Thursday, January 1, 2015

Fallacy of Reasoning: "Post Hoc Ergo Propter Hoc"

"Post hoc ergo propter hoc"  is Latin for the fallacy of reasoning of "after this, therefore because of this."  In an episode of West Wing, President Bartlet  challenged the 27 lawyers in the room that at least one of them should know the meaning of this Latin phrase.  In an episode of Big Bang, Sheldon also mentions this logical fallacy. 

Logical Fallacy

"Post hoc ergo propter hoc" is a logical fallacy referring to questionable causation, that is, "since event Y followed event X, event Y must have been caused by event X." This example is given:  "The rooster crows immediately before the sunrise, therefor the rooster causes the sun to rise." Even Ernie on Sesame Street falls victim to this fallacy of reasoning in this clip where he reached the conclusion that it is the banana in his ear that is keeping the alligators away.  

5th and 11th Circuit

The Court in Huss v. Gayden, 571 F.3d 442 (5th Cir. 2009) refers to this logical fallacy in the determination of the admissibility of expert testimony. The Court in Huss reviewed that it "is axiomatic that causation testimony is inadmissible if an expert relies upon studies or publications, the authors of which were themselves unwilling to conclude that causation had been proven."  

More Logical Fallacies to Avoid

  • "Argumentum ad Hominen" - argument directed to the person - instead of attacking the opponent's argument, the character of the opponent is attacked.
  • "Argumentum ad Misericordian" - an appeal to pity - the audience is asked to accept an argument not due to the strength of the argument but rather because of the speaker's piteous circumstances.
  • "Argumentum ad Populum" - the appeal to emotion - the attempt to establish its conclusion with values the speaker's audience holds dear. 
  • "Ignoratio Elenchi" - proving an irrelevant conclusion - attacking the "straw man"
  • "Petitio Principii" - a circular argument - begging the question. 

The Beatles 

It is now known that the Beatles' song were full, full, full of logical fallacies, such as: sweeping generalizations ("all you need is love"), ad agnorantium/appeal to ignorance ("no where man, please listen, you don't know what you're missing"), oversimplification ("it's easy"), straw man ("everywhere  there's lot of piggies living piggy lives"), and a popular idea must be correct ("I get by with a little help from my friends"). But here Paul actually warns making logical fallacies: ("Think of what you're saying, you can get it wrong, and still you think that it's all right.")  


Here is an explanation of further logical fallacies and critical thinking that may help a lawyer. Here is more and more and more and more and more and finally the logical fallacy that has caused some much trouble:  "Home prices have not fallen since the Great Depression. Therefore, home prices will not fall."
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