Single Idea 13355

[catalogued under 4. Formal Logic / B. Propositional Logic PL / 2. Tools of Propositional Logic / d. Basic theorems of PL]

Full Idea

The Principle of Disjunction says that Γ,φ∨ψ |= iff Γ,φ |= and Γ,ψ |=.

Gist of Idea

'Disjunction' says that Γ,φ∨ψ|= iff Γ,φ|= and Γ,ψ|=

Source

David Bostock (Intermediate Logic [1997], 2.5.G)

Book Reference

Bostock,David: 'Intermediate Logic' [OUP 1997], p.33

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A Reaction

That is, a disjunction leads to a contradiction if they each separately lead to contradictions.