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Glossary 1 min read

Reductio Ad Absurdum

Reductio ad absurdum is a form of argument that demonstrates the falsity of a proposition by showing that it leads to an absurd or contradictory conclusion. In philosophy, it is often used to test the limits of a theory. The Survival Lottery is a classic reductio ad absurdum of act-utilita

By Philosopheasy Published on June 19, 2026

A fundamental argumentative technique used throughout philosophy, exemplified by the Survival Lottery. 3 min read.

Reductio ad absurdum (Latin for "reduction to absurdity") is a method of refuting a claim by showing that it entails an unacceptable consequence. The structure is: assume the claim is true; derive a contradiction or an obviously false conclusion; therefore, the original claim must be false.

In moral philosophy, reductio arguments are used to test ethical theories. The Survival Lottery is a reductio of act-utilitarianism. The argument goes as follows:

  1. Assume act-utilitarianism is true: an action is right if it maximizes overall well-being.
  2. If act-utilitarianism is true, then the Survival Lottery is morally permissible (and perhaps obligatory), because it saves two lives at the cost of one.
  3. But the Survival Lottery is clearly morally impermissible (it involves killing an innocent person against their will).
  4. Therefore, act-utilitarianism is false.

The force of the reductio depends on the premise that the Survival Lottery is obviously wrong. If someone does not share that intuition, the argument loses its force. But for most people, the lottery is a paradigm case of an unjust act. The thought experiment thus serves as a powerful tool for challenging utilitarian theory.

Reductio arguments are common in all areas of philosophy, from metaphysics to ethics. They are a way of showing that a theory has implications that its proponents would not accept, thereby forcing them to revise or abandon the theory.

Referenced Works & Texts

  1. John Harris, "The Survival Lottery," Philosophy, Vol. 50, No. 191 (1975). The thought experiment used as a reductio of act-utilitarianism.

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