"useless" tissue might actually have a use that escaped their detection. The
same with the dietary fiber found in fruits and vegetables: doctors in the
1960s found it useless because they saw no immediate evidence of its necessity,
and so they created a malnourished generation. Fiber, it turns out,
acts to slow down the absorption of sugars in the blood and scrapes the
intestinal tract of precancerous cells. Indeed medicine has caused plenty of
damage throughout history, owing to this simple kind of inferential confusion.
I am not saying here that doctors should not have beliefs, only that
some kinds of definitive, closed beliefs need to be avoided—this is what
Menodotus and his school seemed to be advocating with their brand of
skeptical-empirical medicine that avoided theorizing. Medicine has gotten
better—but many kinds of knowledge have not.
Evidence
By a mental mechanism I call na?ve empiricism, we have a natural tendency
to look for instances that confirm our story and our vision of the
world—these instances are always easy to find. Alas, with tools, and fools,
anything can be easy to find. You take past instances that corroborate
your theories and you treat them as evidence. For instance, a diplomat will
show you his "accomplishments," not what he failed to do. Mathematicians
will try to convince you that their science is useful to society by
pointing out instances where it proved helpful, not those where it was a
waste of time, or, worse, those numerous mathematical applications that
inflicted a severe cost on society owing to the highly unempirical nature of
elegant mathematical theories.
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Even in testing a hypothesis, we tend to look for instances where the
hypothesis proved true. Of course we can easily find confirmation; all we
have to do is look, or have a researcher do it for us. I can find confirmation
for just about anything, the way a skilled London cabbie can find
traffic to increase the fare, even on a holiday.
Some people go further and give me examples of events that we have
been able to foresee with some success—indeed there are a few, like landing
a man on the moon and the economic growth of the twenty-first century.
One can find plenty of "counterevidence" to the points in this book,
the best being that newspapers are excellent at predicting movie and theater
schedules. Look, I predicted yesterday that the sun would rise today,
and it did!
NEGATIVE EMPIRICISM
The good news is that there is a way around this naive empiricism. I am
saying that a series of corroborative facts is not necessarily evidence. Seeing
white swans does not confirm the nonexistence of black swans. There
is an exception, however: I know what statement is wrong, but not necessarily
what statement is correct. If I see a black swan I can certify that all
swans are not whitel If I see someone kill, then I can be practically certain
that he is a criminal. If I don't see him kill, I cannot be certain that he is innocent.
The same applies to cancer detection: the finding of a single malignant
tumor proves that you have cancer, but the absence of such a finding
cannot allow you to say with certainty that you are cancer-free.
We can get closer to the truth by negative instances, not by verification!
It is misleading to build a general rule from observed facts. Contrary to
conventional wisdom, our body of knowledge does not increase from a series
of confirmatory observations, like the turkey's. But there are some
things I can remain skeptical about, and others I can safely consider certain.
This makes the consequences of observations one-sided. It is not
much more difficult than that.
This asymmetry is immensely practical. It tells us that we do not have to
be complete skeptics, just semiskeptics. The subtlety of real life over the
books is that, in your decision making, you need be interested only in one
side of the story: if you seek certainty about whether the patient has cancer,
not certainty about whether he is healthy, then you might be satisfied
with negative inference, since it will supply you the certainty you seek. So
CONFIRMATION SHMONFIRMATIONI 57
we can learn a lot from data—but not as much as we expect. Sometimes a
lot of data can be meaningless; at other times one single piece of information
can be very meaningful. It is true that a thousand days cannot prove
you right, but one day can prove you to be wrong.
The person who promoted this idea of one-sided semiskepticism is Sir
Doktor Professor Karl Raimund Popper, who may be the only philosopher
of science who is actually read and discussed by actors in the real world
(though not as enthusiastically by professional philosophers). As I am
writing these lines, a black-and-white picture of him is hanging on the wall
of my study. It was a gift I got in Munich from the essayist Jochen Wegner,
who, like me, considers Popper to be about all "we've got" among modern
thinkers—well, almost. He writes to us, not to other philosophers.
"We" are the empirical decision makers who hold that uncertainty is our
discipline, and that understanding how to act under conditions of incomplete
information is the highest and most urgent human pursuit.
Popper generated a large-scale theory around this asymmetry, based
on a technique called "falsification" (to falsify is to prove wrong) meant
to distinguish between science and nonscience, and people immediately
started splitting hairs about its technicalities, even though it is not the
most interesting, or the most original, of Popper's ideas. This idea about
the asymmetry of knowledge is so liked by practitioners, because it is obvious
to them; it is the way they run their business. The philosopher maudit
Charles Sanders Peirce, who, like an artist, got only posthumous respect,
also came up with a version of this Black Swan solution when Popper
was wearing diapers—some people even called it the Peirce-Popper
approach. Popper's far more powerful and original idea is the "open" society,
one that relies on skepticism as a modus operandi, refusing and resisting
definitive truths. He accused Plato of closing our minds, according
to the arguments I described in the Prologue. But Popper's biggest idea
was his insight concerning the fundamental, severe, and incurable unpredictability
of the world, and that I will leave for the chapter on prediction.*
Of course, it is not so easy to "falsify," i.e., to state that something is
wrong with full certainty. Imperfections in your testing method may yield
a mistaken "no." The doctor discovering cancer cells might have faulty
* Neither Peirce nor Popper was the first to come up with this asymmetry. The
philosopher Victof Brochard mentioned the importance of negative empiricism in
1878, as if it were a matter held by the empiricists to be the sound way to do
business—ancients understood it implicitly. Out-of-print books deliver many surprises.
