Theorem pm10.253

Description: Theorem *10.253 in WhiteheadRussell p. 149. (Contributed by Andrew Salmon, 17-Jun-2011)

	Ref	Expression
Assertion	pm10.253	$$⊢\neg \forall x\phi \leftrightarrow \exists x\neg \phi$$

Proof

Step	Hyp	Ref	Expression
1		alex	$$⊢\forall x\phi \leftrightarrow \neg \exists x\neg \phi$$
2	1	bicomi	$$⊢\neg \exists x\neg \phi \leftrightarrow \forall x\phi$$
3	2	con1bii	$$⊢\neg \forall x\phi \leftrightarrow \exists x\neg \phi$$