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Metamath Proof Explorer

Theorem pm5.19

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

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

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