To assess theories by whether their assumptions were empirically accurate was, Friedman argued, fundamentally mistaken: "Truly important and significant hypotheses will be found to have assumptions' that are wildly inaccurate descriptive representations of reality.... A hypothesis is important if it explains' much by little ... if it abstracts the common and crucial elements from the mass of complex and detailed circumstances ... and permits valid predictions on the basis of them alone. To be important, therefore, a hypothesis must be descriptively false in its assumptions." (p. 14) The test of a theory was not whether its assumptions were "descriptively `realistic,' for they never are, but ... whether the theory works, which means whether it yields sufficiently accurate predictions" (p. 15).

Donald MacKenzie, An Engine, Not a Camera, loc. 54