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In economics, Knightian uncertainty is a lack of any quantifiable knowledge about some possible occurrence, as opposed to the presence of quantifiable risk (e.g., that in statistical noise or a parameter's confidence interval). The concept acknowledges some fundamental degree of ignorance, a limit to knowledge, and an essential unpredictability of future events.
- "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."
In this matter Knight's own views were widely shared by key economists in the 1920s and 1930s who played a key role distinguishing the effects of risk from uncertainty. They were particularly concerned with the different impact on human behavior as economic agents. Entrepreneurs invest for quantifiable risk and return, savers may mistrust potential future inflation.
Whilst Frank Knight's seminal book elaborated the problem, his focus was on avoiding intervention in markets. Work on estimating and mitigating uncertainty was continued by G. L. S. Shackle who later followed up with Potential Surprise Theory. Shackle, G.L.S. (1938) "Expectations, Investment and Income" (Oxford: Oxford University Press) via  However the concept is largely informal and there is no single best formal system of probability and belief to represent Knightian uncertainty. Economists and management scientists continue to look at practical methodologies for decision under different types of uncertainty.
Common cause and special cause
The difference between predictable variation and unpredictable variation is one of the fundamental issues in the philosophy of probability, and different probability interpretations treat predictable and unpredictable variation differently. The distinction and debate has a long history.
Black swan events
A black swan event, as analyzed by Nassim Nicholas Taleb, is an important and inherently unpredictable event that, once occurred, is rationalized with the benefit of hindsight. Historical developments like the widespread adoption of the personal computer, were entirely impossible to predict but nevertheless had world-changing effects. Another position of the black swan theory is that appropriate preparation for these events is frequently hindered by the pretense of knowledge of all the risks; in other words, Knightian uncertainty is presumed to not exist in day-to-day affairs, often with disastrous consequences.
- Knight, F. H. (1921) Risk, Uncertainty, and Profit. Boston, MA: Hart, Schaffner & Marx; Houghton Mifflin Company
- Köhn, Julia (2017). Uncertainty in economics : a new approach. Cham: Springer. ISBN 978-3-319-55351-1.
- Shackle, G.L.S. (1968). "Expectations, Investment and Income". Oxford: Clarendon P. ISBN 0-19-828157-9.
- Economics as an art of thought : essays in memory of g.l.s. shackle. Place of publication not identified: Routledge. 2013. ISBN 978-0-415-86230-1.