Reductio ad absurdum is a Latin phrase that means “to reduce to the absurd,” referring to an argumentative technique. When someone uses this tactic in a debate, the person uses a series of logical steps to arrive at a ridiculous conclusion, and argues that an original premise must be wrong because of the conclusion. This technique can work for and against people; sometimes, it allows a debater to crush a claim made by the opposition, and at other times, it makes someone's position look poor because the reductio ad absurdum looks like a fallacious argument.
In an example of how such an argument might work, a person could say something like “speed limit laws are important because all vehicles must follow them, and this keeps the road safer.” An opponent might point out that ambulances are exempt from speed laws in emergencies, therefore the original statement is incorrect, because one part of it, “all vehicles must follow them” is not true. In this case, bystanders would probably consider the reasoning fallacious, because there is a compelling reason for emergency vehicles to ignore speed laws to reach people in need.
People may use this technique to try to make a position seem extreme or ridiculous. Someone may assume a claim favorable to the cause of an opponent for the purposes of making an argument, and step through a series of apparently reasonable statements to end up with an outrageous conclusion in a reductio ad absurdum. The person would point to the ridiculous end of the logic chain to argue that the original claim is false or has significant flaws.
One of the key problems with the reductio ad absurdum is that it assumes an absolutist answer to every situation. Many people in casual debate agree that there are often exceptions to rules and for almost any claim, people can come up with compelling evidence to highlight a single exception. If every statement could be rendered utterly false with such arguments, people would be left with few arguments to stand on. Allowing for flexibility in certain situations will allow people to engage with the substance of what someone is arguing, rather than trying to reduce statements to absurdities.
Numerous rhetorical techniques can play on the reductio ad absurdum and people may cry foul when this tactic is used against them. People can try backtracking through the logic a person uses to reach the absurd final argument, in the hopes of finding something to pull apart before the person reaches an inevitable conclusion.