Theorem nfv

Description: If x is not present in ph , then x is not free in ph . (Contributed by Mario Carneiro, 11-Aug-2016) Definition change. (Revised by Wolf Lammen, 12-Sep-2021)

	Ref	Expression
Assertion	nfv	$$⊢Ⅎx\phi$$

Proof

Step	Hyp	Ref	Expression
1		ax5ea	$$⊢\exists x\phi \to \forall x\phi$$
2	1	nfi	$$⊢Ⅎx\phi$$