Adam Cox’s Immigration Law’s Organizing Principles contests the traditional view that immigration law and alienage law—in his terms, “selection rules” and “regulation rules”—are distinct categories with legal and moral salience. Building upon prior scholarship that also called the distinction into question, Cox offers important insights into why this dividing line does not have the sharp conceptual edges that the jurisprudence would suggest exist. Despite the analytical persuasiveness of Cox’s argument, I am not convinced that it will destabilize the entrenched understanding of the dichotomy, at least in the political realm. The relevant and continuing question—for Cox and the rest of us who contest this line drawing—is to discern why the line continues to have such appeal. I want to explore two possible explanations for the tenacity of the categories:(1) the conceptual distinction between the categories that does exist, and (2) the political utility of hewing to the line.