A black swan event is an incident that occurs randomly and unexpectedly and has wide-spread ramifications. The phrase illustrates the frailty of inductive reasoning and the danger of making sweeping generalizations from limited observations.
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For example, if a man saw a thousand swans and they were all white, he might logically conclude that all swans are white. The flaw in his logic is that even when the premises are true, the conclusion can still be false. In other words, just because the man has never seen a black swan, it does not mean they do not exist. As Dutch explorers discovered in 1697, black swans are simply outliers -- rare birds, unknown to Europeans until Willem de Vlamingh and his crew visited Australia.
Statistician Nassim Nicholas Taleb uses the phrase black swan as a metaphor for how humans deal with unpredictable events in his 2007 book, "The Black Swan." According to Taleb, a black swan event is characterized by its ability to surprise the observer, disrupt the status quo and be rationalized in hindsight. Although black swan events are statistically random and cannot be predicted using computer systems or scientific method, they tend to have such a strong emotional impact that humans fully expect them to happen again. People also tend to put a lot of effort into looking back, trying to connect dots after the fact in an effort to figure out how the event could have been predicted. Instead, Taleb maintains, the time and effort would be better spent focusing on identifying what lessons have been learned and reducing the risk that a single event could have catastrophic consequences in the future.