Structurally, the Constitution establishes a federal government with discrete, limited powers, reserving authority not given to the federal government under the Constitution to the states and the people. However, political and legal notions of the limits of federal power and the core of state autonomy change. As congressional assertions of authority have grown, the Supreme Court has had cause to delimit the reach of federal power in several legal contexts. Frequently contested provisions implicating federal versus state power include the Commerce Clause, the Tenth Amendment, and the Fourteenth Amendment. Justice John Paul Stevens has written notable opinions addressing each of these areas. These opinions generally indicate a viewpoint that it is difficult to clearly distinguish between federal and state spheres of power, and a tendency to recognize a limited federal role in issues that affect state prerogatives and local activities. The U.S. Constitution provides that Congress shall have the power to regulate commerce with foreign nations and among the various states. Starting in 1937 with the decision in NLRB v. Jones & Laughlin Steel Corporation , the Supreme Court took a significant doctrinal step in holding that Congress has the ability to protect interstate commerce from burdens and obstructions that "affect" commerce. In the NLRB case, the Court upheld the National Labor Relations Act, finding that control of industrial labor strife was a permissible means of preventing burdens on interstate commerce. Thus, the Court rejected previous distinctions between the economic activities (such as manufacturing) that led up to interstate economic transactions, and the interstate transactions themselves. By allowing Congress to regulate activities that were in the "stream" of commerce, the Court also set the stage for the regulation of a variety of other activities that "affect" commerce. Under this reasoning, the Court upheld many diverse laws, including laws regulating production of wheat on farms, racial discrimination by businesses, and loan-sharking. In United States v. Lopez , however, the Supreme Court brought into question how far Congress can rely on the Commerce Clause as a basis for federal jurisdiction. Under the Gun-Free School Zones Act of 1990, Congress made it a federal offense for "any individual knowingly to possess a firearm at a place that the individual knows, or has reasonable cause to believe, is a school zone." In Lopez , the Court held that, because the act neither regulated a commercial activity nor contained a requirement that the possession was connected to interstate commerce, the act exceeded the authority of Congress under the Commerce Clause. Although the Court did not explicitly overrule any previous rulings upholding federal statutes passed under the authority of the Commerce Clause, the decision did appear to suggest new limits on Congress's legislative authority. Justice Stevens, writing for the Court in Gonzales v. Raich , made clear the extent to which Congress still retained authority after Lopez to govern individual, localized activities so long as they had potential cumulative economic effects on regulated national markets . Raich evaluated an "as applied" challenge to the Controlled Substances Act as regards obtaining, manufacturing, or possessing marijuana for medical purposes. The case was brought by two seriously ill residents of California who used marijuana in compliance with the California Compassionate Use Act of 1996. The challenge was based on the argument that the narrow class of activity being engaged in—the intrastate, noncommercial cultivation and possession of cannabis for personal medical purposes as recommended by a patient's physician pursuant to California state law—did not have a substantial impact on commerce, and thus could not be regulated under the Commerce Clause. In upholding the application of the Controlled Substances Act in the Raich case, Justice Stevens relied on the Court's decision in Wickard v. Filburn , which held that "even if appellee's activity be local and though it may not be regarded as commerce, it may still, whatever its nature, be reached by Congress if it exerts a substantial economic effect on interstate commerce." The Wickard case upheld the application of the Agricultural Adjustment Act of 1938, which was designed to control prices by regulating the volume of wheat moving in interstate commerce. The Court in Wickard held that Congress could regulate not only the wheat sold into commerce, but also wheat retained for consumption on a farm . The Court did so on the theory that the while the impact of wheat consumed on the farm on interstate commerce might be trivial, it was significant when combined with wheat from other farmers similarly situated. Based on Wickard , the Court in Raich held that Congress could consider the aggregate effect that allowing the production and consumption of marijuana for medical purposes would have on the illegal market for marijuana. Of even greater concern was that diversion of marijuana grown for medicinal purposes for other uses would frustrate the federal interest in eliminating commercial transactions in the interstate market. In both cases, the Court found that the regulation was within Congress's commerce power because Congress had a rational basis to determine that production of a commodity meant for home consumption, be it wheat or marijuana, could have a substantial effect on supply and demand. In addition, because