Writing for the Court, Justice John Paul Stevens began by stating the three areas in which Congress can regulate under its commerce power: the channels of interstate commerce, the instrumentalities of interstate commerce, and activities that substantially affect interstate commerce. Stevens then stated that only the third category, activities substantially affecting interstate commerce, was at play in this case.  

Stevens relied on the Court’s holding in Wickard v. Filburn (1942) to establish Congress’ authority to regulate local activity if it exerts a “substantial economic effect” on interstate commerce, even if that activity is purely intrastate and not itself “commercial”. Stevens explained that in affirming such authority, the Court has “never required Congress to legislate with scientific exactitude. When Congress decides that the ‘total incidence’ of a practice poses a threat to a national market, it may regulate the entire class.” 

Stevens stated that the similarities between the facts in this case and those in Wickard were “striking.” Stevens explained that in Wickard, the Agricultural Adjustment Act aimed to control the volume of wheat moving in interstate commerce, just as the CSA aims to control the supply and demand of controlled substances in lawful and unlawful drug markets. He stated, “the regulation is squarely within Congress’ commerce power because production of the commodity meant for home consumption, be it wheat or marijuana, has a substantial effect on supply and demand in the national market for that commodity.‍” 

Stevens asserted that the Court’s task in this case was not to determine whether Raich and Monson’s activities, when taken in the aggregate, “substantially affect” interstate commerce, but rather only whether Congress had a “rational basis” to conclude it does. Stevens pointed out that unique difficulties exist in distinguishing between marijuana grown locally and marijuana grown elsewhere, so the Court had “no difficulty concluding that Congress had a rational basis for believing that failure to regulate the intrastate manufacture and possession of marijuana would leave a gaping hole in the CSA.” Therefore, he explained, Congress was acting within its constitutional authority to “‘make all Laws which shall be necessary and proper’ to ‘regulate Commerce ... among the several States.’” 

Stevens then responded to the argument that the Court should “excise individual applications of a concededly valid statutory scheme.” Stevens distinguished this request from the Court’s holdings in United States v. Lopez(1995) and United States v. Morrison(2000), where two statutes enacted under the Commerce Clause were struck down as an unconstitutional exercise of Congress’s commerce power. Stevens explained stated that “[u]nlike those at issue in Lopez and Morrison, the activities regulated by the CSA are quintessentially economic.” He further explained that since the CSA regulates production, distribution, and consumption of commodities for which an established interstate market already exists, Congress’ prohibition of intrastate manufacture or possession is a rational and commonly utilized regulation. Stevens concluded that because “the CSA is a statute that directly regulates economic, commercial activity,” there is no doubt regarding its constitutionality.