Logical Fallacy

Education and outreach sometimes lead to debate. And if the debaters are not careful with their arguments, they can espouse errors of reason known as logical fallacies. Here are ten of the most common:

  • Ad Hominem - “against the man”; means an attack on the person rather than the idea. It is when the debate becomes “personal”
  • Tu Quoque - “you too” or the appeal to hypocrisy is a form of ad hominem attack. It is an argument that the opponent is guilty of being a part of the problem they are debating
  • Straw Man - attacking an argument the opponent did not put forth - a way to shift the focus away from a difficult point to one that is easy to “win”
  • Appeal to Ignorance - argument based on what is NOT known, what “hasn’t been proven” or “has no evidence”. Arguments based on ignorance can be easily reversed; due to ignorance of what is true.
  • Causal Fallacy - concluding an outcome without evidence that the outcome will arise. A variation of this is Post hoc (short for post hoc ergo propter hoc “after this, therefore because of this”) argument - mistaking something for the cause just because it happened earlier. Sometimes it is true that an earlier event causes a later event, but being earlier is not sufficient by itself.
  • False Dichotomy - argument based on only two options, when a range of options is possible. In perhaps the simplest case, an either-or choice is argued when “both” is also valid.
  • Slippery Slope - argument based on assuming a (generally improbable) causal chain. If we don’t do {something relatively benign}, then a series of effects will cause {something terrible}
  • Petitio principii - “given the initial argument” or circular argument - repeating what was previously claimed as further evidence, or using the claimed conclusion as evidence for reaching that conclusion.
  • Hasty generalization - unsupported claim, generally exaggerated, overstated, or stereotypical. We live in an era of hasty generalization, and often cannot agree on what level of support is appropriate for a claim.
  • Ignoratio elenchi - “ignorance of the chain” or red-herring - an argument that is a diversion, seemingly relevant but taking the debate in a direction more palatable to the debater.