Every attorney is taught that, under Erie Railroad Co. v. Tompkins, federal district courts sitting in diversity must apply the substantive law of the state in which the district court sits. 304 U.S. 64 (1938). But sometimes attorneys forget that not every state law is substantive: some state laws are procedural, and, in those cases, federal procedural rules take precedence.
The U.S. Court of Appeals for the Fifth Circuit’s approach to Texas’s anti-SLAPP law provides a recent and important example of how the substance-procedure distinction can have practical consequences.