 The primary intension of "Water is H2O" is the "a posteriori" component, since it is contingent that the referent of "water" is H2O, while the secondary intension is the "necessary" component of the sentence, since it is necessary that the stuff we in fact call water is H2O.Neither intension gives us both a necessary and an "a posteriori" component.But one gets the false impression that the sentence expresses a "necessary a posteriori" proposition because this single sentence expresses two propositions, one "a posteriori" and one "necessary".