In predicate logic, existential instantiation is a valid rule of inference which says that, given a formula of the form , one may infer for a new constant or variable symbol c. The rule has the restriction that the constant or variable c introduced by the rule must be a new term that has not occurred earlier in the proof.
In one formal notation, the rule may be denoted
where a is an arbitrary term that has not been a part of our proof thus far.
- Hurley, Patrick. A Concise Introduction to Logic. Wadsworth Pub Co, 2008.
- Copi and Cohen
- Moore aand Parker