“We might as reasonably dispute whether it is the upper or the under blade of a pair of scissors that cuts a piece of paper, as whether value is governed by utility or cost of production.”
Alfred Marshall, Principles of Economics (1890) · Book V, chapter III · read it
Kept because it ended a century of argument with a household object. It also settles many arguments that are not about prices. When two factors are each called the real cause, check whether they are two blades.