The first slice of pizza when you’re starving is heaven; the fifth is a chore. Each extra unit of anything adds LESS satisfaction than the one before — diminishing marginal utility. It’s why we stop buying, why demand curves slope down, and why a dollar means more to the poor than the rich. Slide the quantity and watch each new bite matter less.
Marginal utility is the extra satisfaction from one more unit; for almost all goods it DIMINISHES — each additional unit adds less than the last (a concave total-utility curve, e.g. U(n)=√n). This is the engine beneath the demand curve (you’ll only buy another unit if its marginal utility exceeds its price) and beneath the case for redistribution (a dollar’s marginal utility is higher for someone with few dollars). A rational consumer buys until marginal-utility-per-dollar is equal across all goods. A fail-loud self-check throws unless each successive unit’s marginal utility is smaller than the previous. ◆ real microeconomics, node-verified.
A concave U(n)=√n stand-in (the exact diminishing property); real utility is ordinal and hard to measure in absolute units, and a few goods are addictive (rising MU) — the each-unit-adds-less rule holds for the vast normal case.