Exposing PseudoAstronomy

December 11, 2009

Logical Fallacies: Reductio ad absurdum


Introduction

In my continuing series on logical fallacies, this post is about a more subtle fallacy that is usually harder to catch than, say an ad hominem, and that’s the reductio ad absurdum.

What is the “Reductio ad absurdum” Fallacy?

The Latin term reductio ad absurdum literally translates to, “reduction to the absurd.” In formal logic, the reductio ad absurdum is actually a legitimate argument, but it is often applied fallaciously. The fallacy follows the idea that if the premises of someone’s argument are taken as true, then it necessarily will lead to absurd conclusions.

This is a fairly good fallacy to remember when watching courtroom drama series, as lawyers may try to use this fallacy to show that a witness is lying. For example, a witness could make a claim on the stand, such as, “I know she was driving a blue cars.”

Lawyer: “How do you know this?”

Witness: “Because I’m an interior decorator and I always notice the colors of cars on the road.”

Lawyer: “Oh really? Can you tell us then, when you came to court today, what was the color the car that parked in front of you? To your left? Your right? What was the color of the car that was behind you on the freeway? [etc.]”

The lawyer has just used a reductio ad absurdum in this rather contrived example to show that the witness’s testimony that they “always notice the colors of cars” is very likely to be a false premise because when it is followed to its logical extent (that they would be able to answer the lawyer’s question about every car they saw that day) it is an absurd claim.

Example from UFOlogy

An admittedly contrived example from a UFOlogist could be had in the following statement by them: “If you’re so skeptical that you need to see proof with your own eyes of an alien body before you’ll believe that they exist, then how do you believe in the existence of Paris? Or of a dodo bird? Or an echidna? You’ve never seen them, how do you know they exist?”

The person has just used the reductio ad absurdum fallaciously because they assumed there was only one premise – that I required the proof of the alien body to see with my own eyes. Rather, I would accept other evidence, such as a gazillion verifiable photographs, independent corroboration, real hard evidence that has been examined by the bulk of the scientific community that studies such things and has reached the conclusion that it is real.

For example, the existence of Paris is something that I have seen in books, magazines, and movies. I’ve read about it in history books, my parents have been there, and I’ve met people who claim they come from that city. It has apparently been an integral part of the world’s history for at least a few centuries. To me, that is enough evidence that I can trust that Paris exists.

(This example will actually work with any pseudoscientific field where the skeptic actually wants real hard evidence of the phenomenon, I just happened to apply it to UFOs.)

Final Thoughts

The reductio ad absurdum argument can be used logically so long as one understands what they are doing. The false use of it will usually occur when one assumes a limited initial premise to the claim (in the above example, that I would only “believe it when I see it”).

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