This scholarly note examines the groundbreaking contribution of René Descartes, who was the first to recognize and philosophically formulate the fundamental distinction between two different realms of reality. This difference is characterized as essential, specifically as a distinction between two created and finite substances. The text systematically outlines four key areas where this division is manifest. First, it addresses thinking, emphasizing the transition from myth to conceptual reflection, systemic rigor, and criticality. Second, it explores the nature of man, focusing on the tension between human integrity and internal trinity. The third section analyzes the world, nature, and society, distinguishing between true and false beings while examining the role of thought schemes and history. Finally, the note considers the dimension of time. Overall, the text serves as a fundamental framework for a deeper analysis of dualism and the structure of modern consciousness. This concise reflection provided the basis for further elaboration in the author's diary entries, highlighting its significance within the broader context of his philosophical inquiry into the nature of reality and the limits of human knowledge.