Theorem pm5.19

Description: Theorem *5.19 of WhiteheadRussell p. 124. (Contributed by NM, 3-Jan-2005)

	Ref	Expression
Assertion	pm5.19	$$⊢\neg \left(\phi \leftrightarrow \neg \phi \right)$$

Proof

Step	Hyp	Ref	Expression
1		biid	$$⊢\phi \leftrightarrow \phi$$
2		pm5.18	$$⊢\left(\phi \leftrightarrow \phi \right)\leftrightarrow \neg \left(\phi \leftrightarrow \neg \phi \right)$$
3	1 2	mpbi	$$⊢\neg \left(\phi \leftrightarrow \neg \phi \right)$$