黑天鵝

在黑天鵝被發(fā)現(xiàn)之前，生活在十七世紀(jì)歐洲的人們都相信一件事——所有的天鵝都是白色的。因為當(dāng)時所能見到的天鵝的確都是白色的，所以根據(jù)經(jīng)驗主義，那簡直就是一個真理，或者至少可以算是一個公理吧。那么，見到黑色天鵝的幾率是多少呢?根本無法計算，也沒有人想過要計算。直到1697年，探險家在澳大利亞發(fā)現(xiàn)了黑天鵝，人們才知道以前的結(jié)論是片面的——并非所有的天鵝都是白色的。這一事件證明了我們的認(rèn)知是多么地具有局限性——雖然你是在觀察了幾百萬只天鵝之后才得出了“所有的天鵝都是白色的”結(jié)論，但只需要另一個發(fā)現(xiàn)就能將它徹底推翻。

黑天鵝事件是不可預(yù)測的重大事件。它罕有發(fā)生，但一旦發(fā)生，就具有很大的影響力。

Consider a turkey that is fed every day. Every single feeding will firm up the bird’s belief that it is the general rule of life to be fed every day by friendly members of the human race， looking out for its best interests， as a politician would say. On the afternoon of the Wednesday before Thanksgiving， something unexpected will happen to the turkey. It will 1)incur a revision of belief.

The rest of this chapter will outline the Black Swan Problem in its original form: how can we know the future， given knowledge of the past? Or， more generally， how can we figure out properties of the infinite unknown， based on the finite known?

Think of the feeding again. What can a turkey learn about what is in store for it tomorrow from the events of yesterday? A lot， perhaps， but certainly a little less than it thinks. And it is just that little less that may make all the difference.

The Turkey Problem can be generalized to any situation where the same hand that feeds you can be the one that 2)rings your neck.

Let’s go one step further and consider 3)induction’s most worrisome aspect: learning backward.

Consider that the turkey’s experience may have， rather than no value， a negative value. It learned from observation， as we are all advised to do. (Hey， after all， this is what is believed to be the scientific method.) Its confidence increased as the number of friendly feedings grew， and it felt increasingly safe， even though the 4)slaughter was more and more 5)imminent. Consider that the feeling of safety reached its maximum when the risk was at the highest. But the problem is even more general than that. It strikes at the nature of 6)empirical knowledge itself. Something has worked in the past until…well， it unexpectedly no longer does. And what we have learned from the past turns out to be， at best， 7)irrelevant or 1; at worst，8)viciously misleading.

Take， for example， the proto-typical case of The Problem of Induction as encountered in real life. You observe a 9)hypothetical variable for one thousand days. It could be anything， with a few mild transformations: book sales， blood pressure， crimes， your personal income， a given stock， the interest on a loan， or Sunday attendance at a specific Greek Orthodox Church. You subsequently derive， solely from past data， a few conclusions concerning the properties of the pattern， with projections for the next thousand， even five thousand days. On the one thousand and first day， boom， a big change takes place that is completely unprepared for by the past.

Note that after the event you start predicting the possibility of other 10)outliers happening locally， that is in the process you were just surprised by， but not elsewhere. After 11)the stock market crash of 1987， half of America’s traders braced for another one every October， not taking into account that there was no 12)antecedent for the first one. We worry too late—“13)ex post”.Mistaking a naive observation of the past as something definitive or representative of the future is the one and only cause of our inability to understand the “Black Swan.”

Those who believe in the unconditional benefits of past experience should consider this pearl of wisdom， 14)allegedly voiced by a famous ship’s captain.

“But in all my experience I have never been in any accident of any sort worth speaking about. I have seen but one vessel in distress in all my years at sea. I never saw a wreck and never have been wrecked， nor was I ever in any15)predicament that threatened to end in disaster of any sort.”

——E. J. Smith， 1907，

Captain， RMS Titanic

Captain Smith’s ship sank in 1912 in what became the most talked-about shipwreck in history. Statements like those of Captain Smith are so common that it isn’t even funny.

想象一下一只每天有人喂食的火雞。每次喂食都使它更加相信生命的一般法則就是每天得到“為它的最大利益著想”(政客們都這么說)的友善人類的喂食。感恩節(jié)前的星期三下午，一件意料之外的事情將發(fā)生在它身上。這件事將導(dǎo)致一次信念的轉(zhuǎn)變。

本章余下的部分將以原本的形式概括“黑天鵝問題”:如何從過去的知識中預(yù)知未來，或更籠統(tǒng)地說，如何從有限的已知推測無限的未知?

再想想喂食的例子:一只火雞如何通過對昨天情況的觀察知道明天給它喂食的量有多少?可能很多，但肯定比想象的少一點，但就是那“少一點”使事情有完全不同的結(jié)果。

“火雞問題”可以把“喂你的手也可能是擰斷你脖子的那只手”的情況一般化，運用到所有事情上。……