Theorem con2 108

Description: Contraposition. Theorem *2.03 of [WhiteheadRussell] p. 100. (Contributed by NM, 5-Aug-1993.) (Proof shortened by Wolf Lammen, 12-Feb-2013.)

Assertion

Ref	Expression
con2	⊢ ((φ → ¬ ψ) → (ψ → ¬ φ))

Proof of Theorem con2

Step	Hyp	Ref	Expression
1		id 19	. 2 ⊢ ((φ → ¬ ψ) → (φ → ¬ ψ))
2	1	con2d 107	1 ⊢ ((φ → ¬ ψ) → (ψ → ¬ φ))

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:  con2b  324  sp  1747  spOLD  1748