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The first and clearest way is the argument from motion. It is certain and evident to our senses that things are in motion in the world. Now, whatever is in motion is put in motion by another, for nothing can move itself, except insofar as it is in potentiality to that towards which it is moved, while that which moves acts insofar as it is already in actuality. But motion means nothing else than the transition from potentiality to actuality. Yet nothing can bring itself from potentiality to actuality, but only something that is already actual. For example, what is actually hot—such as fire—makes wood, which is only potentially hot, actually hot, and thereby causes motion and change. It is impossible that the same thing should be at once in actuality and in potentiality in the same respect, but only in different respects. For what is actually hot cannot at the same time be potentially hot, but only potentially cold. Therefore, it is impossible that the same thing should be at once, in the same respect and in the same way, both mover and moved—that is, that it should move itself. Therefore, whatever is in motion must be moved by another.
But if the mover itself is moved, then it too must be moved by another, and so on. However, this cannot go on to infinity, for if there were no first mover, there would be no subsequent movers: all subsequent movers move only insofar as they are moved by the first mover, as a stick moves only because it is moved by the hand. Therefore, it is necessary to arrive at a first unmoved mover, moved by no other. And this everyone calls God.
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The second way is based on the nature of efficient cause. In the world of sense, we find an order of efficient causes. There is no case—nor is it possible in principle—in which a thing is the efficient cause of itself, for then it would be prior to itself, which is impossible. Now, in efficient causes, it is not possible to go on to infinity, because in every series of efficient causes, the first is the cause of the intermediate, and the intermediate is the cause of the last—whether there are several intermediates or only one. But if the first cause is removed, so is the effect. Therefore, if there is no first efficient cause among efficient causes, there will be neither an ultimate effect nor any intermediate efficient causes. But if in efficient causes it is possible to go on to infinity, there would be no first efficient cause, and consequently no ultimate effect nor any intermediate causes—which is plainly false. Therefore, it is necessary to admit a first efficient cause, which is itself uncaused by any other. And this everyone calls God.