Supply and demand
One of the first to see this was Adam Smith. In his book The Wealth of Nations (1776), he says that the demand was dependent on the price of a good. He did not see the inverse, that the demand also influences the price. David Ricardo published the book Principles of Political Economy and Taxation in 1817, where he explained market equilibrium as it is explained above. Others who are known for this model are Alfred Marshall and Léon Walras.