Thomas Aquinas: Summa Contra Gentiles

Chapter 3: On the way in which the divine truth is to be known
Some of our beliefs about God are beyond the ability of our nature to comprehend through reason, while others are naturally comprehensible. An example of the former is that God is three in one (triune). Examples of the latter are that God exists and that God is one. Our knowledge of God is limited by the fact that our knowledge derives from the senses, and the objects of our senses (the physical world) fall short of the notion of a divine substance. But they can indicate at least that God is, even if not what God is.

Furthermore, we often do not understand even the nature of sensible things, so how could we understand the nature of something that can never be the object of the senses?

We must acknowledge that there are some aspects of God which reason cannot comprehend, but this is not sufficient grounds for rejecting as false those things said about God (such as that God is triune).

Chapter 4: That the truth about God to which the natural reason reaches is fittingly proposed to men for belief.
If God created human beings, then God makes also determines how He is to be made known to them. Since Aquinas holds that God can be known through reason, the question arises as to why God should also make himself known through faith, in a way that reason cannot grasp. Perhaps supernatural inspiration is "useless" in the face of a superior, rational, way to God.

Aquinas answers by noting that the way of reason is not necessarily superior, and this in three ways.

  1. Few humans would know God, due to the many impediments to the proper exercise of reason: the lack of disposition to such investigation, the exigencies of life, the natural tendency toward laziness. And though God has given us an appetite for knowledge, it is usually not sufficient motivation for the undertaking of a task of the magnitude of proving by reason God's existence.

  2. It takes a long time to train toward the goal of proving God's existence. Such training is generally not undertaken by youth, who are subject more to passion. If the human race had to rely on reason in order to know God, it "would remain in the blackest shadows of ignorance."

  3. Many people are suspicious of the products of reasoning, due to its many failures. So even if God's existence has been proved, they would doubt the power of demonstration.
Chapter 5: That the truths the human reason is not able to investigate are fittingly proposed to man for belief
Many people think that we should not believe what is beyond reaons; that reasonableness is the standard against all belief is to be judged. Aquinas has three responses.

  1. The first is practical: that belief in God is necessary for a higher good than can be found in the objects of reason alone. In particular, there are "spiritual and eternal goods" which are beyond the grasp of reason, yet which should be sought by human beings.

  2. That God is higher than reason can attain gives a true notion of God.

  3. The pretensions of reason are curbed by faith. Many who champion reason are led into error because they presume that their reason is the measure of all things.
Chapter 6: That to give assent to the truths of faith is not foolishness even though they are above reason
Although some truths about God are beyond reason, there is another kind of evidence for them: the working of miracles. Some are extraordinary, such as the raising of the dead, while others occur perpetually, such as the unchangeableness of the heavens. Even more remarkable is the sudden eloquence of simple, untutored people filled with the Holy Spirit. The most remarkable of all is that human beings in great numbers reject the sensible world in favor of the insensible. Finally, that all this would come to pass was prophesized by God.

The signs given by God have been effective, resulting in a massive conversion to Christianity. This makes the further working of miracles unnecessary. Again, Aquinas points to the phenomenon of simple persons believing beyond their means as the greatest miracle of all.

Chapter 7: That the truth of reason is not opposed to the truth of the Christian faith
There is no opposition between faith and reason, as can be shown in several ways.
  1. We are endowed to believe what reason shows to be true, and we are moved by faith insofar as it is the work of God. Both ar true, so they cannot be in conflict.

  2. What we know by nature through reason is given to us by God, who also gives us faith. The teaching of the same teacher will not be in conflict.

  3. God would not paralyze our thinking by opposing reason and faith.

The bottom line is that any arguments brought against the doctrine of faith can be countered, since they cannot show that faith is in conflict with reason.

Chapter 8: How the human reason is related to the truth of faith
What we know through reason is only a faint likeness of what is known through faith. Reason can only supply a cause for a given effect, but generally the cause is less known than is the effect. In the case of God, the distance is great enough that reason is incapable of grasping much of what is given through faith.
Chapter 10: The opinion of those who say that the existence of God, being self-evident, cannot be demonstrated.
Some people believe that God's existence is self-evident, given in the very understanding of what the word 'God' means. Here Aquinas is clearly referring to Anselm of Canterbury's "ontological" argument for God's existence. 'God' is understood to mean that greater than which nothing can be conceived. If God did not exist outside our conception of Him, then a greater reality, an independently existing God, could be conceived, which contradicts the meaning of the word 'God.' So to understand what God is, is to understand that God is.

Another version of the argument is as follows. It is possible to think of something as existing, such that we could not even think of its not existing. A being of that sort would be greater than a being that we could think of as not existing. If God might not exist, He would be a being of the second sort, which contradicts the meaning of 'God.'

Chapter 11: A refutation of the abovementioned opinion and a solution of the arguments
One reason people think that God's existence is self-evident is that all their lives they have believed in God. What is so familiar to us seems self-evident, but it really not so, but only the product of habit.

A more subtle explanation is that there are two senses in which something can be self-evident: absolutely and in relation to us. To us, God's existence is not self-evident, but in an absolute sense ("what God is is His own being) it is self-evident. The problem is that we do not know what God is, so we cannot conclude from what God is, that God is.

Now Aquinas directly attacks the two Anselmian arguments. Against the first, there is some question about the definition of 'God' as 'that greater than which nothing can be conceived." Some people do not understand God in this way, such as the ancient philosophers who identified God with the world. Still, even if the definition be granted, one cannot infer God's existence. It is a fact that we cannot think of anything greater than God, but this shows only that God exists in the intellect of the thinker. It does not prove that there cannot be in reality anything greater than our thought of God. Anselm had claimed that anyone who understand what God is cannot deny that God exists. But Aquinas counters that one can deny that God exists so long as one does not assert that in reality there is something than which nothing greater can be thought.

The same sort of attack is made against the second version of the ontological argument. It is possible to think that God does not exist, and this does not make God the kind of being who might or might not exist. If we think there is no God, it is because of our weak intellects. So to think of God as not existing does not contradict the definition of God, as if a perfect being does not exist. Rather is shows the main point, that we can know God through reason only as the cause of effects in the sensible world.

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