Literally translating as “reduction to absurdity,” this is actually a valid mode of argumentation if used correctly. It is used appropriately when, if the premises lead to a false conclusion, we conclude that we must then reject at least one of the premises. It is a fallacy when, in order to reach the false conclusion, it is extended to an absurd degree using assertions that are not actually in the original argument; in this case, it is not the premise that fails but the logical progression to the conclusion that does. When it is used incorrectly, it often results in a straw man fallacy.
Example of reductio used appropriately:
Refutation: We first assume the premise is true: if “enough” people prayed to God for her successful surgery and speedy recovery, then God would make it so. From this, we can deduce that God responds to popular opinion. However, if God simply granted prayers based on popularity contests, that would be both unjust and absurd. Since God cannot be unjust, then he cannot both respond to popularity and not respond to popularity, the claim is absurd, and thus false.
Examples of reductio used fallaciously:
A UFO enthusiast once argued that if I am skeptical about the existence of alien visitors, I must also be skeptical of the existence of the Great Wall of China, since I have not personally seen either (ignores all other evidence other than personal eyewitness).
Husband to ex-wife: Well, if you want to be completely fair about dividing everything up, you should get one of my testicles and I should get one of your breasts!
Debtor to creditor: Hey, you’ve already repossessed my car and my television. Why don’t you just draw a quart of blood or carve a pound of flesh from my heart too?
There is no way those Girl Scouts could have sold all those cases of cookies in one hour. If they did, they would have to make $500 in one hour, which, based on an 8 hour day is over a million dollars a year. That is more than most lawyers, doctors, and successful business people make! (The Girl Scouts worked just for one hour — not 40 per week for a year. Suggesting the extreme leads to an absurd conclusion; that Girl Scouts are among the highest paid people in the world. Not to mention, there is a whole troop of them doing the work, not just one girl.)
Don’t forget God’s commandment, “thou shall not kill”. By using mouthwash, you are killing 99.9% of the germs that cause bad breath. Prepare for Hell. (It is unlikely that God had mouthwash on his mind when issuing that commandment, but if he did, we’re all screwed.)
Nerdy Ned: Actually, if you just kept exercising and never stopped, you would eventually drop dead. There is a limit to how much exercise you should get.
One of the central tenets of homeopathy is the (unfounded) assertion that water retains a “memory” of substances dissolved in it, even when the solution becomes so weak that no trace of the original substance is present. However, if we accept this hypothesis, then any quantity of tap water would have already acquired all the beneficial chemicals and all the harmful ones too. This patently absurd scenario weakens the hypothesis of water memory.
“If evolution were real, we’d see fish turning into monkeys and monkeys turning into people all the time.”
Earliest known example: Greek poem attributed to Xenophanes of Colophon. Criticizing Homer’s attribution of human faults to the Greek gods, he says that humans also believe that the gods’ bodies have human form. But if horses and oxen could draw, they would draw the gods with horse and oxen bodies. The gods can’t have both forms, so this is a contradiction. Therefore the attribution of other human characteristics to the gods, such as human faults, is also false.
In a 1977 appeal of a U.S. bank robbery conviction, a prosecuting attorney said in his closing argument: “I submit to you that if you can’t take this evidence and find these defendants guilty on this evidence then we might as well open all the banks and say, “Come on and get the money, boys”, because we’ll never be able to convict them.” (The prosecutor was tacitly equating the failure to convict the defendants in one particular trial with the inability to convict any bank robbers, a situation with self-evident unpleasant consequences but very little connection with the outcome of the trial.)
Joe argues that we should disband the military. Jim asks, “What, do you want the terrorists to win? You must hate America!” (It would not have been a fallacious use if Jim had asked, “What do we do with all of the newly unemployed people?”