The Supreme Court began a potentially contentious election-year term Monday in seeming general agreement that juries in state criminal trials must be unanimous to convict a defendant.

The justices took up a quirk of constitutional law, a 47-year-old ruling that requires unanimity in federal, but not state trials. Earlier in the day, the court also wrestled with whether states must allow criminal defendants to plead insanity.

The one minor surprise when the justices took the bench just after 10 o’clock was the absence of Justice Clarence Thomas. The 71-year-old Thomas was at home, likely with the flu, the court said.

Justice Ruth Bader Ginsburg was in her customary seat to the left of Chief Justice John Roberts. The 86-year-old Ginsburg asked the first question in the insanity arguments.