Let's take a look at Descartes' probably best-known concept, which still resonates today and is often regarded as the essential foundation of epistemology:
He established this insight as the first foundation of his philosophy, because at its core, "Cogito, ergo sum" serves as the fundamental truth in Descartes' quest for certainty in the face of skepticism.
He did this, among other things, in the aforementioned work "Meditations on First Philosophy" and thus embarked on a methodical journey in which he doubted everything he could doubt in order to create an indisputable basis for knowledge. He questioned the reliability of sensory perceptions, the existence of the physical world and even the mathematical truths he once held to be certain. This radical skepticism led him to ask what he could be certain of at all. His answer was the "cogito": If he doubts, then he must think, and if he thinks, he must exist. Thus, "I think, therefore I am" turns out to be not just a statement about existence, but a fundamental insight into the nature of consciousness and self-consciousness.
Descartes' methodological skepticism and his subsequent realization of the "cogito" can be seen as a monumental shift from reliance on empirical evidence and external confirmation to an introspective, rationalist approach. He argued that while our senses can deceive us and external realities can be questioned, doubting one's own existence is in itself proof of the reality of one's own mind. The "cogito" is therefore an axiom: it is self-evident, requires no external proof and is the basis for the structure of knowledge.
This insight first appeared in a preliminary form in his earlier work "Discourse on Method" (published in 1637), where he formulated it as "Je pense donc je suis", which in Latin means "Cogito, ergo sum", i.e. "I think, therefore I am". Descartes later refined his reasoning as he realized that formulating the cogito as an argument with a suppressed premise ("All things that think exist; I think, therefore I exist") made it susceptible to doubt.
By the time he wrote the Meditations in 1641, Descartes had further developed his understanding of the cogito. He recognized that the inclusion of the "ergo" ("therefore") represented a contestable logical argument and made it an axiom. Something that always presupposes a certain system in order to be valid. To counter this, Descartes presented the cogito not as an argument but as a direct intuition: "Ego sum, ego existo" ("I am, I exist"), thus affirming its truth whenever it is thought in the mind. This distinction is of crucial importance: for Descartes, the cogito thus went from being an axiom to an immediate and self-evident truth that does not depend on other premises and systems in order to be valid - called a priori.
If you want to learn more about the difference (and similarities) between "axiom" and "a priori", you can find the right article here.
The Cogito marks a decisive turning point in philosophical discourse by establishing the consciousness of the subject as the fundamental anchor of certainty. It laid the foundation for later philosophical inquiry into the nature of the self, knowledge and reality. Descartes' approach influenced countless philosophers and opened up new ways of exploring the mind-body problem, the nature of existence and the possibilities of epistemology.
Despite its fundamental importance, the cogito is not uncontroversial. Critics argue that Descartes' methodological doubt and the cogito introduce a solipsistic view of knowledge that relies too much on the subjective experience of the individual. Furthermore, the leap from the cogito to the proofs for the existence of God and the external world is seen as less convincing, which earns Descartes the accusation of circular reasoning.
Despite his criticism, Descartes' cogito ergo sum remains one of the most important contributions to Western philosophy. It marked a departure from the scholasticism of the Middle Ages and ushered in the era of modern philosophy. The Cogito is a testament to the power of doubt as a philosophical tool and the pursuit of certainty through reason. Although Descartes' broader project in the Meditations - the unquestionable proof of the existence of God and a method for attaining incontestable knowledge - may not have been as successful as he had intended, with the formulation of the cogito he created a new foundation for philosophical inquiry by emphasizing the central role of the thinking self in the search for truth.