Legal positivists maintain that the legality of a rule is fundamentally determined by social facts. Yet for much of legal history, ordinary officials used legal terminology in ways that seem inconsistent with positivism. Judges regularly cited, analyzed, and predicated their decisions on the ‘laws of justice’ which they claimed had universal legal import. This practice, though well-documented by historians, has received surprisingly little philosophical attention; I argue that it invites explanation from positivists. After taxonomizing the positivist’s explanatory options, I suggest that the most viable option appeals to conceptual change: classical Romans, early modern Europeans, founding-era Americans were not using ‘law’ (or ‘lex’ or ‘jus’) to refer to the subject matter of contemporary legal philosophy. But the strategy is costly. It renders positivism’s truth surprisingly parochial. And it supplies new reasons for doubting positivist accounts of contemporary practices, including the treatment of moral principles in modern adjudication.