The fourth cup of coffee helps a lot less than the first

Add more of one input — fertilizer to a field, workers to a factory line — while holding everything else fixed, and each extra unit eventually adds less output than the one before it. Turgot first described the pattern in 18th-century farmland in 1767; Ricardo, Malthus and others built it into the theory of rent in 1815. It's one of the oldest and most durable laws in economics, holding across almost any production process you can name.

— Anne Robert Jacques Turgot; developed by David Ricardo and Thomas Malthus, Observations sur le mémoire de M. de Saint-Péravy — 1767 (Turgot); rent theory, 1815