My Erie question for the day is another puzzle about conflicts between federal procedural common law and state law. In Byrd, Justice Brennan said that it followed from Erie that a state rule bound up with the state’s cause of action must, as a constitutional matter, trump competing federal procedural common law. I’m skeptical. But let’s assume he’s right. Why is it so important that the state’s rule is bound up with a cause of action? Can’t state law trump federal procedural common law in other ways?
Assume that the federal district court in Erie was trying to figure out whether communications made in New York between Erie and its New York counsel were privileged. Under New York law they were. Under federal procedural common law they were not. Why can’t this be a case in which New York law trumps federal procedural common law, even though New York obviously has not bound up its attorney-client privilege law into the Pennsylvania cause of action?*
*For the purpose of this question, assume that the choice-of-law issue had not already been answered by Fed. R. Evid. 501. In addition, ignore the fact that the twin aims of Erie (more on them later) would recommend using New York’s attorney-client privilege law independently of any constitutional considerations.