In Hayburn's Case, 2 U.S. 409, 2 Dall. 409, 1 L. Ed. 436 (1792), the U.S. Supreme Court held that it lacked power to issue an advisory opinion, because federal-court jurisdiction was limited by Article III of the U.S. Constitution to actual "Cases" and "Controversies."
Later the Court affirmed that those two words confine "the business of federal courts to questions presented in an adversary context and in a form historically viewed as capable of resolution through the judicial process." Flast v. Cohen, 392 U.S. 83, 95, 88 S. Ct. 1942, 20 L. Ed. 2d 947 (1968).