When members of an expedition led by Dutch explorer Willem de Vlamingh discovered black swans in Western Australia in 1697, they ruined a perfectly good idiom that had been around for over a thousand years. Roman poet Juvenal was probably responsible for the expression that translated into medieval English as "impossible as a black swan." The Old World presumption was that all swans were white, no evidence appearing to the contrary. These days, a "black swan event" is one that is so unlikely it can't be foreseen, and throws all previous predictions out the window.
Suppose, for instance, I toss a coin 20 times in front of you, and it comes up heads every time. What odds would you give me that it comes up heads on the 21st flip? If you're a logical thinker, you might respond, "50/50. The coin doesn't have a memory." If, however, you trust your intuition more than your mathematical ability, chances are you'll say, "100 percent." Since the odds of 20 heads in a row are about one in a million (two to the 20th power), you could safely assume I'm using a trick coin, so if the next toss came up tails, you would consider it a black swan event.
This is the basis, incidentally, of philosopher Karl Popper's proposal that, for a hypothesis to be scientifically valid, it should be falsifiable. It's obviously impossible to observe all the swans in the world to confirm the hypothesis all swans are white. However, the statement is falsifiable, since finding a single black swan negates it.
In his book The Black Swan, statistician and stock-trader Nassim Nicholas Taleb claims that the most important events in history are unpredictable: They couldn't have been foreseen using the information we had prior to their occurrence, such as the stock market crashes of Oct. 29, 1929, or Oct. 19, 1987. Taleb likens the situation to that of a turkey who has been fed by kindly humans for three years, lulled into complacency by the predictability of his daily vittles. The next day - the day before Thanksgiving, as it happens - his luck runs out. For the turkey, it's a black swan event (although for the butcher, it's entirely predictable, like someone who knows it's a trick coin).
Other black swan events in history might include the assassination in Sarajevo of Archduke Franz Ferdinand of Austria on June 28, 1914, the catalyst that led to the outbreak of the First World War (the black swan event was that Franz Ferdinand's driver took a wrong turn); 9/11 (one of several black swan events was that President Bush's Daily Briefing of Aug. 6, 2001, titled "Bin Ladin Determined To Strike in U.S.," was ignored; Condoleezza Rice later said, "it wasn't something that we felt we needed to do anything about"); and last year's Arab Spring (touched off in the Tunisian city of Sidi Bouzi by a street vendor, Mohamed Bouazizi, who set himself on fire on Dec. 17, 2010, to protest his harassment by a municipal official).
You don't have to review history to find black swan events. Look at the major events of your own life that brought you to this place and time, and ask yourself: Could you reasonably have foreseen them? For myself, unpredictable serendipity was the major player. You?
Barry Evans (firstname.lastname@example.org) invokes induction to predict the sun will rise tomorrow. Or not.