Theorem bj-modalbe 31865

Description: The predicate-calculus version of the axiom (B) of modal logic. See also modal-b 2127. (Contributed by BJ, 20-Oct-2019.)

Assertion

Ref	Expression
bj-modalbe	⊢ (𝜑 → ∀𝑥∃𝑥𝜑)

Proof of Theorem bj-modalbe

Step	Hyp	Ref	Expression
1		modal-b 2127	. 2 ⊢ (𝜑 → ∀𝑥 ¬ ∀𝑥 ¬ 𝜑)
2		df-ex 1696	. . 3 ⊢ (∃𝑥𝜑 ↔ ¬ ∀𝑥 ¬ 𝜑)
3	2	biimpri 217	. 2 ⊢ (¬ ∀𝑥 ¬ 𝜑 → ∃𝑥𝜑)
4	1, 3	sylg 1740	1 ⊢ (𝜑 → ∀𝑥∃𝑥𝜑)

Syntax hints:  ¬ wn 3   → wi 4  ∀wal 1473  ∃wex 1695

This theorem was proved from axioms:  ax-mp 5  ax-1 6  ax-2 7  ax-3 8  ax-gen 1713  ax-4 1728  ax-5 1827  ax-6 1875  ax-7 1922  ax-10 2006  ax-12 2034

This theorem depends on definitions:  df-bi 196  df-ex 1696

This theorem is referenced by: (None)