reductio ad absurdum
(also known as: reduce to absurdity)
Description: A mode of argumentation or a form of argument in which a proposition is disproven by following its implications logically to an absurd conclusion. Arguments that use universals such as, “always”, “never”, “everyone”, “nobody”, etc., are prone to being reduced to absurd conclusions. The fallacy is in the argument that could be reduced to absurdity -- so in essence, reductio ad absurdum is a technique to expose the fallacy.
Assume P is true.
From this assumption, deduce that Q is true.
Also, deduce that Q is false.
Thus, P implies both Q and not Q (a contradiction, which is necessarily false).
Therefore, P itself must be false.
I am going into surgery tomorrow so please pray for me. If enough people pray for me, God will protect me from harm and see to it that I have a successful surgery and speedy recovery.
Explanation: We first assume the premise is true: if “enough” people prayed to God for the patient's 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.
If everyone lived his or her life exactly like Jesus lived his life, the world would be a beautiful place!
Explanation: We first assume the premise is true: if everyone lived his or her life exactly like Jesus lived his, the world would be a beautiful place. If this were true, we would have 7 billion people on this earth roaming from town to town, living off the charity of others, preaching about God (with nobody listening). Without anyone creating wealth, there would be nobody to get charity from -- there would just be 7 billion people all trying to tell each other about God. After a few weeks, everyone would eventually starve and die. This world might be a beautiful place for the vultures and maggots feeding on all the Jesus wannabes, but far from a beautiful world from a human perspective. Since the world cannot be both a beautiful place and a horrible place, the proposition is false.
Exception: Be sure to see the appeal to extremes fallacy.