Theorem frege42

Description: Not not id . Proposition 42 of Frege1879 p. 47. (Contributed by RP, 24-Dec-2019) (Proof modification is discouraged.)

	Ref	Expression
Assertion	frege42	⊢ ¬ ¬ ( 𝜑 → 𝜑 )

Proof

Step	Hyp	Ref	Expression
1		frege27	⊢ ( 𝜑 → 𝜑 )
2		ax-frege41	⊢ ( ( 𝜑 → 𝜑 ) → ¬ ¬ ( 𝜑 → 𝜑 ) )
3	1 2	ax-mp	⊢ ¬ ¬ ( 𝜑 → 𝜑 )