Conventional wisdom holds that there is a connection between (a) the jurisprudential commitment to legal positivism expressed in Erie R.R. v. Tompkins, and (b) Erie's holding that the common law powers of federal courts exercised in Swift v. Tyson are unconstitutional. In this essay we analyze and challenge this conventional wisdom. The conventional wisdom is sometimes expressed as an historical connection between beliefs about positivism and Erie's overruling of Swift. We think this historical claim lacks affirmative support, overlooks significant evidence to the contrary, and misleadingly views Erie's pre-history through the distorting lens of Holmes' dissents. Other times the conventional wisdom is expressed as a conceptual or normative connection between the truth of positivism and Erie's holding. We think these contentions too are wrong. Our claim here is one of irrelevance: Erie's commitment to legal positivism is conceptually and normatively independent of its constitutional holding. Legal positivism is a general theory about the nature of law. Even if true, it has no implications for the allocation of authority between the state and federal governments. This argument shows that Erie is not a decision about the nature of law, but rather reflects a particular time-bound set of constitutional and policy priorities. It helps to explain why some recent philosophical attacks on Erie are groundless. It demonstrates that the many outstanding mysteries about the practical implications of Erie's holding cannot, as many think, be resolved by recourse to legal positivism. And it presents a cautionary lesson about the dangers that inhere in attempting to derive constitutional conclusions from theories about law.