Thomas Aquinas was summarily concerned with the compatibility of faith and reason. In The Summa Against the Gentiles (Summa Contra Gentiles) and the Summa of Theology in particular, Aquinas presents his arguments for the synthesis of faith and reason. Aquinas offers a rather ironic glimpse at the nature of reason, which is both capable of intellectual comprehension of God but simultaneously insufficient for understanding God. Thus, Aquinas argues that God can be ascertained and even logically proven via the use of reason, but that the experience of God is a transcendent, spiritual, and emotional one that requires faith. Faith also fulfills the goals of reason, which is truer and greater understanding of God. Whereas faith fails to provide the means by which to perceive the mundane world, reason is unable to offer a genuine proof or understanding of God.
One of the ways Aquinas reconciles faith and reason is by proving the salience of faith itself. This clever rhetorical trick is most clearly explicated in Aquinas's "five ways" argument. According to Aquinas, there are five ways of knowing God, or five ways of defending faith in God. God may be ascertained, known, experienced, and understood through these five ways. Each of these ways is essentially reasonable in nature, and entail logical proofs. Yet each of these ways can be shown also to be compatible with faith. Faith may reinforce each of the five ways, just as the reason used to analyze God in one or more of these five ways substantiates and enhances faith in God. What Aquinas accomplishes with the Five Ways argument is that faith is a catalyst for a reasoned understanding of God.
The first way is via the perception of motion and the measurability of motion. Because the human sense organs are capable of sensing motion, and because motion is generally accepted to be fact, motion must be explained in some way as having a singular cause. God is the primal or original mover, according to Aquinas. Although Aquinas bases the first way on a gross assumption that there must be a prime mover, the argument has an internal logic. Aquinas claims that things cannot move by themselves, especially cosmic elements like planets. Aquinas goes further to claim that an object cannot move itself because if it could do so, it would contradict the laws of nature. In particular, Aquinas supposes that an object cannot simultaneously be the subject and the object of the same action -- in this case, movement. It may take a leap of faith to feel God, but it does not take a leap of faith to use reason.
The second way of knowing God is via an understanding of causality. The law of cause and effect is related to the first way, movement, in that Aquinas also refers to the impossibility of being both cause and effect just as an object cannot simultaneously be mover and moved. All effects must have causes, and all causes must have effects. Given this is the case, the universe must have an ultimate cause. That cause is, for Aquinas, God. The faith it takes to make the logical leap to God as the primal cause is an ironic way of showing the compatibility of faith and reason. Faith and reason occupy distinct rhetorical domains and yet Aquinas manages to fuse them in several of the "ways" of knowing God.
God can be known in a third way, possibility and necessity. Aquinas presents one of the most complex ways of knowing with the third way, thus inadvertently highlighting the function of faith when applying reason to God. Aquinas claims that most things are dependent on other things for existence, and that most things in the known universe are transient in nature. These are the dependent or contingent things. God is not one of them. Ultimately, there must be something that is not dependent on anything else. Aquinas is uncomfortable with the concept of nothingness coexisting with the potentiality or possibility of being. With this argument, Aquinas also helps to define his terms of God, holding God to be associated with creation.
Extending from Platonic and other classical Greek arguments about the nature of reality, Aquinas also posits that God is a type of ultimate form or ideal. Aquinas muses on perfection and growth, and concludes that God is the ultimate perfection and the end-result of life. Finally, Aquinas points out another possible definition of God other than being the Creator. God is also a sort of universal mind or universal intelligence. Using a reasoned and reasonable argument, Aquinas claims that human beings do not possess limitless knowledge. There are also an abundance of elements in nature that cannot be comprehended by human beings, precisely because they were intelligently designed. Given the implications for the modern intelligence design debate, it is clear that Aquinas reconciles faith and reason in a typically Christian way. One does not need faith to reach God in this way, even if the rational approach to God does not entail one of the core elements of knowing God, which is grace. The five ways avoid the question of grace, which Aquinas deals with elsewhere in the Summa of Theology as well as the Summa Against the Gentiles.
The act of faith is therefore compatible with reason, although Aquinas believes faith to be alone sufficient whereas reason is like a crutch. A key way Aquinas reconciles faith with reason is through his rhetorical debate. Reason can be a valuable stepping-stone toward receiving God's grace. In Summa Contra Gentiles, Aquinas especially elaborates on the insufficiency of reason alone. Reason can surely be useful in one's spiritual journey, implies Aquinas, but faith provides that extra boost that reveals the totality of God. Interestingly, Aquinas is able to frame faith from a rational point-of-view that plays a rhetorical trick on the reader. Setting aside Aquinas's fundamental feeling that faith is sufficient to know God, the philosopher simultaneously presents faith as an act of reason. It is reasonable to have faith in God, is essentially what Aquinas is saying. Moreover, Aquinas is able to lay out the specific reasons why it is reasonable to have faith in God. Those reasons are laid out in the five ways argument, as well as in other passages in both the Summa of Theology and the Summa Contra Gentiles. While faith is typically defined as being divorced from reason, Aquinas manages to reconnect the two elements of human thought.
Aquinas seems to understand the peculiar limitations of the human mind, which is capable of perceiving God, conceiving of God, and engaging in discourse on God. The human mind is also capable of grace, which is linked to faith, and which extends beyond reason. Thus, reason is integral to the totality of being human. The human mind needs both faith and reason. Faith is that component that makes up for what reason cannot accomplish, and reason coaxes the mind by speaking its first language. Aquinas does not outline why it is important to believe in God at all; only that God is real. Similarly, God can be demonstrated through reasoned argument. This is itself important because prior to Aquinas, reason and faith appeared incompatible from a philosophical or epistemological standpoint. Aquinas transformed Christian epistemology by providing logical outlines or proofs of God. There is an essential and insurmountable gap between the human mind and God, which is bridgeable only via faith.
According to Aquinas, reason does not need to stand in the way of faith and nor does faith need to impede reason. It does not take faith to believe in mundane proofs of the senses; likewise, it does not require reason to comprehend the limitless spiritual nature of God. Human beings occasionally need to use faith even for mundane things and…