In his book, "The Demon-Haunted World", Carl Sagan provides
tools for skeptical thinking. This excellent list is a strong
tool to weed out the bad seeds in science.
Quoting ad verbatim:
- Wherever possible there must be independent
confirmation of the "facts".
- Encourage substantive debate on the evidence by
knowledgeable proponents of all points of view.
- Arguments from authority carry little weight -
"authorities" have made mistakes in the past.
They will do so again in the future. Perhaps a better way to
say it is that in science there are no authorities; at most,
there are experts.
- Spin more than one hypothesis.
If there's something to be explained, think of all the
different ways in which it could be explained. Then
think of tests by which you might systematically disprove
each of the alternatives. What survives, the hypothesis that
resists disproof in this Darwinian selection among "multiple
working hypotheses," has a much better chance of being the
right answer than if you have simply run the with first idea
that caught your fancy.
- Try not to get overly attached to a hypothesis just
because it's yours.
It's only a way station in the pursuit of knowledge. Ask
yourself why you like the idea. Compare it fairly with the
alternatives. See if you can find reasons for rejecting it.
If you don't, others will.
If whatever it is you're explaining has some measure, some
numerical quantity attached to it, you'll be much better
able to discriminate among competing hypotheses. What is
vague and qualitative is open to many explanations. Of
course there are thruths to be sought in the many
qualitative issues we are obliged to confront, but finding
them is more challenging.
- If there's a chain of argument, every link in
the chain must work (including the premise) - not just most
- Occam's Razor.
This convenient rule-of-thumb urges us when faced with two
hypotheses that explain the data equally well to
choose the simpler.
- Always ask whether the hypothesis can be, at least in
Propositions that are untestable, unfalsifiable are not
worth much. Consider the grand idea that our Universe and
everything in it is just an elementary particle - an
electron, say - in a much bigger Cosmos. But if we can never
acquire information from outside our Universe, is not the
idea incapable of disproof? You must be able to check
assertions out. Inveterate skeptics must be given the chance
to follow your reasoning, to duplicate your experiments and
see if they get the same result.
- Ad hominem.
Latin for "to the man", attacking the arguer and not the
argument. E.g., The Reverend Dr. Smith is a known
Biblical fundamentalist, so her objections to evolution need
not be taken seriously.
- Argument from authority.
E.g., President Richard Nixon should be re-elected
because he has a secret plan to end the war in Southeast
Asia - but because it was secret, there was no way for
the electorate to evaluate it on its merits; the argument
amounted to trusting him because he was President; a
mistake, as it turned out.
- Argument from adverse consequences.
E.g., A God meting out punishment and reward must exist,
because if He didn't, society would be much more lawless and
dangerous - perhaps even ungovernable.
Or: The defendant in a widely publicized murder trial
must be found guilty; otherwise, it will be an encouragement
for other men to murder their wives.
- Appeal to ignorance.
The claim that whatever has not been proved false must be
true, and vice versa. E.g., There is no compelling
evidence that UFOs are not visiting the Earth; therefore
UFOs exist - and there is intelligent life elsewhere in the
Or: There may be seventy kazillion other worlds, but
not one is known to have the moral advancement of the Earth,
so we're still central to the Universe.
This impatience with ambiguity can be critized in the
phrase: absence of evidence is not evidence of absence.
- Special pleading.
Often to rescue a proposition in deep rhetorical trouble.
E.g., How can a merciful God condemn future
generations to torment because, against orders, one woman
induced one man to eat an apple?
Special plead: You don't understand the subtle Doctrine
of Free Will.
Or: How can there be an equally godlike Father, Son
and Holy Ghost in the same person?
Special plead: You don't understand the Divine Mystery of
Or: How could God permit the followers of Judaism,
Christianity, and Islam - each in their own way enjoined to
heroic measures of loving kindness and compassion - to have
perpetrated so much cruelty for so long?
Special plead: You don't understand Free Will again. And
anyway, God moves in mysterious ways.
- Begging the question.
Also called assuming the answer. E.g., We must institute
the death penalty to discourage violent crime. But does
the violent crime rate in fact fall when the death penalty
Or: The stock market fell yesterday because of a
technical adjustment and profit-taking by investors. But
is there any independent evidence for the causal role of
"adjustment" and profit-taking; have we learned anything at
all from this purported explanation?
- Observational selection.
Also called the enumeration of favorable circumstances, or
as the philosopher Francis Bacon described it, counting the
hits and forgetting the misses. E.g., A state boasts of
the Presidents it has produced, but is silent on its serial
- Statistics of small numbers.
