Logical Fallacies

A good debater can recognise the most common and perilous fallacies of logic and rhetoric. Many good examples can be found in politics, because politicians are so often obliged to justify two contradictory propositions. Among these fallacies are:

• 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);

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  * NOTE: A more cynical formulation by the Roman historian Polybius: Since the masses of the people are inconstant, full of unruly desires, passionate, and reckless of consequences, they must be filled with fears to keep them in order. The ancients did well, therefore, to invent gods, and the belief in punishment after death.

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• 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 Universe. 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 criticized 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 the Trinity. 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 is imposed? 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 killers); *

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  * NOTE: My favorite example is this story, told about the Italian physicist Enrico Fermi, newly arrived on American shores, enlisted in the Manhattan nuclear weapons Project, and brought face-to-face in the midst of World War II with U.S. flag officers.

  So-and-so is a great general, he was told. What is the definition of a great general? Fermi characteristically asked. I guess it's a general who's won many consecutive battles. How many? After some back and forth, they settled on five. What fraction of American generals are great? After some more back and forth, they settled on a few percent.

  But imagine, Fermi rejoined, that there is no such thing as a great general, that all armies are equally matched, and that winning a battle is purely a matter of chance. Then the chance of winning one battle is one out of two, or 1/2, two battles l/4, three l/8, four l/16, and five consecutive battles 1/32 - which is about 3 percent. You would expect a few percent of American generals to win five consecutive battles - purely by chance. Now, has any of them won ten consecutive battles ...?

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• statistics of small numbers

  - a close relative of observational selection (e.g., "They say 1 out of every 5 people is Chinese. How is this possible? I know hundreds of people, and none of them is Chinese. Yours truly." Or: "I've thrown three sevens in a row. Tonight I can't lose.");
  

• 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);
  

• inconsistency

  (e.g., Prudently plan for the worst of which a potential military adversary is capable, but thriftily 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] pills." Or: Before women got the vote, there were no nuclear weapons);
  

• 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 the problem");
  

• 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 streets. 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); *

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  * NOTE: Children who watch violent TV programs tend to be more violent when they grow up. But did the TV cause the violence, or do violent children preferentially enjoy watching violent programs? Very likely both are true. Commercial defenders of TV violence argue that anyone can distinguish between television and reality. But Saturday morning children's programs now average 25 acts of violence per hour. At the very least this desensitizes young children to aggression and random cruelty. And if impressionable adults can have false memories implanted in their brains, what are we implanting in our children when we expose them to some 100,000 acts of violence before they graduate from elementary school?

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Knowing the existence of such logical and rhetorical fallacies rounds out our debating skills. Like all skills, such knowledge can be misused, applied out of context, or even employed as a rote alternative to thinking. But applied judiciously, it can make all the difference in a debate - not least in evaluating our own arguments before we present them to others.