exempting the use of medical marijuana could undercut enforcement of the Controlled Substances Act, the Court found that the application in this case was within Congress's authority to "make all Laws which shall be necessary and proper" to effectuate its powers. As Wickard has traditionally been seen as one of the most expansive interpretations of the Commerce Clause, the Court's reliance on that case seemed to limit the possibility that the Commerce Clause would be likely to be subjected to a major retrenchment. The powers of the federal government, while limited to those enumerated in the Constitution, have been interpreted expansively at times, so as to create a large potential overlap with state authority. For instance, Article I, § 8, cl. 18 provides that "[t]he Congress shall have Power ... To make all Laws which shall be necessary and proper for carrying into Execution the foregoing Powers and all other Powers vested by this Constitution in the Government of the United States, or in any Department or Officer thereof." Early in the history of the Constitution, the Supreme Court characterized how this clause supplemented the powers of Congress. Recognizing that the Constitution nevertheless does place limits on the powers of the federal government, the Court has found it difficult to articulate clear principles to define these limits. For instance, for a time, the Court discerned it significant that the Tenth Amendment provides that "powers not delegated to the United States by the Constitution, nor prohibited by it to the States, are reserved to the States respectively, or to the people." While this language would appear to represent a clear example of a federalist principle in the Constitution, it has ultimately had only limited impact in defining the reach of federal authority. Thus, in National League of Cities v. Usery , the Court struck down federal wage and price controls on state and local employees as involving the regulation of core state functions. The author of the opinion, then-Justice Rehnquist, wrote that the statute at issue was unconstitutional because it "displaced the states' freedom to structure integral operations in areas of traditional governmental functions." The opinion first suggested there had to be a limiting principle to guide federalism-based restrictions on national power, and that such a limit must be judicially enforceable. Second, Justice Rehnquist opined that an essential part of state sovereignty was flexibility to determine the compensation state employees would be paid and the hours they would work. Third, he argued that federal wage and price controls substantially interfered with state sovereignty by imposing significant new costs that could "force[] relinquishment of important governmental activities" or "displace[] state policies regarding the manner in which they will structure delivery of those governmental services which their citizens require." Justice Stevens, who had only recently joined the Court, dissented from the holding in National League of Cities , arguing that it was difficult to perceive a defining principle within the majority opinion to distinguish between which state regulations were valid and which were not. The Court, by limiting the application of wage and price controls, was holding that the federal government could not interfere with a state's right to "pay a substandard wage to the janitor at the state capitol." Yet, Justice Stevens argued, there were many ways that the janitor could still be regulated: The Federal Government may, I believe, require the State to act impartially when it hires or fires the janitor, to withhold taxes from his paycheck, to observe safety regulations when he is performing his job, to forbid him from burning too much soft coal in the capitol furnace, from dumping untreated refuse in an adjacent waterway, from overloading a state-owned garbage truck, or from driving either the truck or the Governor's limousine over 55 miles an hour. Even though these and many other activities of the capitol janitor are activities of the State qua State, I have no doubt that they are subject to federal regulation. Justice Stevens's concerns proved to be well-founded. After National League of Cities , the lower courts struggled for a number of years to define which "core state functions" were not amenable to federal regulation. For instance, lower federal courts found that providing ambulance services, licensing automobile drivers, and operating a municipal airport were immune from government regulation. On the other hand, the courts had found that issuance of industrial development bonds, regulation of traffic on public roads, and regulation of air transportation were subject to federal regulation. This was the state of the law nine years after National League of Cities when the Court decided Garcia v. San Antonio Metropolitan Transit Authority . In Garcia , Justice Harry Blackmun reversed course and, in an opinion that Justice Stevens joined, overruled National League of Cities . The Court picked up the theme that had been introduced by Justice Stevens: We find it difficult, if not impossible, to identify an organizing principle that places each of the cases in the first group on one side of a line and each of the cases in the second group on the other side. The constitutional distinction between licensing drivers and regulating traffic, for example, or between operating a highway authority and operating a mental health facility, is elusive at best.