The Concept of Substance in Modern Western Rationalism explores the evolving philosophical notion of substance within the framework of modern Western rationalist thought. It examines the works of key thinkers, such as Descartes, Spinoza, and Leibniz, who shaped the metaphysical and epistemological underpinnings of substance in rationalist philosophy.The study critically analyzes how the concept of substance was central to addressing fundamental questions about reality, existence, and causality, while also assessing its limitations and challenges in light of contemporary philosophical critiques. By bridging classical perspectives with modern interpretations, this work provides a nuanced understanding of the enduring relevance of substance in the rationalist tradition.