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Is there anyone who could explain to me why these errors occur? It seems to me the rule was used properly.

Mauro ALLEGRANZA
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sannelavinia
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1 Answers1

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Existential Elimination requires that the witness (a) does not occur in the conclusion or in any undischarged assumptions.

At those points, you have an undischarged assumption that includes a on line 2.

Suggestion: Place the negation introduction subproofs inside each case of the v-elimination subproofs.

|  |_ Ex -Px
|  |  |_ -Pa
|  |  |  |_ Pa & Qa
|  |  |  |  :
Graham Kemp
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