Rational Soul, Faith, and Reason: Aquinas and Descartes
Rational Soul and the Limits of Knowledge
According to St. Thomas, the rational soul of man is limited because it must consider the senses of the body. It must start from the concrete particular to reach the abstract universal. These limits are evident when considering God; our knowledge is imperfect and analogical. Where reason fails, faith begins. Each has complementary truths and they do not have to conflict. The knowledge of faith, revealed by God in scripture, expands our knowledge gained by rational means. The contents of reason (philosophy) and faith (theology) are distinct, but there are shared truths:
- The existence of God. St. Thomas believed that reason could prove God’s existence (the five ways).
- The immortality of the soul. Influenced by Plato, he thought the immortal soul could be explained by both faith and reason.
Proofs of God’s Existence
For Aquinas, the proposition “God exists” is evident in itself, but not for us due to our limited understanding of the divine nature. Augustine believed it was possible to attain knowledge of God’s existence from the idea of God. St. Anselm’s ontological argument (11th century) in his work Proslogion states that all men have an idea of God, which exists not only in our thinking but also in reality. Thus, we start from the idea of God as a reason to conclude its existence as a fact, using reason, typical of Christian Platonism.
Aquinas distinguishes two kinds of truths:
- Those evident in themselves and for us (propositions whose truth becomes apparent from the mere analysis of concepts).
- Those evident in themselves but not for us.
Aquinas’s five ways to prove God’s existence:
- Motion: To reach God as the unmoved mover of the universe.
- Causation: To reach God as the first uncaused cause.
- Contingency: To reach God as the necessary being who justifies the existence of contingent beings.
- Degrees of Perfection: To reach God as the absolutely perfect being.
- Order: To reach God as the intelligent designer of the universe.
The Methodical Doubt
The key method is based on evidence. Like a mathematician, we must reject everything that is not obvious, “not accept as true anything that is not.” How do we find these “simple ideas”? How can we be sure of the evidence? Descartes’s primary purpose is to find a starting point: a truth so immediately apparent that even the most extravagant skeptics cannot doubt it. The way to achieve this is “to reject as absolutely false everything in which I could imagine the least doubt, to see if, after this is done, in my belief would not be something that was entirely indubitable.”
Therefore, we must doubt all knowledge we have acquired. Are our senses sometimes deceiving us? Could it be that we are always deceived? Can we say the same about reason? Could it all be a dream from which we have not yet woken up? To make the doubt universal, Descartes proposed the hypothesis of an “evil genius” of extreme power and intelligence, who makes every effort to mislead us. It seems that nothing can be excluded from doubt.
Descartes clarifies that his doubt is methodical, not skeptical. Skeptics doubt for the sake of doubt. Cartesian doubt is provisional, a way out of the question, a means to evidence. The doubt will disappear when we find something that cannot be doubted.
Descartes’ Scientific Method
Descartes’ scientific method was devised after deciding to reject everything that had been taught as true and could not be proven. For him, only logic, geometry, and algebra are sciences that can be proven. From these sciences, Descartes derived four basic rules:
- Never accept anything that is not clear and distinct.
- Divide the problem into as many parts as needed for analysis.
- Order thoughts from simple to complex, even where no order is apparent.
- Check everything carefully to ensure nothing has been overlooked.