Economists’ insistence that their discipline is like physics sounds a little nervous. Did you ever hear a physicist boast to the world that physics is like economics? More important, when they talk about economics this way, Samuelson, Nicholson, and other economists are misrepresenting what they do and what economics is. From Adam Smith to Karl Marx, from John Maynard Keynes to Milton Friedman, economists have sought to gain insight into economies by building models of them. They make simplified assumptions about the economic world we inhabit and construct imaginary economies—in other words, models—based on those assumptions. They use these imaginary economies to draw practical conclusions about the actual economies we inhabit.