 Condition (i) ensures that nothing is implied by an unsatisfiable antecedent. The evaluation clauses for the other connectives are classical. A formula A is true in a model (M ⊨ A) iff M, g ⊨ A; and A is valid iff A is true in every model. Condition (ii) ensures that the law of contraposition is valid. A set Δ of formulas is true in a model iff every element of Δ is true in the model.