Theorem frege26

Description: Identical to idd . Proposition 26 of Frege1879 p. 42. (Contributed by RP, 24-Dec-2019) (Proof modification is discouraged.)

	Ref	Expression
Assertion	frege26	$$⊢\phi \to \left(\psi \to \psi \right)$$

Proof

Step	Hyp	Ref	Expression
1		ax-frege1	$$⊢\psi \to \left(\phi \to \psi \right)$$
2		ax-frege8	$$⊢\left(\psi \to \left(\phi \to \psi \right)\right)\to \left(\phi \to \left(\psi \to \psi \right)\right)$$
3	1 2	ax-mp	$$⊢\phi \to \left(\psi \to \psi \right)$$