The federal question put forward in a case taken to the Supreme Court from a state court cannot be abstract or moot.  Therefore, the federal question must raise a live controversy and must affect the rights of the litigants in the proceeding.  It must be substantial in nature.

A federal question is not considered substantial if it is without any foundation or merit, if it is a matter of conjecture, if it rests upon false assumptions, if it is not debatable and requires no analysis or exposition, and if it is explicitly foreclosed by a previous decision of the Supreme Court leaving no room for controversy.

In Tyler v. Judges of Court of Registration, 179 U.S. 405, 409 (U.S. 1900), the court observed that it becomes necessary to give an opinion upon a question of law when determining rights of person or property.  The opinion thus given can have weight as a precedent for future decisions.  But at the same time, the court is not empowered to decide moot questions or abstract propositions, or to declare principles or rules of law which have no connection in issue in the case before it.