Friedman distinguished "positive" economics (the study of "what is") from "normative" economics (the study of "what ought to be"). The goal of positive economics, he wrote, "is to provide a system of generalizations that can be used to make correct predictions about the consequences of any change in circumstances. Its performance is to be judged by the precision, scope, and conformity with experience of the predictions it yields. In short, positive economics is, or can be, an `objective' science, in precisely the same sense as any of the physical sciences."

Donald MacKenzie, An Engine, Not a Camera, loc. 53