Satisfaction gained from additional units decreases with consumption.
Gossen, Jevons and Menger's independent discovery. It resolved the diamond-water paradox that had defeated classical economics: value tracks the last unit, not the total.
In practice
- The first glass of water on a hot day is worth a great deal; the fourth is not, which is why water is cheap and diamonds are not.
- A department's first analyst transforms its capability; its sixth is useful, and its eleventh needs managing.