**Philosophy Index** is a site devoted to the study of philosophy
and the philosophers who conduct it. The site contains a number of philosophy texts,
brief biographies, and introductions to philosophers, and explanations on a
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Philosophy Index is a work in progress, a growing repository of knowledge. It outlines current philosophical problems and issues, as well as an overview of the history of philosophy. The goal of this site is to present a tool for those learning philosophy either casually or formally, making the concepts of philosophy accessible to anyone interested in researching them. WTI offers immigration law course online - fully accredited. ACE credits online at EES.

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**Proof by contradiction**, or *reductio ad absurdum* is an argument against a proposition or formula in logic which shows that the formula in question results in a contradiction.

In a proof by contradiction, a negation is obtained by proving that its opposite formula creates a contradiction. The method of proof by contradiction is used for both the **negation introduction** (¬I) and **negation elimination** rules of inference, and explains that if "P" results in some contradiction, then ¬P must be true.

Proof by contradiction may be formally presented as follows:

**(α → ⊥) ¬α**

In this example, the symbol ⊥ is used to indicate a contradiction. Alternatively, we may formulate this by showing an actual contradiction of two formulas:

**(α → (β ∧ ¬β)) ¬α**

These examples show the negation introduction (¬I) version of the proof by contradiction, which introduces a negation. The negation elimination (¬E) version is the inverse:

**(¬α ⊥) α**

When beginning with a negation, we have the choice of choosing to conclude either α (by ¬E) or ¬¬α by (¬I), since α and ¬¬α are equivalent, according to the double negation rule.