The black swan theory or theory of black swan events is a metaphor that describes an event that comes as a surprise, has a major effect, and is often inappropriately rationalized after the fact with the benefit of hindsight. The term is based on an ancient Roman saying expressing the European presumption that black swans did not exist until Dutch mariners saw them in Australia in 1697, and the term was then reinterpreted to mean an unforeseen and consequential event.

Decimus Junius Juvenalis, known in English as Juvenal, was a Roman poet active in the late first and early second century AD. He is the author of the collection of satirical poems known as the Satires. The details of Juvenal's life are unclear, although references within his text to known persons of the late first and early second centuries AD fix his earliest date of composition. One recent scholar argues that his first book was published in 100 or 101. A reference to a political figure dates his fifth and final surviving book to sometime after 127.