Some descriptions of modal logic speak of possible universes. This sounds very metaphysical, but it plays a vital role in the engineering of epistemology when coping with uncertainty. The mathematical literature on modal logic normally starts with axioms and derives theorems. There are many varieties of axioms with many different theorem sets even when they use identical notation. (For example “⟡⟡p ≡ ⟡p” and “◇◇p ≡ ◇p” ♢ are theorems in some systems but not others.) ☐☐p ▢▢p ⎔⎔p The philosophical literature ranges from very practical to very theoretical (and metaphysical)).