 The standard theory makes use of the fact that in classical logic, the truth-function “p ⊃ q” (“If p, q”) is false only when p is true and q is false. The relation between “p” and “q” in this case is sometimes referred to as material implication (see the entry on  the logic of conditionals).  On this account of “if p, q”, if the conditional “p ⊃ q” is true, and p holds, then q also holds; likewise if q is false, then p must also be false (if the conditional is true). The standard theory claims that when the conditional “p ⊃ q” is true the truth of the consequent, “q”, is necessary for the truth of the antecedent, “p”, and the truth of the antecedent is in turn sufficient for the truth of the consequent. This reciprocal relation between necessary and sufficient conditions matches the formal equivalence between a conditional formula and its contrapositive (“~q ⊃ ~p” is the contrapositive of “p ⊃ q”). Descending from talk of truth of statements to speaking about states of affairs, we can equally correctly say, on the standard theory, that using the key was a necessary condition of opening the door.