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Flaws in Arguments (1). Critical Thinking D Gray Feb 2007 Updated September 2009. Post Hoc fallacy. Post hoc ergo propter hoc “After this, therefore because of this.” X occurs before Y Therefore X is the cause of Y Causality Sequential Coincidence Superstition. Post Hoc fallacy.

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flaws in arguments 1

Flaws in Arguments(1)

Critical Thinking

D Gray

Feb 2007

Updated September 2009

post hoc fallacy
Post Hoc fallacy
  • Post hoc ergo propter hoc
  • “After this, therefore because of this.”
  • X occurs before Y
  • Therefore X is the cause of Y
  • Causality
  • Sequential
  • Coincidence
  • Superstition
post hoc fallacy3
Post Hoc fallacy

Walking under a ladder …

Early to bed, early to rise, …

Lucky rabbit’s foot

Your sins will find you out.

Watching television gives you square eyes.

Watching violence on TV causes violence in young people.

My alarm goes off at 5 am and the Sun rises shortly afterwards. Therefore my alarm causes the sun to rise.

Good exam results lead to happiness.

ad hominem fallacy
Ad Hominem fallacy
  • “Against the man”
  • Person X makes a claim
  • Person Y attacks the character of Person X
  • Person Y concludes that Person X’s claim is therefore false.
  • Attacking personality
  • Historical context
  • Reputation
  • Credibility
ad hominem fallacy5
Ad Hominem fallacy
  • Look for:
  • Attacks on the person, not the opinion.
  • Ignoring the argument
  • Talk about past reputation or character
ad hominem fallacy6
Ad Hominem fallacy
  • Political debates
  • Family arguments
  • Interviews on the news
  • Racism, sexism, homophobia etc…
why are these examples of post hoc fallacies
Why are these examples of Post Hoc fallacies?

Breaking a mirror leads to seven years bad luck.

Finding a four leaf clover brings good luck

A black cat crossing your path brings bad luck

post hoc fallacy8
Post Hoc fallacy?

In Copenhagen, in Denmark, some people believe that the more storks that nest in the city, the more human babies are born.

They point to the fact that during the 12 years following WW2, the more storks that nested in Copenhagen in the Spring, the more babies were born in the year.

post hoc fallacy9
Post Hoc fallacy?

Many people warned that the liberalisation of drink laws, which allowed pubs and clubs to extend drinking hours, would fuel disorder.

However, the few months after their introduction, serious violent crime has fallen 21% - and it is down twice that in some towns. Also, there were 14% fewer woundings in the same period.

This shows that the new licensing laws have resulted in less violent crime.

ad hominem
Ad Hominem

Write a short Ad hominem argument as to why:

  • Wayne Rooney should not be England football captain.
  • Sophie should not have won BB9
  • Gordon Brown should not be prime minister
  • Freddy Flintoff should not be England cricket captain
  • Oasis should stop touring
straw man fallacy
Straw man fallacy
  • Straw targets are easier to knock down
  • Person X presents an argument
  • Person Y represents the argument in a distorted fashion
  • Person Y then refutes the argument
  • This undermines the original argument of Person X
  • Misrepresentation
  • Refute
  • Ridicule
  • Exaggerate
straw man fallacy12
Straw man fallacy
  • Ridiculous!
  • Wild!
  • Ludicrous!
  • Absurd!
  • Unbelievable!
  • Stupid!
tu quoque fallacy
Tu quoque fallacy
  • “You too” or “You also”
  • Do as I say, not as I do!
  • Person A makes a claim
  • Person B asserts that A’s actions or past claims are inconsistent with the claim
  • Therefore the claim is false
tu quoque fallacy14
Tu quoque fallacy

Rejecting an argument because of what the person does, not because of the strength of the argument.

Seeking to retain the status quo


Staying out late

Speeding on a motorway

White lies

Everyone does it