Judges and academics have long relied on the work of a small number of Enlightenment political theorists—particularly Locke, Montesquieu, and Blackstone—to discern meaning from vague and ambiguous constitutional provisions. This Essay cautions that Enlightenment political theory should rarely, if ever, be cited as an authoritative source of constitutional meaning. There are three principal problems with constitutional interpretation based on eighteenth-century political theory. First, Enlightenment thinkers developed distinct and incompatible theories about how to structure a republican form of government. That makes it difficult to decide which among the conflicting theories should possess constitutional significance. Second, the Framers did not write the Constitution in the image of the philosophy of Montesquieu, Locke, or Blackstone. Instead, they developed a new form of government to meet what they perceived to be the needs of a nascent republic. And third, the Constitution itself departs from the dominant strands of Enlightenment political theory in crucial respects. For example, while some Enlightenment theorists advocated for precisely divided federal powers, the Framers favored a system of procedural checks, not formal separation. Thus, while Enlightenment works can be normatively persuasive or act as a guide to historical meaning, they should be treated as presumptively irrelevant in constitutional interpretation. Unless the party who would invoke an Enlightenment political theorist can produce evidence of consensus or common ground about that theory from an episode of American constitutional debate, the theorist’s prescriptions are no more probative than any other work of normative political theory.