 Ricardo's formulation of the law was the first clear exposition of the source and magnitude of rent, and is among the most important and firmly established principles of economics.John Stuart Mill called it the ""pons asinorum"" of economics.The law of rent states that the rent of a land site is equal to the economic advantage obtained by using the site in its most productive use, relative to the advantage obtained by using marginal (i.e., the best rent-free) land for the same purpose, given the same inputs of labor and capital.