In the Summa Contra Gentiles Thomas Aquinas discusses the relationship between reason and revelation. The term for this is scholastic theology. This is when a person combines reason and faith so it can be accepted by all people. This was especially true in the time of Thomas Aquinas when philosophy was a prominent area of learning and Aristotle's works were being rediscovered. In this particular example of scholastic theology Thomas Aquinas discusses two primary truths. The first is the first condition: The ability of the mind to know the truth. The second is the first principle: the principle of non-contradiction. These two primary truths are used to make plain the fact that the natural order and the spiritual order could balance out, or in other words to show that there could be harmony between faith and reason.
Thomas Aquinas starts out Chapter 7 of Summa Contra Gentiles by discussing how the mind is able to know the truth. This is the first condition of the three primary truths. He uses this truth to illustrate that revelation cannot be without reason. He states that the human brain knows certain things. With just this knowledge a person cannot understand the Christian faith. He says, "Now the afore mentioned truth of the Christian faith surpasses the ability of human reason, never the less those things which are naturally instilled in human reason cannot be opposed to the truth" (333). This is in reference to the knowledge humans have as part of the natural order. It is not something that people have been given a revelation about. There is also knowledge of the spiritual order. That is knowledge that God gave to people when he made them. This is what Thomas Aquinas says about the spiritual knowledge, "Now the knowledge of naturally known principles is instilled into us by God, since God Himself is the author of our nature" (333-334). In this way it is easy to see that both the natural and spiritual nature of human knowledge can work together. Since something cannot be true if it does not fit into the spiritual knowledge that was given to humans by God then it is not true. However, just because the natural knowledge cannot understand does not mean it is false. For example, though the natural knowledge of the human brain cannot understand fully how Jesus was true God and true man it is a truth that can be understood with the spiritual nature God gave people when He created them. Since this is true if something is cannot be grasped as true through the spiritual nature it cannot be a Divine Truth from God.
The next thing Thomas Aquinas talks about in the Summa Contra Gentiles is the first principle: The principle of non-contradiction. If this principle is not held as truth than a human can learn nothing because the natural knowledge would be unable to grasp a fact without this truth. The fact that the natural order is not able to understand something if it has a contrary argument is not something that God did. Thomas Aquinas says, "Our intellect is stayed by contrary arguments, so that it cannot advance to the knowledge of truth" (334). God also does not give revelations to humans if they are the opposite of the knowledge that is part of the natural order. This is said in the text by Thomas Aquinas, "Now contrary opinions cannot be together in the same subject. Therefore God does not instill into man any opinion or belief contrary to natural knowledge" (334). Thomas Aquinas is talking about the fact that if something is said to fit into the spiritual knowledge but cannot fit into the natural knowledge it cannot be from God because God would not do something like that to the human race because they would get confused and would never reach the true knowledge. He says that if, "...whatever arguments are alleged against the teaching of faith, they do not rightly proceed from the first self-evident principles instilled by nature" (334). This is his way of saying that if the argument a person hears goes against the Old testament or the New Testament (he quotes a verse of the Bible in the previous paragraph) then it cannot possibly in the spiritual order or the natural order of things.
Thomas Aquinas uses the first contradiction and the first principle to show that there could be harmony between faith and reason. He did this because he lived in a time when philosophy was a science and Aristotle's works were being rediscovered. The method he used was called scholastic theology because it combined reason and revelation. Summa Contra Gentiles was the work of a genius which is evident through its concise arguments that show that faith and reason can be in harmony with one another.