Exposing PseudoAstronomy

December 11, 2009

Logical Fallacies: Reductio ad absurdum


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”).



  1. What’s that line Christians use… there’s more historical evidence for Jesus (or maybe his divinity) than there is for the existence of Julius Caesar. The implication no sane person would doubt Julius’s existence so why doubt Zombie Jesus?

    Comment by Karl — December 15, 2009 @ 1:02 pm | Reply

    • That argument is directed at people who deny that Jesus ever existed, in that context it is perfectly valid. That particular argument has nothing to do with the issue of the resurrection and anyone who uses it as such is mistaken. The amount of historical literary evidence even outside the New Testament for the existence of Jesus is substantial. Even critical scholars rarely ever deny that Jesus existed, because they know full well that to do so means throwing out the rest of ancient history. You can accept that Jesus existed and still reject the resurrection.

      As for proving the resurrection, or showing that it is more plausibly true than false, no the argument doesn’t work for that. It was never designed to do that. Just like how my chainsaw does a poor job at pounding in nails. Other arguments are used for that purpose.

      Comment by Benjamin — August 17, 2012 @ 2:55 pm | Reply

  2. What do mean by “Zombie Jesus”

    Comment by Sarah — December 16, 2009 @ 5:19 am | Reply

  3. In my culture mythological beings that die and return from the dead and walk among humans are called zombies.

    Comment by Karl — December 17, 2009 @ 12:18 pm | Reply

  4. Actually a ‘zombie’ is someone who comes back from the dead without a soul. i.e. the living dead.

    Comment by Sarah — December 18, 2009 @ 5:20 am | Reply

  5. Yes, in my culture too a zombie has no soul (as well as no pink unicorn friend, no invisible blue fairy, no pocket bigfoot). But then in my culture a living human being has no soul, no pink unicorn friend, no invisible blue fairy, no pocket bigfoot either. To wit, souls, zombies, pink unicorns, and magical zombie Jesus are all myths in my culture.

    So, Zombie Jesus seems a good term to me in the context of my culture.

    Comment by Karl — December 18, 2009 @ 10:42 am | Reply

  6. I suspected that in your culture a human being has no soul. It explains a lot. But NO PINK UNICORN FRIEND is unforgivable!! You will go to Narnia for that one.

    Comment by sarah — December 19, 2009 @ 6:06 am | Reply

  7. Not believing in things there is no evidence for I think explains why I make right choices in life that have led me to be happy and find friends and mates who enrich my life. I don’t chase multi-level marketing schemes. I don’t gamble away my pay. I don’t spend money on strippers thinking one of them is going to date me.

    Comment by Karl — December 21, 2009 @ 1:00 pm | Reply

    • And here folks we have a classic real world example of the fallacious use of reductio ad absurdum:
      “…make right choices” – according to whom? By what unbiased measure?
      “…led me to be happy” – I’m pleased for you that you feel happy. It’s still merely your opinion though.
      “…friends […] who enrich my life” – This is the exclusive experience of “rationalists”? Once again, by what unbiased measure?
      “…multi level marketing schemes” – Again, not exclusively the choice of “rationalists”.
      “…gamble…” – You (like me) simply have a low appetite for risk. Gambling is a trade off of risk for potential reward, and the reward is genuine based on evidence, just extremely elusive.
      “… strippers […] date me. – Neither does anyone who does spend money on them.
      These are all non-sequiturs to the reliance on evidence, but rather indicate a tendency to use unrelated evidence to support a previously determined belief. I think this could be categorised as cognitive bias.

      Comment by Chris — March 15, 2019 @ 2:37 am | Reply

  8. Why DO you spend money on strippers then??

    Comment by Sarah — December 23, 2009 @ 5:45 am | Reply

  9. Who spends money on strippers?

    Comment by Karl — December 23, 2009 @ 11:37 am | Reply

  10. A very interesting article, truthfully I know a lot more information from this article

    Comment by Gavin — January 2, 2010 @ 9:22 pm | Reply

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