Thomas Aquinas claims that the existence of God can be proven in five ways. I will first quote the text from the Summa, and then outline the five arguments.
Summa Theologiae, Question 2, Article 3.
It seems that God does not exist, for if one of two contrary things were infinite, its opposite would be completely destroyed. By “God,” however, we mean some infinite good. Therefore, if God existed evil would not. Evil does exist in the world, however. Therefore God does not exist.
Furthermore, one should not needlessly multiply elements in an explanation. It seems that we can account for everything we see in this world on the assumption that God does not exist. All natural effects can be traced to natural causes, and all contrived effects can be traced to human reason and will. Thus there is no need to suppose that God exists.
But on the contrary God says, “I am who I am” (Ex. 3:14).
Response: It must be said that God’s existence can be proved in five ways. The first and most obvious way is based on the existence of motion. It is certain and in fact evident to our senses that some things in the world are moved. Everything that is moved, however, is moved by something else, for a thing cannot be moved unless that movement is potentially within it. A thing moves something else insofar as it actually exists, for to move something is simply to actualize what is potentially within that thing. Something can be led thus from potentiality to actuality only by something else which is already actualized. For example, a fire, which is actually hot, causes the change or motion whereby wood, which is potentially hot, becomes actually hot. Now it is impossible that something should be potentially and actually the same thing at the same time, although it could be potentially and actually different things. For example, what is actually hot cannot at the same moment be actually cold, although it can be actually hot and potentially cold. Therefore it is impossible that a thing could move itself, for that would involve simultaneously moving and being moved in the same respect. Thus whatever is moved must be moved by something, else, etc. This cannot go on to infinity, however, for if it did there would be no first mover and consequently no other movers, because these other movers are such only insofar as they are moved by a first mover. For example, a stick moves only because it is moved by the hand. Thus it is necessary to proceed back to some prime mover which is moved by nothing else, and this is what everyone means by “God.”
The second way is based on the existence of efficient causality. We see in the world around us that there is an order of efficient causes. Nor is it ever found (in fact it is impossible) that something is its own efficient cause. If it were, it would be prior to itself, which is impossible. Nevertheless, the order of efficient causes cannot proceed to infinity, for in any such order the first is cause of the middle (whether one or many) and the middle of the last. Without the cause, the effect does not follow. Thus, if the first cause did not exist, neither would the middle and last causes in the sequence. If, however, there were an infinite regression of efficient causes, there would be no first efficient cause and therefore no middle causes or final effects, which is obviously not the case. Thus it is necessary to posit some first efficient cause, which everyone calls “God.”
The third way is based on possibility and necessity. We find that some things can either exist or not exist, for we find them springing up and then disappearing, thus sometimes existing and sometimes not. It is impossible, however, that everything should be such, for what can possibly not exist does not do so at some time. If it is possible for every particular thing not to exist, there must have been a time when nothing at all existed. If this were true, however, then nothing would exist now, for something that does not exist can begin to do so only through something that already exists. If, therefore, there had been a time when nothing existed, then nothing could ever have begun to exist, and thus there would be nothing now, which is clearly false. Therefore all beings cannot be merely possible. There must be one being which is necessary. Any necessary being, however, either has or does not have something else as the cause of its necessity. If the former, then there cannot be an infinite series of such causes, any more than there can be an infinite series of efficient causes, as we have seen. Thus we must to posit the existence of something which is necessary and owes its necessity to no cause outside itself. That is what everyone calls “God.”
The fourth way is based on the gradations found in things. We find that things are more or less good, true, noble, etc.; yet when we apply terms like “more” and “less” to things we imply that they are closer to or farther from some maximum. For example, a thing is said to be hotter than something else because it comes closer to that which is hottest. Therefore something exists which is truest, greatest, noblest, and consequently most fully in being; for, as Aristotle says, the truest things are most fully in being. That which is considered greatest in any genus is the cause of everything is that genus, just as fire, the hottest thing, is the cause of all hot things, as Aristotle says. Thus there is something which is the cause of being, goodness, and every other perfection in all things, and we call that something “God.”
The fifth way is based on the governance of things. We see that some things lacking cognition, such as natural bodies, work toward an end, as is seen from the fact hat they always (or at least usually) act the same way and not accidentally, but by design. Things without knowledge tend toward a goal, however, only if they are guided in that direction by some knowing, understanding being, as is the case with an arrow and archer. Therefore, there is some intelligent being by whom all natural things are ordered to their end, and we call this being “God.”
To the first argument, therefore, it must be said that, as Augustine remarks, “since God is the supreme good he would permit no evil in his works unless he were so omnipotent and good that he could produce good even out of evil.”
To the second, it must be said that, since nature works according to a determined end through the direction of some superior agent, whatever is done by nature must be traced back to God as its first cause. in the same way, those things which are done intentionally must be traced back to a higher cause which is neither reason nor human will, for these can change and cease to exist and, as we have seen, all such things must be traced back to some first principle which is unchangeable and necessary, as has been shown.”
Argument Analysis of the Five Ways
The First Way: Argument from Motion
- Our senses prove that some things are in motion.
- Things move when potential motion becomes actual motion.
- Only an actual motion can convert a potential motion into an actual motion.
- Nothing can be at once in both actuality and potentiality in the same respect (i.e., if both actual and potential, it is actual in one respect and potential in another).
- Therefore nothing can move itself.
- Therefore each thing in motion is moved by something else.
- The sequence of motion cannot extend ad infinitum.
- 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: Argument from Efficient Causes
- We perceive a series of efficient causes of things in the world.
- Nothing exists prior to itself.
- Therefore nothing is the efficient cause of itself.
- If a previous efficient cause does not exist, neither does the thing that results.
- Therefore if the first thing in a series does not exist, nothing in the series exists.
- The series of efficient causes cannot extend ad infinitum into the past, for then there would be no things existing now.
- Therefore it is necessary to admit a first efficient cause, to which everyone gives the name of God.
The Third Way: Argument from Possibility and Necessity (Reductio argument)
- We find in nature things that are possible to be and not to be, that come into being and go out of being i.e., contingent beings.
- Assume that every being is a contingent being.
- For each contingent being, there is a time it does not exist.
- Therefore it is impossible for these always to exist.
- Therefore there could have been a time when no things existed.
- Therefore at that time there would have been nothing to bring the currently existing contingent beings into existence.
- Therefore, nothing would be in existence now.
- We have reached an absurd result from assuming that every being is a contingent being.
- Therefore not every being is a contingent being.
- Therefore some being exists of its own necessity, and does not receive its existence from another being, but rather causes them. This all men speak of as God.
The Fourth Way: Argument from Gradation of Being
- There is a gradation to be found in things: some are better or worse than others.
- Predications of degree require reference to the “uttermost” case (e.g., a thing is said to be hotter according as it more nearly resembles that which is hottest).
- The maximum in any genus is the cause of all in that genus.
- 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: Argument from Design
- We see that natural bodies work toward some goal, and do not do so by chance.
- Most natural things lack knowledge.
- But as an arrow reaches its target because it is directed by an archer, what lacks intelligence achieves goals by being directed by something intelligence.
- Therefore some intelligent being exists by whom all natural things are directed to their end; and this being we call God.