The Constitution is primarily about power, the capacity of government actors to change legal rules and legal relations. It is also to some extent about the duties of government actors and institutions, and imposes on them obligations that it is wrong not to fulfill. These two functions of constitutional rules, setting out powers and imposing duties, are distinct from one another and it is important not to confuse them. The provisions of the Constitution that grant and limit the power of Congress are concerned exclusively with power and do not create duties. They therefore are not the source of the obligations that Senators and Representatives have to ensure that their legislation is consistent with the Constitution. Members of Congress do have obligations of that kind, but those obligations come from their role as legislators, and because of that role are flexible and not absolute. There are circumstances in which a conscientious legislator may vote for legislation that the legislator believes to be partly unconstitutional. By contrast, the constitutional provisions that grant authority to act conclusively, such as the provisions that empower the courts, do bring with them unqualified obligations to follow the law. Power and duty interact differently depending on the kind of power involved. Because the obligations of legislators concerning the constitutionality of legislation are flexible, they do not support the inference that legislation is necessarily valid or invalid on its face and not as applied. But the text of the First Amendment, understood as dealing solely with the power and not the duty of Congress, does strongly suggest that it makes rules valid or invalid as such, so that all invalidity is facial invalidity. The well established phenomenon of as-applied invalidity, and the sever-ability of valid from invalid applications, can be explained consistently with this reading of the First Amendment. Sever-ability happens when Congress has implicitly or explicitly provided a fallback rule that is valid as such and that replaces the primary rule that is invalid on its face. The First Amendment operates at the level of rules and not applications.