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Indirect Method for Existential Formulas

Instead of proving (exists x: G), we assume (~exists x: G), i.e., (forall x: ~G) and deriving a contradiction:

exists x: G
K union {forall x: ~G}

"We prove (exists x: G). Assume (forall x: ~G). Then ... (derivation of a contradiction with additional knowledge (forall x: ~G))."

We will see later how to work with universal formulas in knowledge.

Author: Wolfgang Schreiner
Last Modification: November 30, 1999

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