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equipment causing optical illusions; or he could be a bell-curve-using
economist disguised as a doctor. An eyewitness to a crime might be drunk.
But it remains the case that you know what is wrong with a lot more confidence
than you know what is right. All pieces of information are not
equal in importance.
Popper introduced the mechanism of conjectures and refutations,
which works as follows: you formulate a (bold) conjecture and you start
looking for the observation that would prove you wrong. This is the alternative
to our search for confirmatory instances. If you think the task is
easy, you will be disappointed—few humans have a natural ability to do
this. I confess that I am not one of them; it does not come naturally to me.
Counting to Three
Cognitive scientists have studied our natural tendency to look only for
corroboration; they call this vulnerability to the corroboration error the
confirmation bias. There are some experiments showing that people focus
only on the books read in Umberto Eco's library. You can test a given rule
either directly, by looking at instances where it works, or indirectly, by focusing
on where it does not work. As we saw earlier, disconfirming instances
are far more powerful in establishing truth. Yet we tend to not be
aware of this property.
The first experiment I know of concerning this phenomenon was done
by the psychologist P. C. Wason. He presented subjects with the threenumber
sequence 2, 4, 6, and asked them to try to guess the rule generating
it. Their method of guessing was to produce other three-number
sequences, to which the experimenter would respond "yes" or "no" depending
on whether the new sequences were consistent with the rule.
Once confident with their answers, the subjects would formulate the
rule. (Note the similarity of this experiment to the discussion in Chapter 1
of the way history presents itself to us: assuming history is generated according
to some logic, we see only the events, never the rules, but need to
guess how it works.) The correct rule was "numbers in ascending order,"
nothing more. Very few subjects discovered it because in order to do so
they had to offer a series in descending order (that the experimenter would
say "no" to). Wason noticed that the subjects had a rule in mind, but gave
him examples aimed at confirming it instead of trying to supply series that
were inconsistent with their hypothesis. Subjects tenaciously kept trying to
confirm the rules that they had made up.
CONFIRMATION SHMONFIRMATION! 59
This experiment inspired a collection of similar tests, of which another
example: Subjects were asked which questions to ask to find out whether
a person was extroverted or not, purportedly for another type of experiment.
It was established that subjects supplied mostly questions for which
a "yes" answer would support the hypothesis.
But there are exceptions. Among them figure chess grand masters,
who, it has been shown, actually do focus on where a speculative move
might be weak; rookies, by comparison, look for confirmatory instances
instead of falsifying ones. But don't play chess to practice skepticism. Scientists
believe that it is the search for their own weaknesses that makes
them good chess players, not the practice of chess that turns them into
skeptics. Similarly, the speculator George Soros, when making a financial
bet, keeps looking for instances that would prove his initial theory wrong.
This, perhaps, is true self-confidence: the ability to look at the world without
the need to find signs that stroke one's ego.*
Sadly, the notion of corroboration is rooted in our intellectual habits
and discourse. Consider this comment by the writer and critic John Updike:
"When Julian Jaynes . . . speculates that until late in the second millennium
B.C. men had no consciousness but were automatically obeying
the voices of gods, we are astounded but compelled to follow this remarkable
thesis through all the corroborative evidence." Jaynes's thesis may be
right, but, Mr. Updike, the central problem of knowledge (and the point
of this chapter) is that there is no such animal as corroborative evidence.
Saw Another Red Mini!
The following point further illustrates the absurdity of confirmation. If
you believe that witnessing an additional white swan will bring confirmation
that there are no black swans, then you should also accept the statement,
on purely logical grounds, that the sighting of a red Mini Cooper
should confirm that there are no black swans.
Why? Just consider that the statement "all swans are white" implies
* This confirmation problem pervades our modern life, since most conflicts have at
their root the following mental bias: when Arabs and Israelis watch news reports
they see different stories in the same succession of events. Likewise, Democrats and
Republicans look at different parts of the same data and never converge to the
same opinions. Once your mind is inhabited with a certain view of the world, you
will tend to only consider instances proving you to be right. Paradoxically, the
more information you have, the more justified you will feel in your views.
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that all nonwhite objects are not swans. What confirms the latter statement
should confirm the former. Therefore, the sighting of a nonwhite object
that is not a swan should bring such confirmation. This argument,
known as Hempel's raven paradox, was rediscovered by my friend the
(thinking) mathematician Bruno Dupire during one of our intense meditating
walks in London—one of those intense walk-discussions, intense to
the point of our not noticing the rain. He pointed to a red Mini and
shouted, "Look, Nassim, look! No Black Swan!"
Not Everything
We are not na?ve enough to believe that someone will be immortal because
we have never seen him die, or that someone is innocent of murder because
we have never seen him kill. The problem of na?ve generalization
does not plague us everywhere. But such smart pockets of inductive skepticism
tend to involve events that we have encountered in our natural
environment, matters from which we have learned to avoid foolish generalization.
For instance, when children are presented with the picture of a single
member of a group and are asked to guess the properties of other unseen
members, they are capable of selecting which attributes to generalize.
Show a child a photograph of someone overweight, tell her that he is a
member of a tribe, and ask her to describe the rest of the population: she
will (most likely) not jump to the conclusion that all the members of the
tribe are weight-challenged. But she would respond differently to generalizations
involving skin color. If you show her people of dark complexion
and ask her to describe their co-tribesmen, she will assume that they too
have dark skin.
So it seems that we are endowed with specific and elaborate inductive
instincts showing us the way. Contrary to the opinion held by the great
