Reductio ad Absurdum: Defeating Arguments by Following Them to Their Logical End

How Reductio Works

The structure is: Assume proposition P is true. Show that P logically leads to conclusion Q. Show that Q is absurd, contradictory, or obviously false. Therefore, P must be false.

For example, suppose someone argues: 'No laws should restrict individual freedom.' Using reductio: if no laws should restrict freedom, then laws against murder restrict freedom and should be abolished. But a society without murder laws would be untenable. Therefore, the original claim that no laws should restrict freedom is too broad.

Reductio does not show that the general direction of the opponent's thinking is wrong -- it shows that their specific formulation is flawed. In the example above, the opponent may well be right that individual freedom is important, but their absolute claim does not survive logical scrutiny.

Reductio vs. Slippery Slope

Reductio ad absurdum is sometimes confused with the slippery slope fallacy, but they are fundamentally different. A slippery slope argues that a moderate action will lead to extreme consequences through a chain of causation. Reductio argues that a stated principle, taken to its logical conclusion, produces absurd results.

The difference is that reductio follows the logic of the opponent's stated position, not a causal chain of events. It asks: 'If we consistently apply your stated principle, where does it lead?' This is a legitimate logical operation, not a speculative prediction about future events.

However, a reductio can be misapplied if the 'absurd' conclusion is not actually entailed by the premise, or if the intermediate steps contain hidden assumptions. A valid reductio must follow strict logical entailment, not loose association.

Using Reductio Effectively

The key to an effective reductio is making sure the absurd conclusion genuinely follows from the opponent's premises. If there is a logical gap, your opponent can simply say 'my position does not actually lead to that conclusion.' Show each step explicitly.

Choose your 'absurd' conclusion carefully. It should be something that virtually everyone would reject, not something controversial. If your reductio leads to a conclusion that some people might actually accept, it loses its force.

You can also use reductio defensively. When your opponent presents a counterexample to your position, check whether their counterexample, taken to its logical conclusion, leads to absurd results for their own position. This counter-reductio can be devastatingly effective.

• •Reductio assumes the opponent's position and shows it leads to absurd or contradictory conclusions.
• •It differs from slippery slope because it follows logical entailment, not speculative causal chains.
• •Each step in the reductio must follow logically from the previous step.
• •Choose absurd conclusions that virtually everyone would reject for maximum impact.
• •Reductio shows a specific formulation is flawed, not necessarily that the general direction is wrong.