St Thomas Aquinas (1224-1274), an Italian Dominican priest, was one of the greatest philosophers and theologians that ever lived. Over the centuries his writings have had a profound influence on Christian, especially Catholic, thought. Among his many brilliant and extensive works, the Summa Theologiae, in which he sets out systematically to explain Christian theology, is considered one of the greatest.

Aquinas believed certain theological truths, including the existence of God, can be established by the use of reason. In Part One of Summa Theologiae, in Article 3 (titled Whether God Exists?), Aquinas presents the Quinquae viae, the Five Ways to demonstrate the existence of God.

Below you will find, first, an attempt to summarize the Five Ways of St Thomas. This is followed by the same arguments expressed in Aquinas's own words, taken directly from Summa Theologiae.

 

The Five Ways

1. Motion

"The first and most manifest way," writes Aquinas, "is the argument from motion."

This argument centers on the fact that nothing in the universe can move itself. We are all aware, writes Aquinas, that things in motion require to be "moved by another." The pins in the bowling alley, for example, are knocked over, set in motion by the ball. The ball itself is in motion, traveling down the lane as a result of the force applied by the motion of the bowler's arm. The word 'motion' as used here also includes, not only change involving movement, but also change of state. Wood, for example, will change its state when fire is applied to it. It will become hot. Wood always has the potential to become hot, just as the pins in the bowling alley have the potential to move, but the wood requires something that is actually hot, such as fire, to change it, just as the pins require something that is actually moving in order to change their position.

Here lies the crux of Aquinas's argument; namely, that "nothing can be reduced from potentiality to actuality, except by something in a state of actuality." The pins in the bowling alley, which have the potential to move, cannot be moved by a ball lying stationary in the gutter, which also possesses only the potential to move. The pins can be moved only by a ball which is moving in actuality.

By themselves, the pins in the bowling alley cannot simultaneously have the potential to move and yet actually move at the same time. It is impossible, as Aquinas says, for a thing be both the mover and the moved; that is, the pins cannot move themselves. "Therefore," says Aquinas, "whatever is moved must be moved by another," and in turn that 'another' requires to be moved by yet another again, and so on.

This cannot go on for eternity, says Aquinas, otherwise "there would be no first mover, and consequently, no other mover, seeing that subsequent movers move only inasmuch as they are move by the first mover.."

That First Mover, says Aquinas, is God.

2. Efficient Cause

"The second way," writes Aquinas," is from the nature of efficient cause."

The argument used here, which relates to the causation of existence, is somewhat similar to the argument from motion. There never has been a case, states Aquinas, of a thing being the cause of its own existence. For that to happen, that thing would have to exist prior to itself, which is impossible. Everything has to have a cause, and if you take away the cause, you take away the effect. Every cause has to be caused by another, which in turn has to be caused by yet another, and if you trace the series of causes back in time, you cannot go on for eternity, but must reach the first event in the series.

That first event is the "first efficient cause," says Aquinas, "to which everyone gives the name of God."

3. Possibility and Necessity

"The third way is taken from possibility and necessity, " writes Aquinas, " and runs thus."

Things in nature, Aquinas says, have the possibility of existing or not existing. He states that things "are found to be generated, and to be corrupted...," meaning that things come into being at a certain time (are generated), and cease to exist at a later time (are corrupted). If everything were like this, capable of not existing, then "at one time there was nothing in existence." This can not be true, however, for if at one time there was nothing in existence, and since something cannot come out of nothing, then today there would be nothing in existence. There must therefore exist a necessary being; that is, a being which cannot possibly not exist, and which causes the existence of other things or beings.

"This all men speak of as God," writes Aquinas.

4. Gradation

"The fourth way is taken from the gradation to be found in things," says Aquinas.

We make judgments about the relative perfection of different things, including the attributes of people. In making such judgments, we are making comparisons, indicating that one thing or person has a greater or lesser degree of some characteristic or quality. "Among beings," writes Aquinas, "there are some more and some less good, true, noble, and the like." We make these comparisons "according as they resemble in their different ways something which is the maximum...". In other words, somewhere there must be some perfect standard against which all things are measured. The most perfect in any category, writes Aquinas, is the cause of all in that category, just as fire, the maximum of heat, is the cause of all hot things.

"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," concludes Aquinas.

5. Governance of the World

"The fifth way is taken from the governance of the world," writes Aquinas.

