 A logic for which Tarski's method is applicable, is called "algebraizable".There are however a number of logics where this is not the case, for instance the modal logics S1, S2, or S3, which lack the rule of necessitation (⊢φ implying ⊢□φ), so ~ (defined above) is not a congruence (because ⊢φ→ψ does not imply ⊢□φ→□ψ).Another type of logics where Tarski's method is inapplicable are relevance logics, because given two theorems an implication from one to the other may not itself be a theorem in a relevance logic.