There has been considerable debate in legal philosophy about how to attribute purposes to rules. Separately, within cognitive science, there has been a growing body of research concerned with questions about how people ordinarily attribute purposes. Here, we argue that these two separate fields might be connected by experimental jurisprudence. Across four studies, we find evidence for the claim that people use the same criteria to attribute purposes to physical objects and to rules. In both cases, purpose attributions appear to be governed not so much by original intention or by moral value as by current practice. We argue that these findings in the cognitive science of purpose attribution have implications for jurisprudential questions involving purposivist legal interpretation.