One thing that has bothered me for a long time is the imprecision in the commonly tossed around idea that courts are supposed to act as “faithful agents” when they interpret statutes.  E.g., Amy Coney Barrett, Substantive Canons and Faithful Agency, 90 B.U. L. Rev. 109, 110 (2010).  The basic idea of this theory is that the goal of courts when they interpret statutes is to implement the will of Congress.  Both purposivists and textualists subscribe to the theory; their disagreements are about which method is better at faithfully implementing the will of Congress.  

But to say that courts should act as faithful agents really does not tell us very much. Agents come in many different forms.  Compare, for example, real estate agents and attorneys. Both are agents, but they have different powers.

So too with statutory interpretation.  Another group of institutions that is supposed to act as faithful agents in interpreting statutes are administrative agencies. See Jerry L. Mashaw, Norms, Practices, and the Paradox of Deference: A Preliminary Inquiry into Agency Statutory Interpretation, 57 Admin. L. Rev. 501, 505 (2005). But they have different interpretive powers than the courts.  Unlike with courts, the goal of agency interpretation is not simply to implement the will of Congress.  Agencies can seek to implement their own policy preferences when they interpret statutes. 

When people say that courts should act as faithful agents, they are really making a claim about the type of power courts exercise when interpreting statutes. So they are actually making arguments about the scope of the judicial power in Article III (leaving aside questions about whether Congress can direct how courts interpret statutes).   They are saying that, in their view, the judicial power requires courts to interpret statutes in a way that implements the will of Congress—as opposed to an agency, which has different power assigned to it and is not obliged as a faithful agent to try to implement Congress’s will when interpreting a statute.

When talking about faithful-agency theories of interpretation, it would be helpful to disaggregate the fact that the court is an agent from the scope of the agency.  Just saying that courts have to be “faithful agents” tends to obscure more than illuminate.