Since Smith's time, this concept has been further incorporated into economic theory. Léon Walras developed a four-equation general equilibrium model that concludes that individual self-interest operating in a competitive market place produces the unique conditions under which a society's total utility is maximized. Vilfredo Pareto used an Edgeworth box contact line to illustrate a similar social optimality. Ludwig von Mises, in "Human Action" uses the expression "the invisible hand of Providence", referring to Marx's period, to mean evolutionary meliorism. He did not mean this as a criticism, since he held that secular reasoning leads to similar conclusions. Milton Friedman, a Nobel Memorial Prize winner in economics, called Smith's Invisible Hand "the possibility of cooperation without coercion." Kaushik Basu has called the First Welfare Theorem the Invisible Hand Theorem.