Reductio ad Absurdum
Reductio ad absurdum tests a proposition by assuming it and deriving a contradiction. A valid refutation requires the consequences to follow and the other premises to be justified; merely finding an outcome strange or unwelcome is insufficient.
THE IDEA
Reductio ad Absurdum
Description: A reasoning technique that tests a proposition by following its implications to a contradiction. Universal claims using terms such as “always”, “never”, “everyone”, or “nobody” may be vulnerable to counterexamples, but their wording alone does not make them false. Reductio is not itself a fallacy.
Logical Form:
- Assume the proposition is true.
- Derive its consequences using justified premises.
- If a contradiction follows, reject the assumption or identify which additional premise failed.
Example #1: The source considers a claim that if “enough” people pray for successful surgery, God will grant it. The proposed refutation adds that this would make God respond unjustly to popularity, and that God cannot be unjust. Those added theological premises and the interpretation of prayer need justification before the argument refutes the claim.
Example #2: The source interprets everyone living like Jesus as 7 billion people relying on charity while nobody produces food or wealth. Starvation conflicts with its premise of a beautiful world. The objection depends on that particular interpretation; it does not refute every possible meaning of living like Jesus.
Caution: Do not replace the proposition with an extreme caricature. An unpleasant consequence, without a contradiction or an independently justified premise ruling it out, does not complete a reductio.
When to use it
Testing an opponent’s universal claim by checking whether its actual implications contradict another justified premise.
Testing the consistency of a proposed ethical principle by applying it to extreme scenarios.
Analyzing scientific hypotheses by examining if they lead to logical self-contradictions.
FROM THEORY TO EVERYDAY LIFE
In practice
Prayer and Popularity
The proposed argument against guaranteed answers to collective prayer depends on additional premises about divine justice and popularity. Calling the result absurd alone does not disprove the original claim.
Everyone Like Jesus
The source argues that universal dependence on charity with no production would cause starvation. Applying that objection to living like Jesus requires showing that the phrase really entails universal dependence without production.
A MOMENT TO REFLECT
Put it to work.
Think of an argument that uses a universal term like 'always' or 'never'. How could it be reduced to an absurd conclusion?
In what situations might reductio ad absurdum be misused to create a straw man?
How does this technique differ from simply pointing out consequences that are unlikely rather than logically impossible?
Often confused with
CONTINUE THE CONVERSATION
Source texts
- Logically Fallacious - The Ultimate Collection of Over 300 Logical Fallacies