Prof. Barnett's example was poorly chosen--and yet it supports his argument. It was poorly chosen because Congress does have the constitutional authority to require individuals to purchase firearms--under Art. I sec. 8 cl. 12, which gives Congress power "to provide for...arming...the militia." To "arm" is a verb one employs to indicate requiring a person to act when that person is not currently acting. It differs from the language of the commerce clause, which gives Congress only power to "regulate commerce," a word that tends to apply to commercial traffic that is already ongoing. One may find this argument unconvincing, but it is at least a reasonable interpretation of the langauge, and the difference between the more active verb "arming" and the more passive description of a noun--"commerce"--in fact supports Barnett's argument that the commerce clause does not allow Congress to compel people who are not currently engaged in commerce to do so.