The term black swan was a Latin expression — its oldest known reference comes from the poet Juvenal's characterization of something being "rara avis in terris nigroque simillima cygno".
In English, this Latin phrase means "a rare bird in the lands, and very like a black swan." When the phrase was coined, the black swan was presumed not to exist. The importance of the simile lies in its analogy to the fragility of any system of thought. A set of conclusions is potentially undone once any of its fundamental postulates is disproven. In this case, the observation of a single black swan would be the undoing of the phrase's underlying logic, as well as any reasoning that followed from that underlying logic.
Juvenal's phrase was a common expression in 16th century London as a statement of impossibility. The London expression derives from the Old World presumption that all swans must be white because all historical records of swans reported that they had white feathers. In that context, a black swan was impossible or at least nonexistent.
Black Swan Events were characterized by Nassim Nicholas Taleb in his 2007 book (revised and completed in 2010), The Black Swan. Taleb regards almost all major scientific discoveries, historical events, and artistic accomplishments as "black swans" — undirected and unpredicted. He gives the rise of the Internet, the personal computer, World War I, and the September 11 attacks as examples of Black Swan Events.
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