Reductio ad Absurdum

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Proof Rule

Reductio ad Absurdum is a valid argument in certain types of logic dealing with negation $\neg$ and contradiction $\bot$.

This includes classical propositional logic and predicate logic, and in particular natural deduction, but for example not intuitionistic propositional logic.


Proof Rule

If, by making an assumption $\neg \phi$, we can infer a contradiction as a consequence, then we may infer $\phi$.
The conclusion $\phi$ does not depend upon the assumption $\neg \phi$.


Sequent Form

The Reductio ad Absurdum can be symbolised by the sequent:

$\paren {\neg p \vdash \bot} \vdash p$


Explanation

Reductio ad Absurdum can be expressed in natural language as:

If, by making an assumption that a statement is false, a contradiction can be deduced, that statement must in fact be true.


Variants

The following forms can be used as variants of this theorem:

Variant 1

$\neg p \implies \bot \vdash p$

Variant 2

$\neg p \implies \paren {q \land \neg q} \vdash p$


Law of the Excluded Middle

This theorem depends on the Law of the Excluded Middle.

This is one of the logical axioms that was determined by Aristotle, and forms part of the backbone of classical (Aristotelian) logic.

However, the intuitionist school rejects the Law of the Excluded Middle as a valid logical axiom.

This in turn invalidates this theorem from an intuitionistic perspective.


Also see

From the point of view of purely classical logic, this is acceptable. However, in the context of intuitionistic logic, it is essential to bear in mind that only the Proof by Contradiction is valid.

This is because Proof by Contradiction starts with a positive assumption $\phi$.

As a result, it does not depend on the Law of Excluded Middle.


Linguistic Note

Reductio ad Absurdum is Latin for reduction to an absurdity.


Sources

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