“rara avis in terris nigroque simillima cygno”

“a bird as rare upon the earth as a black swan”

From this old roman expression, the phrase “black swan” was a common saying in 16th century London as a statement of impossibility. It derives from the presumption that all swans must be white because all historical records of swans reported that they had white feathers. Thus, a black swan was believed to be impossible or at least nonexistent.

However, in 1697, Dutch explorers led by Willem de Vlamingh became the first Europeans to see black swans, in Western Australia. The term then changed its meaning to connote the idea that a perceived impossibility might later be disproved.