2014 Reprint of 1921 Edition. Full facsimile of the original edition, not reproduced with Optical Recognition Software. In economics, "Knightian uncertainty" is risk that is immeasurable, impossible to calculate. Knightian uncertainty is named after University of Chicago economist Frank Knight (1885-1972), who distinguished risk and uncertainty in his work "Risk, Uncertainty, and Profit":

"Uncertainty must be taken in a sense radically distinct from the familiar notion of Risk, from which it has never been properly separated.... The essential fact is that 'risk' means in some cases a quantity susceptible of measurement, while at other times it is something distinctly not of this character; and there are far-reaching and crucial differences in the bearings of the phenomena depending on which of the two is really present and operating.... It will appear that a measurable uncertainty, or 'risk' proper, as we shall use the term, is so far different from an unmeasurable one that it is not in effect an uncertainty at all."

Knight's works remains a classic text to this day.