Theorem pm5.19

Description: Theorem *5.19 of WhiteheadRussell p. 124. (Contributed by NM, 3-Jan-2005)

	Ref	Expression
Assertion	pm5.19	⊢ ¬ ( 𝜑 ↔ ¬ 𝜑 )

Proof

Step	Hyp	Ref	Expression
1		biid	⊢ ( 𝜑 ↔ 𝜑 )
2		pm5.18	⊢ ( ( 𝜑 ↔ 𝜑 ) ↔ ¬ ( 𝜑 ↔ ¬ 𝜑 ) )
3	1 2	mpbi	⊢ ¬ ( 𝜑 ↔ ¬ 𝜑 )