I know that this sequent is true for classical logic. But is it true in minimal or intuitionistic logic?
What would be a Kripke model that doesn't satisfy the property?
I would appreciate some (as many as possible) Kripke model examples where classically true formulas are false.
I already have one where $\forall x\neg\phi\lor\neg\neg\phi$ is not true:
Here we have $\alpha\nvDash \neg P(a)\lor\neg\neg P(a)$
Yes, it is.
We can prove it without LEM or Double Negation.
1) $∃xψ(x) → ∀xθ(x)$ --- premise
2) $ψ(a)$ --- assumed [a]
3) $∃xψ(x)$ --- from 2) by $\exists$-I
4) $∀xθ(x)$ --- from 3) and 1)
5) $θ(a)$ --- from 4)
6) $ψ(a)→θ(a)$ --- from 1) and 5) by $\to$-I, discharging [a]