Metamath Proof Explorer


Theorem bj-axdd2

Description: This implication, proved using only ax-gen and ax-4 on top of propositional calculus (hence holding, up to the standard interpretation, in any normal modal logic), shows that the axiom scheme |- E. x T. implies the axiom scheme |- ( A. x ph -> E. x ph ) . These correspond to the modal axiom (D), and in predicate calculus, they assert that the universe of discourse is nonempty. For the converse, see bj-axd2d . (Contributed by BJ, 16-May-2019) (Proof modification is discouraged.) (New usage is discouraged.)

Ref Expression
Assertion bj-axdd2 ( ∃ 𝑥 𝜑 → ( ∀ 𝑥 𝜓 → ∃ 𝑥 𝜓 ) )

Proof

Step Hyp Ref Expression
1 ala1 ( ∀ 𝑥 𝜓 → ∀ 𝑥 ( 𝜑𝜓 ) )
2 exim ( ∀ 𝑥 ( 𝜑𝜓 ) → ( ∃ 𝑥 𝜑 → ∃ 𝑥 𝜓 ) )
3 1 2 syl ( ∀ 𝑥 𝜓 → ( ∃ 𝑥 𝜑 → ∃ 𝑥 𝜓 ) )
4 3 com12 ( ∃ 𝑥 𝜑 → ( ∀ 𝑥 𝜓 → ∃ 𝑥 𝜓 ) )