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16 June, 2015 - 16:26
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The complementary ∃Elim rule corresponds to giving a (new) name to a witness. Thus if you know "there exists some prime bigger than 5", then by ∃Elim we can think of giving some witness the name (say) c, and end up concluding "c is a prime bigger than 5". The caveats are that c must be a new name not already used in the proof, and different from any variables free in the conclusion we're aiming for. However, we will be able to use that variable c along with universal formulas to get useful statements.

Thus the general form of the rule is that \exists p :(\phi ) \vdash \phi [p\rightarrow c]. That is, we can rewrite the body of the exists, replacing the quantified variable p with any new variable name c, subject to the restrictions just mentioned.