Sometimes we get an opinion back from a court, and the reasoning
leaves us scratching our heads and wondering, "Where did that
come from?" In the opinion, the court has decided the case on
something that neither party ever argued. We blogged about a case
like that once, here. In that case at least, we could tell
where the court got the reasoning that it found dispositive - from
an amicus curiae brief. But usually, when that happens to
us, we have no idea where the court got the idea - probably from
some sharp judicial clerk's research.
Well, it turns out there are limits on a court's ability to
do that.
Last term, in United States v. Sineneng-Smith, 140 S. Ct.
1575 (2020), a criminal case, the Court (in one of Justice
Ginsburg's last opinions), reversed the Ninth Circuit for going
off on a tangent that neither party had asserted. The opinion in
Sineneng-Smith invoked something called the "party
presentation" principle (the interesting thing we had never
heard
