In metaphysics, the concept of substance has been central to philosophical inquiry for centuries, tracing back to the works of Aristotle, Descartes, Spinoza, and many modern thinkers. Substance is generally understood as that which exists independently and serves as the fundamental reality underlying all phenomena. It is what underpins the properties and changes of objects, providing a stable, enduring entity in contrast to the transitory nature of appearances. In other words, substance is the “what” that remains when all accidental or changeable characteristics are stripped away. Philosophers have often debated whether substance is material, immaterial, or both, and how it relates to properties, causation, and existence itself. Aristotle provides one of the earliest systematic treatments of substance. According to him, substance, or *ousia*, is that which exists in itself and not in another. He distinguishes substance from accidents, which are the attributes or properties that a substance may have but which do not exist independently. For instance, the redness of a rose is an accident, but the rose itself, as an independent entity, is a substance. Aristotle further categorizes substances into primary and secondary: primary substances are individual entities, such as a particular tree or person, while secondary substances are the species or genera to which these individuals belong, such as “tree” or “human.” For Aristotle, substances are essential for understanding the nature of reality because they are the bearers of properties and the subjects of predication. He emphasizes that substances are concrete entities that undergo change while maintaining their identity through form and matter; the form provides the essence, while matter provides the substratum that allows change. Descartes approaches substance from a rationalist perspective, focusing on the dualistic distinction between mind and body. For him, substance is something that exists independently and requires no other entity to exist. He famously identifies two fundamental types of substances: thinking substance (*res cogitans*), which is the mind, and extended substance (*res extensa*), which is matter. This dualistic view posits that the mind and body are fundamentally different in nature—one immaterial and conscious, the other material and extended in space—but both qualify as substances because they can exist independently in their essential aspects. Descartes also allows for the existence of God as a substance, characterized by absolute independence and infinite attributes. His definition underscores the metaphysical emphasis on independence and self-sufficiency as criteria for substance. Spinoza extends the notion of substance to a monistic framework in which only one