My parents were Owen and Constance Papineau.Could I have had different parents?Surely not.A Person with different parents would not have been me.So it is necessary that David Papineau's parents were Owen and Constance Papineau.But this statement is surely not a priori.You need evidence to know who my parents were.It's not something you can find out just by thinking.So that's one emample of an a posteriori necessity.