Descartes and Aquinas: Reason, Faith, and Existence

Descartes

Descartes introduces a mathematical method into philosophy, giving reason as the definitive criterion for human truth.

Doubt and the First Truth

Descartes makes a radical critique of all knowledge, considering all previous knowledge unreliable. His universal method is based on the following reasons:

• Sense-data uncertainty: The senses have deceived us many times. Therefore, Descartes doubts all data originating from the senses, including his own body and the material reality of the world.
• The errors of reasoning: Humans often commit errors. A good part of traditional knowledge is based on reason, but this knowledge has become confused. Therefore, there is reason to doubt all knowledge taken as demonstrated.
• The difficulty distinguishing sleep from wakefulness: Descartes considers the possibility that all waking thoughts are really dreams, and we cannot recognize them as such.
• The hypothesis of the evil genius: To make his doubt universal, he posits the existence of an evil genius, cunning and deceitful, who leads him to regard as obvious things that are not.

His first certainty is the truism he hopes to find. The philosopher concludes that if he is thinking, then he exists ("I think, therefore I am," or "Cogito, ergo sum"). The cogito ergo sum does two things: it justifies the existence of a distinct thinking self and becomes a model principle.

The Three Substances

Through his method, Descartes distinguishes three substances that make up all of reality: the thinking self, God, and the world.

• The thinking self: The existence of a thinking self, a substance that thinks, is called res cogitans. Descartes doubts the existence of his body and the world around him because he receives information through the senses, which are unreliable. However, he cannot doubt the existence of his thoughts, his subjectivity, which is the collection of thoughts, ideas, and representations that flow within himself. He sorts ideas as follows: adventitious or acquired, fictitious or artificial, and innate or natural.
• God: Descartes believes that the thinking self is not perfect, and methodical doubt has been established to correct its mistakes. Despite this, the thinking self has the idea of perfection. The idea of perfection is innate in us, the idea of a perfect being, which is the idea of God. This idea of perfection cannot come from us; therefore, it must have come from a divine reality that has given rise to our minds. God overcomes our subjectivity. Outside of ourselves, there is another reality, which asserts that God is perfect. Descartes makes God the guarantee of knowledge.
• The World: There is another type of finite and created substance: that of bodies, each with a fundamental attribute, extension. Matter, or res extensa, is the third substance of Cartesian metaphysics.

Aquinas

Reason and Faith

Both reason and faith are faculties that collaborate in a common area. Aquinas thinks that truths are treated by both faith and reason, but in different ways. Reason and faith, despite being distinct, arrive at the same point: truth. Both reason and faith are independent. Reason offers a system, but the main criterion of truth is when reason arrives at a conclusion that faith guarantees. Faith has the last word. Reason must not commit an error in its procedure. Aquinas believes that a conflict between faith and reason is impossible because truth is unique and founded in God.

Demonstrating God

To demonstrate God, Aquinas develops several ways, known as the five Thomistic ways. These demonstrations are based on arguments and observations of empirical phenomena from the sensible world. Each way starts with a cause and effect, splitting the data from the empirical world. The principle of causality is applied, rejecting an infinite process, and arriving at God as the observed data. The first, second, and fifth ways start from the consideration of sensible things as a changing world. The third and fourth ways start from the period and limitation of perfection in sensible things.