This is often referred to as the argument from 'intelligent design'. Natural things, says Aquinas, which themselves have no intelligence, act in an intelligent manner to achieve their end, "so as to obtain the best result." This does not happen by chance, he says, but by design. Natural things are being directed by a being with knowledge and intelligence.

"Therefore some intelligent being exists by whom all natural things are directed to their end; and this being we call God."

Summa Theologiae (Part One, Article 3): Whether God Exists?
by St Thomas Aquinas

The existence of God can be proved in five ways.

The first and more manifest way is the argument from motion. It is certain, and evident to our senses, that in the world some things are in motion. Now whatever is in motion is put in motion by another, for nothing can be in motion except it is in potentiality to that towards which it is in motion; whereas a thing moves inasmuch as it is in act. For motion is nothing else than the reduction of something from potentiality to actuality. But nothing can be reduced from potentiality to actuality, except by something in a state of actuality. Thus that which is actually hot, as fire, makes wood, which is potentially hot, to be actually hot, and thereby moves and changes it. Now it is not possible that the same thing should be at once in actuality and potentiality in the same respect, but only in different respects. For what is actually hot cannot simultaneously be potentially hot; but it is simultaneously potentially cold. It is therefore impossible that in the same respect and in the same way a thing should be both mover and moved, i.e. that it should move itself. Therefore, whatever is in motion must be put in motion by another. If that by which it is put in motion be itself put in motion, then this also must needs be put in motion by another, and that by another again. But this cannot go on to infinity, because then there would be no first mover, and, consequently, no other mover; seeing that subsequent movers move only inasmuch as they are put in motion by the first mover; as the staff moves only because it is put in motion by the hand. 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 is from the nature of the efficient cause. In the world of sense we find there is an order of efficient causes. There is no case known (neither is it, indeed, possible) in which a thing is found to be the efficient cause of itself; for so it would be prior to itself, which is impossible. Now in efficient causes it is not possible to go on to infinity, because in all efficient causes following in order, the first is the cause of the intermediate cause, and the intermediate is the cause of the ultimate cause, whether the intermediate cause be several, or only one. Now to take away the cause is to take away the effect. Therefore, if there be no first cause among efficient causes, there will be no ultimate, nor any intermediate cause. But if in efficient causes it is possible to go on to infinity, there will be no first efficient cause, neither will there be an ultimate effect, nor any intermediate efficient causes; all of which is plainly false. Therefore it is necessary to admit a first efficient cause, to which everyone gives the name of God.

The third way is taken from possibility and necessity, and runs thus. We find in nature things that are possible to be and not to be, since they are found to be generated, and to corrupt, and consequently, they are possible to be and not to be. But it is impossible for these always to exist, for that which is possible not to be at some time is not. Therefore, if everything is possible not to be, then at one time there could have been nothing in existence. Now if this were true, even now there would be nothing in existence, because that which does not exist only begins to exist by something already existing. Therefore, if at one time nothing was in existence, it would have been impossible for anything to have begun to exist; and thus even now nothing would be in existence--which is absurd. Therefore, not all beings are merely possible, but there must exist something the existence of which is necessary. But every necessary thing either has its necessity caused by another, or not. Now it is impossible to go on to infinity in necessary things which have their necessity caused by another, as has been already proved in regard to efficient causes. Therefore we cannot but postulate the existence of some being having of itself its own necessity, and not receiving it from another, but rather causing in others their necessity. This all men speak of as God.

The fourth way is taken from the gradation to be found in things. Among beings there are some more and some less good, true, noble and the like. But "more" and "less" are predicated of different things, according as they resemble in their different ways something which is the maximum, as a thing is said to be hotter according as it more nearly resembles that which is hottest; so that there is something which is truest, something best, something noblest and, consequently, something which is uttermost being; for those things that are greatest in truth are greatest in being, as it is written in Metaph. ii. Now the maximum in any genus is the cause of all in that genus; as fire, which is the maximum heat, is the cause of all hot things. 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 is taken from the governance of the world. We see that things which lack intelligence, such as natural bodies, act for an end, and this is evident from their acting always, or nearly always, in the same way, so as to obtain the best result. Hence it is plain that not fortuitously, but designedly, do they achieve their end. Now whatever lacks intelligence cannot move towards an end, unless it be directed by some being endowed with knowledge and intelligence; as the arrow is shot to its mark by the archer. Therefore some intelligent being exists by whom all natural things are directed to their end; and this being we call God.