does not exist, does not take its beginning except through something that does exist. If then nothing existed, it would be impossible for anything to begin, and there would now be nothing existing, which is admittedly false. Hence not all things are mere accidents, but there must be one necessarily existing being. Now every necessary thing either has a cause of its necessary existence, or has not. In the case of necessary things that have a cause for their necessary existence, the chain of causes cannot go back infinitely, just as not in the case of efficient causes, as proved. Hence there must be presupposed something necessarily existing through its own nature, not having a cause elsewhere but being itself the cause of the necessary existence of other things---which all call God.
The fourth proof arises from the degrees that are found in things. For there is found a greater and a less degree of goodness, truth, nobility, and the like. But more or less are terms spoken of various things as they approach in diverse ways toward something that is the greatest, just as in the case of hotter (more hot) which approaches nearer the greatest heat. There exists therefore something that is the truest, and best, and most noble, and in consequence, the greatest being. For what are the greatest truths are the greatest beings, as is said in the Metaphysics Bk. II. 2. What moreover is the greatest in its way, in another way is the cause of all things of its own kind (or genus); thus fire, which is the greatest heat, is the cause of all heat, as is said in the same book (cf. Plato and Aristotle). Therefore there exists something that is the cause of the existence of all things and of the goodness and of every perfection whatsoever---and this we call God.
The fifth proof arises from the ordering of things for we see that some things which lack reason, such as natural bodies, are operated in accordance with a plan. It appears from this that they are operated always or the more frequently in this same way the closer they follow what is the Highest; whence it is clear that they do not arrive at the result by chance but because of a purpose. The things, moreover, that do not have intelligence do not tend toward a result unless directed by some one knowing and intelligent; just as an arrow is sent by an archer. Therefore there is something intelligent by which all natural things are arranged in accordance with a plan---and this we call God.
