n. the connective of modal logic usually defined in terms of the impossibility of the truth of its antecedent at the same time as the falsehood of its consequent, that is,
P ⇒ Q ≡ - ◊(P & -Q),
where ◊ is the possibility operator. It is the relation that holds between two sentences precisely when one is validly deducible from the other, and is not truth-functional. Compare material implication.