This book provides a strong introduction to the principal domains of legal knowledge by examining a structured list of legal maxims, many originating in medieval Roman and canon law. Oriented by historical and methodological approaches, it explores legal thought and reasoning through a comparative lens. Geoffrey Samuel explains the differences between common law and civil continental traditions, outlining preceding works on regulae iuris across the centuries. With an emphasis on English law, the book analyses basic principles and addresses the law of obligations, the law of actions and the law of property and public law. Delving into fundamental legal remedies, substantive law and legal institutions, Samuel questions whether some maxims are more memorable or indisposable than others and shows the impact they have had on legal theory. Exploring historical developments, the book also maps the original sources of key theories, utilising a civil law taxonomical plan to classify them.