This Article applies the method of text and principle to an important problem in constitutional interpretation: the constitutional legitimacy of the modem regulatory state and its expansive definition of federal commerce power Some originalists argue that the modem state cannot be justified, while others accept existing precedents as a “pragmatic exception” to originalism. Nonoriginalists, in turn, point to these difficulties as a refutation of originalist premises. Contemporary originalist readings have tended to view the commerce power through modem eyes. Originalists defending narrow readings offederal power have identified “commerce” with the trade of commodities; originalists defending broad readings of federal power have identified “commerce” with all gainful economic activity. In the eighteenth century, however “commerce” did not have such narrowly economic connotations. Instead, “commerce” meant “intercourse” and it had a strongly social connotation. “Commerce” was interaction and exchange between persons or peoples. To have commerce with someone meant to converse with them, meet with them, or interact with them. Thus, commerce naturally included all trade and economic activity because economic activity was social activity. But the idea of commerce-as-intercourse was broader than economics narrowly conceived-it also included networks of transportation and communication through which people traveled, interacted, and corresponded with each other. Understanding “commerce” in its original sense of “intercourse” is consistent with all of the evidence offered by rival theories of commerce as trade or economic activity; but it better explains the source of Congress’s powers over immigration and foreign affairs. It also better explains Congress’s broad powers over transportation and communications networks, whether or not these networks are used for purposes of business or trade. Congress’s power to regulate commerce “among the several states” is closely linked to the general structural purpose behind Congress’s enumerated powers as articulated by the Framers-to give Congress power to legislate in all cases where states are separately incompetent or where the interests of the nation might be undermined by unilateral or conflicting state action. Properly understood, the commerce power authorizes Congress to regulate problems or activities that produce spillover effects between states or generate collective action problems that concern more than one state. This basic structural principle explains why Congress’s commerce power inevitably expanded with the rise of a modern integrated economy and society, and it explains and justifies most if not all of modern doctrine. This approach justifies the constitutionality of federal regulation of labor law, consumer protection law, environmental law, and antidiscrimination law; it even shows why a federal mandate for individuals to purchase health insurance is constitutional. Finally, this approach shows why there are still areas where federal commerce power does not extend-these are areas where Congress cannot reasonably claim that an activity produces interstate spillovers or collective action problems, and does not involve networks of transportation and communication.