Check out Michael Green's Erie, Swift, and Legal Positivism, discussing A Critical Guide to Erie Railroad Co. v. Tompkins by Caleb Nelson. Here is a taste:
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Legal positivism is identified, in part, by the social fact thesis: the law of a jurisdiction is fundamentally a matter of social facts concerning officials (or the general population) within the jurisdiction. In his opinion in Erie, Justice Brandeis rejected the regime of Swift v. Tyson, in part, because he thought Swift was incompatible with this thesis. In Swift, Justice Story opined about the common law prevailing in New York without deferring to the decisions of New York state courts. That sounds anti-positivist. Story apparently understood the common law to be binding in New York independently of social facts about New York (or, indeed, any other jurisdiction’s) officials.