The Black Swan theory (in Nassim Nicholas Taleb’s version) refers to a large-impact, hard-to-predict, and rare event beyond the realm of normal expectations. The term black swan comes from the commonplace Western cultural assumption that all swans are white. In that context, a black swan was a metaphor for something that could not exist. The 17th Century discovery of black swans in Australia metamorphosed the term to connote that the perceived impossibility actually came to pass.
In risk management, we need to deal with black swans that have consequences. Further, a search in the literature in the philosophy and history of probability shows the depressing fact that large impact events are absent from discussions. Probabilities by themselves do not matter. They can be very small, but their results are not. What matters in life is the equation probability x consequence. This point may appear to be simple, but its consequences are not.
If small probability events carry large impacts, and these small probability events are more difficult to compute from past data itself, then our empirical knowledge about the potential contribution – or role – of rare events (probability x consequence) is inversely proportional to their impact.