William Stanley Jevons (1 September 1835 – 13 August 1882) was a British economist and logician.
Irving Fisher described his book The Theory of Political Economy (1871) as beginning the mathematical method in economics. It made the case that economics as a science concerned with quantities is necessarily mathematical. In so doing, it expounded upon the "final" (marginal) utility theory of value. Jevons' work, along with similar discoveries made by Carl Menger in Vienna (1871) and by Léon Walras in Switzerland (1874), marked the opening of a new period in the history of economic thought. Jevons' contribution to the marginal revolution in economics in the late 19th century established his reputation as a leading political economist and logician of the time.
Jevons broke off his studies of the natural sciences in London in 1854 to work as an assayer in Sydney, where he acquired an interest in political economy. Returning to the UK in 1859, he published General Mathematical Theory of Political Economy in 1862, outlining the marginal utility theory of value, and A Serious Fall in the Value of Gold in 1863. For Jevons, the utility or value to a consumer of an additional unit of a product is inversely related to the number of units of that product he already owns, at least beyond some critical quantity.
It was for The Coal Question (1865), in which he called attention to the gradual exhaustion of the UK's coal supplies, that he received public recognition, in which he put forth what is now known as Jevon's paradox, i.e. that increases in energy production efficiency leads to more not less consumption. The most important of his works on logic and scientific methods is his Principles of Science (1874), as well as The Theory of Political Economy (1871) and The State in Relation to Labour (1882).