"It is not from the benevolence of the butcher, the brewer, or the baker, that we expect our dinner, but from their regard to their own interest."
— Adam Smith, 1776
"Economists view the individual as the unit of agency and investigate the interaction of the purposeful behaviors of different individuals within various economic institutions. The purpose of economics is to analyze institutions, such as trading mechanisms and organization structures, and to ask how those institutions mediate the interests of different economic agents." — Faruk Gul and Wolfgang Pesendorfer (2005).
"Economics is not about the intersection of supply and demand curves, and about what direction prices move if a curve 'shifts.' It is a quantitative theory of human behavior both individually and in interaction with other people." — David Levine (2012).
"It is only then because land is of different qualities with respect to its productive powers, and because in the progress of population, land of an inferior quality, or less advantageously situated, is called into cultivation, that rent is ever paid for the use of it. When, in the progress of society, land of the second degree of fertility is taken into cultivation, rent immediately commences on that of the first quality, and the amount of that rent will depend on the difference in the quality of these two portions of land."
— David Ricardo (1817): Principles of Political Economy and Taxation
We will consider three different ways of looking at the behavior of a function $f$ at some point $x_0$. We shall refer to $\dfrac{d}{dx}f(x_0)$ as the:
"Trump's steel tariffs cost U.S. consumers $900,000 for every job created, experts say." — Washington Post (May 7, 2019)
"This time is different." — Reinhart and Rogoff (2009).