Common fallacies of logic and rhetoric (by Carl Sagan)
Look out for these sure signs of fallacious arguments when having a discussion!
  • Ad hominem - attacking the arguer and not the argument.
  • Argument from “authority”.
  • Argument from adverse consequences (putting pressure on the decision maker by pointing out dire consequences of an “unfavorable” decision).
  • Appeal to ignorance (absence of evidence is not evidence of absence).
  • Special pleading (typically referring to god’s will).
  • Begging the question (assuming an answer in the way the question is phrased).
  • Observational selection (counting the hits and forgetting the misses).
  • Statistics of small numbers (such as drawing conclusions from inadequate sample sizes).
  • Misunderstanding the nature of statistics (President Eisenhower expressing astonishment and alarm on discovering that fully half of all Americans have below average intelligence!)
  • Inconsistency (e.g. military expenditures based on worst case scenarios but scientific projections on environmental dangers thriftily ignored because they are not “proved”).
  • Non sequitur - “it does not follow” - the logic falls down.
  • Post hoc, ergo propter hoc - “it happened after so it was caused by” - confusion of cause and effect.
  • Meaningless question (”what happens when an irresistible force meets an immovable object?).
  • Excluded middle - considering only the two extremes in a range of possibilities (making the “other side” look worse than it really is).
  • Short-term v. long-term - a subset of excluded middle (”why pursue fundamental science when we have so huge a budget deficit?”).
  • Slippery slope - a subset of excluded middle - unwarranted extrapolation of the effects (give an inch and they will take a mile).
  • Confusion of correlation and causation.
  • Caricaturing (or stereotyping) a position to make it easier to attack.
  • Suppressed evidence or half-truths.
  • Weasel words - for example, use of euphemisms for war such as “police action” to get around limitations on Presidential powers. “An important art of politicians is to find new names for institutions which under old names have become odious to the public”

18 Responses to this post
I know this isn’t your list but I must point out that “Misunderstanding the nature of statistics (President Eisenhower expressing astonishment and alarm on discovering that fully half of all Americans have below average intelligence!)” demonstrates exactly what it purporst to describe - a misunderstanding of the nature of statistics.

Let’s say that there are a total of 10 Americans. Nine have an IQ of 90 and the tenth has an IQ of 150. The average IQ is 96. Therefore nine (well less than half) of all Americans have IQ less that the average.

Eisenhower’s astonishment was entirely reasonable and logical.
Speaking of below average intelligence, I should have proof-read my submission. Change “purporst” to “purports” and change “well less than half” to “well over half”.
David, thanks for the comments.

However, I think you might have missed something here. The mean IQ should be 100 in a large enough population.

This is true especially considering that the scores are adjusted to express the average improvements in population intelligence over time -I am not sure about the USA though :-).

Therefore, from the curve that describes the IQ distribution, approximately half of the population should be on the left side of 100 (i.e. less than average) and half should be on the right side of 100 (more than average).

Hence, Eisenhower’s astonishment is NOT logical since this is going to be the normal distribution in pretty much all large populations!
If someone had said to President Eisenhower that half of the population was less bright than the other half, I doubt he’d have been astonished, except at the temerity of the person taking the President’s time for the purpose of telling him such a noninformative thing.

Presumably Eisenhower did not assume a mere tautology was being reported to him in the situation.

The term “average” can refer to arithmetic mean, to the median, or to the mode (most often occuring).

In the reported instance, President Eisenhower probably visualized something like the bell curve, and probably recognized that it should be the case that most persons were around average, which in fact was and is the case.

So, presumably, he thought it was being reported to him that the top of the curve was lower than at the 50% mark, or that the curve was not an even bell curve, but was skewed toward the lower end, which would indeed be alarming.

Why would anyone think he needed to be told that 50=50?

I like Carl sagan, but I don’t think he fully thought this through before sideswipingly impugning the intellect of the brilliant General Officer and great American President, Dwight D. Eisenhower, whom history will always recognize as one the foremost men responsible for the victory of the allies in WWII.
Ok, I admit that the statistical thing is too much for my destroyed brain. But the list of fallacies sounds like a good guidance :)
“I don’t think he fully thought this through before sideswipingly impugning the intellect of the brilliant General Officer and great American President, Dwight D. Eisenhower, whom history will always recognize as one the foremost men responsible for the victory of the allies in WWII.”

Perhaps Sagan should have fit in something about not worshiping idols to this list as well.
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