In Part I, Questions 1-13 of Summa Theologica, Aquinas propounds five arguments for the existence of God and then discusses the essence of God.
The first argument for the existence of God is the following: some things in the world are in motion. Whatever is in motion is put in motion by another. However, there must be a first mover that is not put in motion by another; otherwise, the chain of events would go on to infinity, which is impossible. This unmoved mover is God. This first argument tells us nothing about the essence of God other than that he is an unmoved mover. There might be several unmoved movers in the world. Certain matter or energies might be unmoved movers. The Christian notion of God is very different from the mere description of an unmoved mover.
The second argument is the following: in the world there are causes and effects. The chain of causes and effects cannot go on to infinity; for there will be no first cause, which means there will be no intermediate causes and no ultimate causes. Thus, there must be a first cause, which is God. This second argument, like the first, tells us very little about the essence of God. According to this argument, God is the first cause of all causes. However, it is possible to imagine many first causes. In other words, one can imagine a number of chains of causes and effects beginning and merging with each other.
The third argument is the following: there are things that can exist and not exist. Things that have the potential to not exist cannot always exist. Therefore, at one time, there could have been nothing in existence if all things potentially might not exist. Nothing comes from nothing; therefore, something’s existence must be necessary and eternal. That thing is God. This third argument is similar to the first and second arguments, and it suffers from the same ambiguity.
The fourth argument is the following: there are gradations among beings. Some things are more or less hot than others, more or less noble than others, and more or less true than others. ‘More’ and ‘less’ are predicated based upon a thing’s relation to a maximum. A thing ‘X’ is hotter than ‘Y’ when ‘X’ more closely resembles that which is hottest than ‘Y’ does. As the maximum in any genus is the cause of all in that genus (fire is the maximum of hot and the cause of heat in all things), the maximum of truth, which is the maximum of being, is the cause of all being. The thing which is the truest is God. This fourth argument is similar to Plato’s Idea of the Forms. Plato asserts that a perfect ideal exists for every notion and the degree to which an object partakes of that ideal is the degree to which it is that ideal. For example, Beauty is an Ideal. Objects that partake in the Ideal of Beauty to a higher degree than other objects are said to be more beautiful than the other objects. I do not believe that the maximum of a genus is the cause of all things in the genus. Aquinas’ example of fire is clearly wrong. Heat is not caused by fire, heat in an object is cause by thermal energy being applied to an object.
The fifth argument is the following: there are things which lack intelligence, but they move toward an end nearly always in the same way to obtain the best result. This is the result of design, not chance. Therefore, some intelligent being must move unintelligent bodies toward their ends. This intelligent being is God. This fifth argument is flawed for two reasons. The first is that man often moves unintelligent bodies toward ends. Does this make man God? The second reason is that man is a pattern-finding and meaning-generating animal. Man will often impose meaning on an unintentional system. For example, a single true dice will roll a 5 once every six rolls. Many men will interpret some unintended meaning or agent behind this phenomenon. Actually, it is the result of pure chance due to the physical properties of the dice.
Demonstration can be made in two ways: One is through the cause, and is called “a priori,” and this is to argue from what is prior absolutely. The other is through the effect, and is called a demonstration “a posteriori”; this is to argue from what is prior relatively only to us. When an effect is better known to us than its cause, from the effect we proceed to the knowledge of the cause. And from every effect the existence of its proper cause can be demonstrated, so long as its effects are better known to us; because since every effect depends upon its cause, if the effect exists, the cause must pre-exist. Hence the existence of God, in so far as it is not self-evident to us, can be demonstrated from those of His effects which are known to us.
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.