Logic and Critical Thinking

Look at this classic example of logical reasoning:

All men are mortal. (Premise 1)
Socrates is a man. (Premise 2)
Socrates is mortal. (Conclusion)

Now look at this example of a classic fallacy:

All dogs have four legs. (Premise 1)
A chair has four legs. (Premise 2)
A chair is a dog. (Conclusion)

What went wrong?

There are two basic types of fallacies: ones that draw an irrelevant conclusion, and ones that are based on unclear wording. The most common types are those that are irrelevant  in some ways - like the conclusion above.

Here are a few very common types of logical fallacies:

  • A false generalization -- often based on using 'all' or 'never' and applying it more broadly than you should
  • See the example above.
  • An argument ad hominem -- where you comment about the person rather than about the argument
  • "She makes no sense - she's just a dumb blonde."
  • An argument from ignorance
  • I've never seen a UFO, so I don't believe they exist.
  • A false cause
  • He walked under a ladder, so he got sick later that day.
  • An implied assumption
  • When did you stop beating your wife?

    Here are a few of my favorites from the Online Writing Lab's Logic in argumentative writing - take a look there for more. What are the premises in these? What's wrong with these arguments?

    Cooks have been preparing food for generations, so our cook must be a real expert.

    More young people are attending high schools and colleges than ever before in the history of our nation. But there is more juvenile delinquency than ever before. This makes it clear that to eliminate delinquency among the youth we must abolish the schools.

    You say we ought to discuss whether or not to buy a new car now. All right, I agree. Let's discuss the matter. Which should we get, a Ford or a Chevy?

    Our nation is a democracy and dedicated to the proposition that all men are created equal. We believe in equality of opportunity for everyone, so our colleges and universities should admit every applicant, regardless of his economic or educational background.

    (from the Online Writing Lab)

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    Last updated 9 July 1998 by D.Healey, Deborah.Healey@orst.edu.