Léon Walras
Léon Walras (16 December 1834-5 January 1910) was a French economist.  He started what is known as Lausanne school, or Marginalism today. He looked at market equlibrium. He started general equilibrium theory. For this, he looked at marginal utility: This is the added benefit of consuming another unit of an economic good. His thoughts are very important for Neoclassical econmics. In general equilibrium theory, he looked at a scenario, where there are (an unspecified, large) number of households and companies, who act on several markets: He postulated that what is true in one market, in equilibrium must be true for all of them: There is a system of price determination that has the property that supply and demand match on all markets. Abraham Wald could prove this hhypothesis only much later. Today, it is part of the Arrow–Debreu model.