Theorem pm2.5 144

Description: Theorem *2.5 of [WhiteheadRussell] p. 107. (Contributed by NM, 3-Jan-2005.) (Proof shortened by Wolf Lammen, 9-Oct-2012.)

Assertion

Ref	Expression
pm2.5	⊢ (¬ (φ → ψ) → (¬ φ → ψ))

Proof of Theorem pm2.5

Step	Hyp	Ref	Expression
1		simplim 143	. 2 ⊢ (¬ (φ → ψ) → φ)
2	1	pm2.24d 135	1 ⊢ (¬ (φ → ψ) → (¬ φ → ψ))

Syntax hints:  ¬ wn 3   → wi 4

This theorem was proved from axioms:  ax-mp 5  ax-1 6  ax-2 7  ax-3 8

This theorem is referenced by:  pm5.11  854