The following is an excerpt from Proofs of God by Matthew Levering.
In his Summa theologiae, Aquinas very briefly offers five ways of demonstrating God’s existence. These are found in question 2, article 3 of the Prima Pars.
The first way is the argument from motion or change. Aquinas states that “motion is nothing else than the reduction of something from potentiality to actuality,” and he adds that “nothing can be reduced from potentiality to actuality, except by something in a state of actuality.” The same thing cannot be in potency and actuality in the same respect, and so a thing cannot be both mover and moved in the same respect. This shows that “whatever is in motion must be put in motion by another,” but this cannot proceed to infinity, since if there is no first mover, there can be no intermediate movers either. The first unmoved mover, who is pure actuality and the source of all act and potency composites, is God.
The second way is the argument from efficient causality, understood again in terms of act and potency. Nothing is the efficient cause of its own finite actuality (or act of existing), and it is not possible to proceed to infinity in essentially ordered efficient causes, since without a first cause—which itself needs no efficient cause of its act of existing and is therefore pure actuality—there can be no intermediate causes and no ultimate effect. Since there obviously are intermediate causes and an ultimate effect, there must be a first cause, which is God.
Necessarily, then at some time (given infinite time on an endless continuum) everything would have not existed, since “that which is possible to be at some time is not.” If so, then there would now be nothing in existence, since nothing can come from nothing. The fact that something now exists, therefore, means that there must be some thing or things whose existence is necessary. As shown by the argument from efficient causality (the second way), it is impossible to proceed to infinity in necessary things that are caused by another. There must be one uncaused necessary being that causes all others, and this is God.
The fourth way is from the degrees of perfection found in finite things: “Among beings there are some more and some less good, true, noble, and the like.” The predication of “more” or “less” good or true requires that there be a measure of the degree to which something “resembles” goodness or truth. This measure must be maximal goodness or truth, for otherwise it would itself be measured rather than being the measure. For a maximum in perfection to exist, it must be maximal actuality, “for those things that are greatest in truth are greatest in being.” This maximum, as perfect actuality, “is to all beings the cause of their being, goodness, and every other perfection.”
The fifth and final way comes from the governance of the world. Nonrational things cannot direct themselves to an end, and yet nonrational things in the universe generally repeat the same actions to achieve the same ends. This could not be the result of chance. Thus nonrational things are ordered to their ends by an intelligent orderer who, as the one who orders this-worldly things to their end, transcends and governs this world.
©2016 by Matthew Levering. Published by Baker Academic. Unauthorized use of this material without express written permission is strictly prohibited.
For more information on Proofs of God, click here.