Léon Walras
In the textbook story, favored by the followers of Léon Walras, if the quantity demanded does not equal the quantity supplied in a market, "price adjustment" is the rule: if there is a market "surplus" or glut (excess supply), prices fall, ending the glut, while a "shortage" (excess demand) causes price to rise. A simple model for price adjustment is the "Evans price adjustment model", which proposes the differential equation: