This text analyzes Descartes' distinction between two spheres of reality, res extensa and res cogitans, which the author interprets as a pivotal moment in the history of European philosophy. Although Descartes is often labeled as the founder of European subjectivism, the text clarifies and refines this common misconception. It emphasizes that subjectivity was recognized as early as ancient Greek thought through the distinction between opinion and knowledge, but Descartes introduced a fundamental shift from an emphasis on truth to an emphasis on subjective certainty. The main contribution of Cartesian philosophy is not the creation of subjectivism itself, which is viewed as a mere side product or byproduct, but rather opening the way to understanding the subject as a non-object. This discovery fundamentally challenged traditional Greek philosophical objectivism and established the subject as a primary philosophical theme of the highest importance. The text demonstrates that Descartes' legacy lies in problematizing the relationship between extended and thinking substances, ultimately leading to the recognition of the subject as a sphere that cannot be reduced to a mere object.