This work aims to explore how Charles S. Peirce achieved unity in his pragmatist philosophy through Scholastic Realism. The author presents this doctrine as a guide to navigate the various stages of Peirce's philosophical journey. By delving into his realist doctrine, we can understand why Peirce considered it a viable theory for addressing the problem of Universals, a concern that has historically influenced philosophy. The argument emphasizes the significance of Scholastic Realism in comprehending Peirce's philosophy, marking a novel approach in Peirce scholarship. It offers a framework for questioning reality akin to Peirce's own methods. Consequently, the author illustrates how Peirce's realism tackles interconnected philosophical issues, ultimately leading him to label his final philosophy as "Scientific Metaphysics." The conclusion interprets Scholastic Realism as a solution to Peirce's concerns, providing a pathway to a more robust theory of reality in his quest for metaphysics a posteriori. The discussion also highlights the doctrine's applications, particularly in abstraction theory and foundational principles of mathematics. This work aims to enhance our understanding of Peirce's philosophical challenges while illuminating the roots of pragmatism and the ongoing debate between realism and nominalism.
