Economics during the 19th century.

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The problem with the classicals such as Smith, Malthus, Ricardo, Marx, and Mill are that they couldn't come up with a convincing theory of value. Smith articulated the labor theory of value, which bases value on the amount of labor or difficulty of labor in making something, and pretty much everyone especially Marx followed him. The problem with the labor theory of value is that something isn't more valuable just because it's harder to make. You can spend all day building a mound of dirt in your backyard, but that doesn't mean someone will buy it from you. What's missing is the demand side: how many people want this thing?

In the 1870s, Carl Menger first articulated the concept of marginal utility in a non mathematical way. Leon Walras proved that prices of goods come into a general equilibrium across many markets. William Stanely Jevons independently arrived at marginal utility and defined it mathematically. These three, Menger, Walras, and Jevons started the so called marginal revolution in economics. In 1890, Alfred Marshall synthesized the ideas of marginal utility for both supply and demand and created modern neoclassical supply and demand theory, which, unlike the labor theory of value, determines value (or price) by using the meeting place of both producers and consumers. J.M. Keynes was a student of Alfred Marshall. Marshall had hoped that Keynes would become a chair of economics and wrote him several letters urging him to that effect, but Keynes instead wanted to go into public service, which he did before WWI and again during.