Theorem pm2.521

Description: Theorem *2.521 of WhiteheadRussell p. 107. Instance of pm2.521g and of pm2.521g2 . (Contributed by NM, 3-Jan-2005)

	Ref	Expression
Assertion	pm2.521	$$⊢\neg \left(\phi \to \psi \right)\to \left(\psi \to \phi \right)$$

Proof

Step	Hyp	Ref	Expression
1		pm2.521g	$$⊢\neg \left(\phi \to \psi \right)\to \left(\psi \to \phi \right)$$