The Summa Theologiae of Thomas Aquinas

Question 2: Concerning God, whether God exists

As to this question, there are three points of inquiry:

1. Whether God’s existence is self-evident

2. Whether it can be demonstrated

3. Whether God exists.
Article 1: Whether God’s existence is self-evident.

Thus we proceed to the first point. It seems that God’s existence is self-evident, for those things are said by us to be self-evident the knowledge of which is naturally within us, as is the case with first principles. But, as John of Damascus says, “The knowledge of God’s existence is naturally implanted in all things.” Therefore God’s existence is self-evident.

Furthermore, those things are said to be self-evident the truth of which is obvious once the meaning of the words is clear. For example, when we understand the means of the words “whole” and “part,” we immediately realize that every whole is greater than its part. Once we understand the meaning of the word “God,” however, it immediately follows that God exists. The words itself signifies “that being a greater than which cannot be signified.” That which exists in fact and in the mind is greater than that which exists in the mind alone. Thus, since the moment we understand the meaning of the word “God” he exists in our minds, it follows that he must also exist in fact. Thus God’s existence is self- evident.

Furthermore, it is self-evident that truth exists, for whoever denies the existence of truth simultaneously concedes its existence. If truth does not exist, then it is true that truth does not exist; yet if something is true, then truth exists. God, however, is truth itself. “I am the way, the truth and the life” (Jn. 14:6). Therefore God’s existence is self-evident.

But on the contrary, no one can think the opposite of what is self-evident, as Aristotle remarks. One can, however, think the opposite of the proposition “God exists,” for, as the Psalm says, “The fool says in his heart, ‘there is no God.” (Ps. 13:1, 52:1). Thus it is not self-evident that God exists.

Response: It must be said that a thing can be called “self-evident” in two- ways, in itself and in relation to us. A proposition is self-evident when its predicate is included in the definition of its subject. For example, in the proposition “man is an animal,” the idea of “animal” is included in the definition of “man.” Thus if everyone knows the definitions of both subject and predicate, the proposition will be self-evident to all, as is the case with the first principles of demonstration, the terms of which are so common that no one is ignorant of them, such as “being” and “nonbeing,” “whole” and “part,” etc. If, the proposition may be self-evident in itself, but not to them. Thus it happens, as Boethius says, that some things are common conceptions of the mind” and are self-evident “among the learned only, such as that incorporeal beings do not occupy a place.”

I say, therefore, that this proposition, “God exists,” is self-evident in itself, since the predicate is the same as the subject. For God is his own existence, as will be seen later. Nevertheless, because we do not know what is involved in being God, the proposition is not self-evident to us, but needs to be demonstrated through those things that are more evident to us though less evident to themselves, namely God’s effects.

To the first argument, therefore, it must be said that a general and confused knowledge of God’s existence is naturally infused within us, for God is man’s beatitude and man naturally desires beatitude. What man naturally desires he naturally knows. This is not to know God’s existence specifically, however. It is one thing to know that someone is approaching and quite another to know that Peter is approaching, even though that someone may actually be Peter. Many people think that the perfect good of man called “beatitude” is wealth, some imagine it to be pleasure, and so on.

To the second argument it must be said that he who hears the name “God” may perhaps not know that it signifies “something greater than which cannot be conceived,” since some people have thought of God as a body. Granting, however, that someone should think of God in this way, namely as “that being a greater than which cannot be conceived, “it does not follow on this account that the person must understand what is signified to exist in the world of fact, but only in the mind. Nor can one argue that it exists in fact unless one grants that there actually exists in fact something a greater than which cannot be conceived. It is, however, precisely this assertion the atheist denies.

To the third, it must be said that the existence of truth in general is self- evident to us, but it is not self-evident that this particular being is the primal truth.

Article 2: Whether God’s existence is demonstrable.

We proceed thus to the second point. It seems that God’s existence is not demonstrable, for it is an article of faith. What is a matter of faith cannot be demonstrable, for demonstration allows one to know, whereas faith, as Paul says, is in “things not seen” (Heb. 11:1). Therefore God’s existence is not demonstrable.
Furthermore, the central link in any demonstration is a definition; yet we cannot know what God is, but only what he is not, as John of Damascus says. Therefore we cannot demonstrate God’s existence.

Furthermore, if God’s existence were demonstrable, this could only be through his effects; yet his effects are not proportionate to him, for he is infinite, his effects are infinite, and there is no proportion between the two. Therefore, since a cause cannot be demonstrated through an effect which is not proportioned to it, it seems that God’s existence cannot be demonstrated.

But on the contrary Paul says, “The invisible things of God are understood by the things that are made” (Romans 1:20). Such could not be the case unless God’s existence could be demonstrated by the things that are made, for the first thing to be understood about a thing is whether it exists.

Response: It must be said that there are two types of demonstration. One is through the cause, is called a demonstration propter quid, and argues from what is prior in an absolute sense. The other is through the effect, is called a demonstration quia, and argues from what is prior according to our perspectives; for when an effect is better known to us than its cause, we proceed from the effect to knowledge of the cause. In situations where the effect is better know to us than the cause, the existence of the cause can be demonstrated form that of the effect, since the effect depends on the cause and can only exist if the cause already does so. Thus God’s existence, though not self-evident to us, can be demonstrated through his effects.

To the first argument, therefore, it must be said that God’s existence and other things about him which (as Paul says) can be known by natural reason are not articles of faith but preambles to the articles of faith. For faith presupposes natural knowledge just as grace presupposes nature and perfection presupposes something which can be perfected. Nothing prohibits what is demonstrable and knowable in itself from being accepted on faith by someone who does not understand the demonstration.

To the second it must be said that, when a cause is demonstrated through its effect, the effect substitutes for the definition of the cause within the demonstration. This is particularly true in arguments concerning God. When we prove that something exists, the middle term in the demonstration is what we are taking the word to mean for purposes of the demonstration, not what the thing signified by the word actually is (since the latter, the actual nature of the thing in question, is determined only after we determine that it exists). In demonstrating that God exists, we can take as our middle term definition of what this word “God” means for us, for, as we shall see, the words we use in connection with God are derived from his effects.

To the third, it must be said that perfect knowledge of a cause cannot be derived from an effect that is not proportionate to the cause. Nevertheless, the existence of the cause can be demonstrated clearly from the existence of the effects, even though we cannot know the cause perfectly according to its essence.

Article 3: Whether God exists.

Thus we proceed to the third point. 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.