Theorem pm3.13

Description: Theorem *3.13 of WhiteheadRussell p. 111. (Contributed by NM, 3-Jan-2005)

	Ref	Expression
Assertion	pm3.13	$$⊢\neg \left(\phi \wedge \psi \right)\to \left(\neg \phi \vee \neg \psi \right)$$

Proof

Step	Hyp	Ref	Expression
1		pm3.11	$$⊢\neg \left(\neg \phi \vee \neg \psi \right)\to \left(\phi \wedge \psi \right)$$
2	1	con1i	$$⊢\neg \left(\phi \wedge \psi \right)\to \left(\neg \phi \vee \neg \psi \right)$$