PHILADELPHIA—Justice Anthony Kennedy, in a speech in Philadelphia, says the Supreme Court is left to decide too many issues unresolved in the political arena.

Kennedy is teaching this week at the University of Pennsylvania law school. He spoke Thursday evening to faculty members, days before the court's new term starts Monday.

Kennedy, a 77-year-old centrist from California, is often the swing vote in high-profile 5-4 decisions. He says any nation that leaves its toughest issues for courts to decide is not a functioning democracy.

Justice Kennedy says the nation is in a period of "time compression," with changes in technology and social mores coming at stunning speeds. As an example, he says he never expected attitudes toward gay civil rights to evolve so quickly.