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Existential instantiation

Type	Rule of inference
Field	Predicate logic
Symbolic statement	${\displaystyle \exists xP\left({x}\right)\implies P\left({a}\right)}$

In predicate logic, existential instantiation (also called existential elimination) is a rule of inference which says that, given a formula of the form ${\displaystyle (\exists x)\phi (x)}$, one may infer ${\displaystyle \phi (c)}$ for a new constant symbol c. The rule has the restrictions that the constant c introduced by the rule must be a new term that has not occurred earlier in the proof, and it also must not occur in the conclusion of the proof. It is also necessary that every instance of ${\displaystyle x}$ which is bound to ${\displaystyle \exists x}$ must be uniformly replaced by c. This is implied by the notation ${\displaystyle P\left({a}\right)}$, but its explicit statement is often left out of explanations.

In one formal notation, the rule may be denoted by

${\displaystyle \exists xP\left({x}\right)\implies P\left({a}\right)}$

where a is a new constant symbol that has not appeared in the proof.

See also

• Existential fallacy
• Universal instantiation
• List of rules of inference

References

1. Hurley, Patrick. A Concise Introduction to Logic (11th ed.). Wadsworth Pub Co, 2008. Pg. 454. ISBN 978-0-8400-3417-5
2. Copi, Irving M.; Cohen, Carl (2002). Introduction to logic (11th ed.). Upper Saddle River, N.J.: Prentice Hall. ISBN 978-0-13-033737-5.

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