Philip Sales, Lord Sales, is a justice of the Supreme Court of the United Kingdom. This article is based on the text of the Sir Henry Brooke lecture for the British and Irish Legal Information Institute, delivered on Nov. 12, 2019. Lord Sales is grateful to Philippe Kuhn for his research assistance and to Richard Susskind for his comments on a draft of the lecture. The views expressed and any errors are the sole responsibility of Lord Sales. A revised and updated version of the lecture was published in the British journal Judicial Review in 2020.