ubstance is a longstanding concept in metaphysics used to describe what underlies or supports the properties and characteristics of a thing. Philosophers have differed over whether substance should be understood as a physical entity, an immaterial reality, a combination of matter and form, or simply the enduring subject of change. Questions about substance often involve the relationship between identity, properties, essence, matter, mind, and persistence through change, making substance central to many theories about the structure of reality.