Descartes’ Philosophy: Reason, Doubt, and Certainty
Descartes’ Rationalist Philosophy
The Quest for Certainty Through Reason
Descartes’s philosophy rests on the principle that there is a unique foundation, a conclusion adopted ultimately: the existence of a unique reason. He is convinced that everything can be known. In fact, in his Discourse on Method, he goes so far as to say that ultimately there is nothing so obscure that we cannot shed light upon it. Therefore, Descartes asks how we can reach that knowledge and concludes that we can only be sure our knowledge is true if we apply reason. Reason is the only thing we cannot doubt, while the senses may lead to error. Based on these principles, Descartes attempts to develop a simple and rational method to exclude false or unnecessary judgments, based on two qualities of reason: intuition and deduction.
Methodical Doubt and the Cogito
The goal becomes, therefore, to find a first certainty, which harbors no shadow of doubt, employing only reason and without recourse to empirical data. And so, after doubting everything, even what seems obvious at first sight (methodical doubt), Descartes understands that if there is something impossible to doubt, it is that he exists as a thinking subject (the Cogito: “I think, therefore I am”).
Demonstrating God and the Material World
He then tries to demonstrate rationally the existence of God and the world. Descartes is aware that, for this, he only has ideas, which are but the objects of his thoughts—realities that, therefore, he cannot doubt, even when the objects to which they refer might not exist. In this process, he distinguishes three kinds of ideas:
- Adventitious ideas: Originating from sense experience.
- Factitious ideas: Constructed by the imagination.
- Innate ideas: Absolutely independent of experience.
Only the latter, innate ideas, will serve as the basis for his purpose. Thus, from the innate idea of infinity, and following an argument similar to that of Saint Anselm, Descartes believes he has established the existence of God, from which the existence of the material world follows.
The Three Substances
This discovery leads Descartes to further develop the idea of substance as that which needs nothing else to exist, thereby distinguishing between three substances:
- Thinking substance (res cogitans – mind)
- Extended substance (res extensa – matter/body)
- Divine substance (God)
The Cartesian Method for Unified Knowledge
Here we analyze Descartes’ approach where he proposes a method inspired by mathematics, logic, and algebra.
The Cartesian Project: Unifying Knowledge
Descartes famously said that philosophy is like a tree whose roots are metaphysics, the trunk is physics, and the branches emerging from that trunk are all the other sciences, which are reduced to three main ones: medicine, mechanics, and morals. From this analogy, we understand what is called the Cartesian project.
What did Descartes seek with this project? To unify all knowledge and all science into one system. To understand this well, Descartes thought that the different fields of knowledge or science (physics, mathematics, medicine, etc.) are but small pieces of human knowledge, our human wisdom. Therefore, they are just manifestations of a knowledge that is unique, just as our reason is unique, though applied to different objects.
Inspiration from Mathematics
Moreover, as we know, Descartes believed that mathematics is a model of knowledge to follow. It inspires him to develop his method, as suggested in the text we are analyzing. The philosopher should follow the same argumentative style as mathematics, i.e., starting from self-evident truths (principles) and deducing the rest of the truths that the mind clearly perceives as true.
Prioritizing Reason Over Sensory Experience
As in mathematics, philosophical truth will only be achievable if we renounce our senses, if we set aside the sensible and turn to the intelligible. From here stems the radical Cartesian distrust of the testimony of the senses and the scope of imagination when seeking knowledge. We are talking about an author who relies exclusively on the power of reason. It corresponds only to reason to judge the truth.
Defining the Method: Rules for Truth
But of course, the Cartesian project requires a method. The word ‘method’ comes from the Greek word for ‘way’. According to Descartes, we need a way to lead us to truth, meaning the certainty that the knowledge we acquire follows a set of orderly, easy, safe, and simple rules. Our author is convinced that by applying these rules, we can be free from error and doubt.