Modern economic theory distorts the original ideas
How does modern economic theory distort the original ideas
Modern economic theories often distort the original ideas of the early authors, such as Jevons and Walras. These distortions are the very source of confusion in the current economic theory. This is especially true for the theory of value, the very foundation of the economic theory.
My prompt: Jevons explicitly stated that economic value is exchange value. And he developed the theory of exchange value as marginal utility. It is purely quantitative. If you read his book itself instead of later interpretations. You can see that clearly. By the way, Leon Walras didn't propose the utility theory of value. He was against it. He proposed the scarcity theory of value. Again, if you go over his book directly, you will not claim he proposed the utility theory of value.
Gemini: You are making an accurate distinction about primary historical texts versus modern secondary summaries. Going directly to William Stanley Jevons's The Theory of Political Economy (1871) and Léon Walras's Elements of Pure Economics (1874) reveals essential details that are often smoothed over in introductory textbooks.
1. Jevons, Exchange Value, and Cardinal Utility
You are spot on regarding Jevons. Jevons explicitly argued that economics is a mathematical science of quantities and that value in exchange is the core subject of political economy.
"Value is Exchange Value": Jevons explicitly rejected classical notions of intrinsic or labor-based value. He defined exchange value strictly as the ratio of exchange between quantities of two commodities ($d y / d x$).
The Mathematical Link to Final Degree of Utility: Jevons formulated his fundamental equation of exchange:
$$\frac{\phi(x)}{\psi(y)} = \frac{y}{x}$$
Here, the ratio of exchange ($\frac{y}{x}$) equals the inverse ratio of what he called the "final degree of utility" (his term for marginal utility).
Explicitly Quantitative (Cardinal): In Chapter III of The Theory of Political Economy, Jevons treated pleasure and pain as measurable quantities on a single continuous scale with dimensions. His mathematical derivations using differential calculus (derivatives of utility functions) required utility to be treated as a continuous, cardinal variable. He explicitly wrote that economics, dealing with quantities of pleasure and satisfaction, must be a mathematical science.
Historians of economic thought agree: Jevons did not view utility as an abstract, unmeasurable ordering (ordinalism came much later with Pareto, Hicks, and Allen). For Jevons, marginal utility was a cardinal, quantitative engine that directly dictated the quantitative ratio of exchange.
2. Walras, Rareté, and the Scarcity Theory of Value
Your point on Léon Walras clarifies a critical terminology distinction that many general accounts obscure.
Walras took his core framework directly from his father, Auguste Walras, who formulated the scarcity theory of value. In Elements of Pure Economics, Léon Walras explicitly defined social wealth as consisting of scarce items—things that are both useful and limited in quantity.
The Concept of Rareté: Walras chose the specific French term "rareté" to define the cause of value. He explicitly defined rareté as the intensity of the last want satisfied by a quantity of a commodity.
Why "Scarcity" rather than Pure "Utility": Walras explicitly rejected the idea that utility alone creates value. A good can be immensely useful (like air or water), but if it is abundant, its exchange value is zero. Value arises only when utility encounters limitation in quantity (scarcity).
Mathematical Equivalence to Marginal Utility: While Walras insisted on calling his concept rareté (scarcity) to ground it in the dual conditions of utility and limited supply, modern economists translate rareté mathematically as "marginal utility." However, as you rightly note, in Walras’s own words and framing, he viewed himself as proposing a theory of scarcity-driven exchange value, explicitly opposing writers who attempted to attribute value to utility in isolation.
Summary
Primary texts reveal that:
Jevons built a strictly cardinal, quantitative theory where "exchange value" is an objective ratio dictated by the mathematical derivative of pleasure (final degree of utility).
Walras centered his entire system on rareté—a specific synthesis of utility and quantitative limitation—and maintained that scarcity, not raw utility, is the true origin of exchange value.