René Descartes, John Locke, and Baruch Spinoza each present distinctive conceptions of substance within the framework of Western modern philosophy, yet their approaches differ fundamentally in metaphysical assumptions and epistemological commitments. Descartes’ view of substance is grounded in his dualism, which sharply distinguishes between mind and body. For Descartes, substances are entities that exist independently and do not require any other entity to exist. He famously identifies two types of finite substances: res cogitans (thinking substance, mind) and res extensa (extended substance, matter), alongside the infinite substance, God. Descartes posits that the mind is essentially defined by thought and is immaterial, whereas matter is characterized by spatial extension and is devoid of consciousness. In this schema, the essence of a substance is tied to its defining property: thought for mind and extension for matter. Each substance has self-sufficiency and causal independence, yet the two finite substances interact through a union orchestrated by God, creating the mind-body problem that dominates Cartesian metaphysics. John Locke, in contrast, approaches substance from an empiricist perspective, focusing on experience as the basis for knowledge. For Locke, substance is more of an epistemological concept than a clear metaphysical entity. He is skeptical about the notion of knowing the real essence of substance. In his “Essay Concerning Human Understanding,” Locke