In contrast, in an adversarial system, the onus of framing the case rests on the parties, and judges generally decide the case presented to them, rather than acting as active investigators, or actively reframing the issues presented. "In our adversary system, in both civil and criminal cases, in the first instance and on appeal, we follow the principle of party presentation. That is, we rely on the parties to frame the issues for decision and assign to courts the role of neutral arbiter of matters the parties present." This principle applies with force in all issues in criminal matters, and to factual issues: courts seldom engage in fact gathering on their own initiative, but decide facts on the evidence presented (even here, there are exceptions, for "legislative facts" as opposed to "adjudicative facts"). On the other hand, on issues of law, courts regularly raise new issues (such as matters of jurisdiction or standing), perform independent research, and reformulate the legal grounds on which to analyze the facts presented to them. The United States Supreme Court regularly decides based on issues raised only in amicus briefs from non-parties. One of the most notable such cases was "Erie Railroad v. Tompkins", a 1938 case in which neither party questioned the ruling from the 1842 case "Swift v. Tyson" that served as the foundation for their arguments, but which led the Supreme Court to overturn "Swift" during their deliberations. To avoid lack of notice, courts may invite briefing on an issue to ensure adequate notice. However, there are limits—an appeals court may not introduce a theory that contradicts the party's own contentions.