Theorem frege27 40164

Description: We cannot (at the same time) affirm 𝜑 and deny 𝜑. Identical to id 22. Proposition 27 of [Frege1879] p. 43. (Contributed by RP, 24-Dec-2019.) (Proof modification is discouraged.)

Assertion

Ref	Expression
frege27	⊢ (𝜑 → 𝜑)

Proof of Theorem frege27

Step	Hyp	Ref	Expression
1		ax-frege1 40143	. 2 ⊢ (𝜑 → (𝜓 → 𝜑))
2		frege26 40163	. 2 ⊢ ((𝜑 → (𝜓 → 𝜑)) → (𝜑 → 𝜑))
3	1, 2	ax-mp 5	1 ⊢ (𝜑 → 𝜑)

Syntax hints:   → wi 4

This theorem was proved from axioms:  ax-mp 5  ax-frege1 40143  ax-frege8 40162

This theorem is referenced by:  frege42  40199