Theorem pm2.52

Description: Theorem *2.52 of WhiteheadRussell p. 107. (Contributed by NM, 3-Jan-2005) (Proof shortened by Wolf Lammen, 8-Oct-2012)

	Ref	Expression
Assertion	pm2.52	⊢ ( ¬ ( 𝜑 → 𝜓 ) → ( ¬ 𝜑 → ¬ 𝜓 ) )

Proof

Step	Hyp	Ref	Expression
1		conax1k	⊢ ( ¬ ( 𝜑 → 𝜓 ) → ( ¬ 𝜑 → ¬ 𝜓 ) )