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NEGATED QUANTIFIER DECOMPOSITION RULES

As an example, let’s determine the validity of the sequent:

Notice that here we applied the -∣ v rule to line 2 first, since this is a non-branching rule.

So isbut had we applied that before Ξ (which branches), then we would not have been able to applywithout either breaking the restriction on it that we cannot have used the same name previously on the same path, or using a different name, which would have

got us nowhere. The economy of the Truth Tree method can be seen by comparing the succinct 8 line tree-proof above with a rule-of-inference proof of the same sequent:1

23.1.4

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Source: Arthur R.T.W.. An Introduction to Logic: Using Natural Deduction, Real Arguments, a Little History, and Some Humour. Broadview Press,2016. — 456 p.. 2016

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