jats:titleAbstract</jats:title>jats:pThis article aims to clarify what is meant by “a source of law” argument. A source of law argument justifies an action by showing that it has as its legal basis the best interpretation of a rule, principle or value identified in a material source of law. Such an argument is authority-based in that it appeals for its correctness to a collective decision to adopt a particular rule. The identification comes from an analysis of the practices within a specific legal community. The concept of “a rule of recognition” is not helpful since it glosses over the contestability of what is a source of law and its revisability over time. In a second part, the article illustrates the dynamics of change by reference to the status of EEC/EU law in a number of national laws and the 1966 Practice Statement on precedent in the House of Lords.</jats:p>