Proof. Assume that L is disjunctive. Let m be a dense element of L. That is
[m]∗ = {0}. For any x ∈ L, [m∧ x]∗∗ = [m]∗∗ ∩ [x]∗∗ = L ∩ [x]∗∗ = [x]∗∗ and hence
[m ∧ x]∗∗∗ = [x]∗∗∗. So that [m ∧ x]∗ = [x]∗. Since L is disjunctive, we get that
m∧ x = x. Therefore m is a left identity element of L.