Theorem exnal

Description: Existential quantification of negation is equivalent to negation of universal quantification. Dual of alnex . See also the dual pair df-ex / alex . Theorem 19.14 of Margaris p. 90. (Contributed by NM, 12-Mar-1993)

	Ref	Expression
Assertion	exnal	⊢ ( ∃ 𝑥 ¬ 𝜑 ↔ ¬ ∀ 𝑥 𝜑 )

Proof

Step	Hyp	Ref	Expression
1		alex	⊢ ( ∀ 𝑥 𝜑 ↔ ¬ ∃ 𝑥 ¬ 𝜑 )
2	1	con2bii	⊢ ( ∃ 𝑥 ¬ 𝜑 ↔ ¬ ∀ 𝑥 𝜑 )