A close relative of observational selection. E.g., They
say 1 out of 5 people is Chinese. How is this possible? I
know hundreds of people, and none of them is Chinese.
Or: I've thrown three sevens in a row. Tonight I can't
- Misunderstanding of the nature of statistics.
E.g., President Dwight Eisenhower expressing astonishment
and alarm on discovering that fully half of all Americans
have below average intelligence.
E.g., Prudently plan for the worst of which a potential
military adversary is capable, but thriftly ignore
scientific projections on environmental dangers because
they're not "proved".
Or: Attribute the declining life expectancy in the
former Soviet Union to the failures of communism many years
ago, but never attribute the high infant mortality rate in
the United States (now highest of the major industrial
nations) to the failures of capitalism.
Or: Consider it reasonable for the Universe to
continue to exist forever into the future, but judge absurd
the possibility that it has infinite duration into the past.
- Non Sequitur.
Latin for "It doesn't follow". E.g., Our nation will
prevail because God is great. But nearly every nation
pretends this to be true; the German formulation was "Gott
mit uns". Often those falling into the non sequitur fallacy
have simply failed to recognize alternative possibilities.
- Post hoc, ergo propter hoc.
Latin for "It happened after, so it was caused by". E.g.,
Jaime Cardinal Sin, Archbishop of Manila: "I know of...a
26-year-old who looks 60 because she takes [contraceptive]
Or: Before women got the vote, there were no nuclear
- Meaningless question.
E.g., What happens when an irresistible force meets an
immovable object? But if there is such a thing as an
irresistible force there can be no immovable objects, and
- Excluded middle, or False Dichotomy.
Considering only the two extremes in a continuum of
intermediate possibilities. E.g., Sure, take his side; my
husband's perfect; I'm always wrong.
Or: Either you love your country or you hate it.
Or: If you're not part of the solution, you're part of
- Short-term vs. long-term.
A subset of the excluded middle, but so important I've
pulled it out for special attention. E.g., We can't
afford programs to feed malnourished children and educate
pre-school kids. We need to urgently deal with crime on the
Or: Why explore space or pursue fundamental science
when we have so huge a budget deficit?
- Slippery slope.
Related to excluded middle. E.g., If we allow abortion in
the first weeks of pregnancy, it will be impossible to
prevent the killing of a full-term infant.
Or, conversely: If the state prohibits abortion even in
the ninth month, it will soon be telling us what to do with
our bodies around the time of conception.
- Confusion of correlation and causation.
E.g., A survey shows that more college graduates are
homosexual than those with lesser education; therefore
education makes people gay.
Or: Andean earthquakes are correlated with closest
approaches of the planet Uranus; therefore - despite the
absence of any such correlation for the nearer, more massive
planet Jupiter - the latter causes the former.
- Straw man.
Caricaturing a position to make it easier to attack. E.g.,
Scientists suppose that living things simply fell
together by chance - a formulation that willfully
ignores the central Darwinian insight, that Nature ratches
up by saving what works and discarding what doesn't.
Or - this is also a short-term/long-term fallacy -
environmentalists care more for snail darters and spotted
owls than they do for people.
- Suppressed evidence, or half-truths.
E.g., An amazingly accurate and widely quoted "prophecy"
of the assasination attempt on President Reagan is shown on
television; but - an important detail - was it recorded
before or after the event?
Or: These government abuses demand revolution, even if
you can't make an omelette without breaking some eggs.
Yes, but is this likely to be a revolution in which for more
people are killed than under the previous regime? What does
the experience of other revolutions suggest? Are all
revolutions against oppressive regimes desirable and in the
interests of the people?
- Weasel words.
E.g., The separation of powers of the U.S. Constitution
specifies that the United States may not conduct a war
without a declaration by Congress. On the other hand,
Presidents are given control of foreign policy and the
conduct of wars, which are potentially powerful tools for
getting themselves re-elected. Presidents of either
political party may therefore be tempted to arrange wars
while waving the flag and calling the wars something else -
"police actions," "armed incursions," "protective reaction
strikes," "pacification," "safeguarding American interests,"
and a wide variety of "operations," such as "Operation Just
Cause." Euphemisms for war are on of a broad class of
reinventions of language for political purposes. Talleyrand
said, "An important art of politicians is to find new names
for institutions which under old names have become odious to
Carl Sagan, astronomer and storyteller extraordinaire, showed
how Nature and science could be explained so everyone could
understand it. He was truly one of a kind.
He will be sorely missed.