Theorem pm2.5

Description: Theorem *2.5 of WhiteheadRussell p. 107. (Contributed by NM, 3-Jan-2005)

	Ref	Expression
Assertion	pm2.5	⊢ ( ¬ ( 𝜑 → 𝜓 ) → ( ¬ 𝜑 → 𝜓 ) )

Proof

Step	Hyp	Ref	Expression
1		pm2.5g	⊢ ( ¬ ( 𝜑 → 𝜓 ) → ( ¬ 𝜑 → 𝜓 ) )