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It's difficult to read the business news without encountering the expression "black swan." The reference is to an influential 2007 book by options trader Nassim Nicholas Taleb, who wrote that humans are quick to devise explanations for everything after the fact but are woefully unprepared for randomness. He began The Black Swan: The Impact of the Highly Improbable by noting that Europeans once swore that all swans were white - until they found black swans in Australia. The earlier certainty "illustrates a severe limitation to our learning from observations or experience and the fragility of our knowledge." From this Taleb developed the image of the black swan as an unexpected event with an enormous impact, either positive (the rise of the Internet, though some did expect that) or negative (the terrorist attacks of Sept. 11, 2001).

The image has taken flight, appropriately enough. In an interview last July, Janice Fukakusa, chief financial officer of Royal Bank of Canada, referred to "the black-swan events - very low probability, but high impact." In The Chicago Sun-Times on Oct. 10, Terry Savage wrote, in reference to the current financial crisis, that "unless this is the proverbial 'black swan' - the unimaginable and unique event that annihilates capitalism - this panic will subside."

Of word-play interest is the direct connection between Taleb's image and a Latin phrase common in English: rara avis, a rare bird, referring to something or someone exceptional. The Times of London referred to the late geographer David Hooson in June as "that rara avis, a Soviet specialist who was not a Cold Warrior."

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The phrase dates from 1,900 years ago, in the Satires of Roman author Juvenal, who had a way with memorable phrases. (We may thank him for "bread and circuses" and "a healthy mind in a healthy body.") The original Latin was " Rara avis in terris nigroque simillima cycno" (also given as cygno, and I will leave it to Latin scholars to debate the discrepancy). Its meaning: "a bird rarely seen on Earth, much like a black swan." That Juvenal hedged his bets - using rarely rather than never, despite not having heard of Australia or the existence of black swans - indicates a commendable openness to the possibility of the unpredictable.

The swan isn't alone among proverbial black animals. The black cat was once considered Satan's vessel, and a woman with a black cat stood a greater chance of being branded as a witch. The superstition persists in the warning not to let a black cat cross one's path. The black sheep of the family - the unruly or scandalous one - originates in the relative rarity of black sheep a millennium or so ago. There weren't enough of them to create a market for black wool, and their fleece couldn't be dyed white, so to have a black sheep was considered inconvenient and a source of bad luck. There were also stories that a black sheep would disrupt the flock. By the 1700s the image was being used for children and rogue uncles who brought shame to their families. English has also adopted the French phrase bĂȘte noire, black beast, to describe an object of fear or dislike, and British leader Winston Churchill's use of "black dog" to describe his bouts of depression was arresting enough that others have borrowed it.

Then there's the dark horse, the one that nobody expects to win the race but that surprises the crowd by coming first. The image's popularity dates from an 1831 novel by Benjamin Disraeli, who was later to be Britain's prime minister. "A dark horse which had never been thought of," he wrote in The Young Duke, "and which the careless St. James had never even observed in the list, rushed past the grandstand in sweeping triumph." It is probable that Disraeli had no intention of creating a metaphor, and was simply referring to a dark-coloured horse, but he was a popular author and the image spread. It was in general use in American politics by the 1860s to describe the triumph of everyone's last choice. Possibly a dark horse could be so unexpected, and create such turmoil, that he would qualify as a black swan, but that way lies unethical cross-breeding.

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