Theorem pm2.6

Description: Theorem *2.6 of WhiteheadRussell p. 107. (Contributed by NM, 3-Jan-2005)

	Ref	Expression
Assertion	pm2.6	$$⊢\left(\neg \phi \to \psi \right)\to \left(\left(\phi \to \psi \right)\to \psi \right)$$

Proof

Step	Hyp	Ref	Expression
1		id	$$⊢\left(\neg \phi \to \psi \right)\to \left(\neg \phi \to \psi \right)$$
2		idd	$$⊢\left(\neg \phi \to \psi \right)\to \left(\psi \to \psi \right)$$
3	1 2	jad	$$⊢\left(\neg \phi \to \psi \right)\to \left(\left(\phi \to \psi \right)\to \psi \right)$$