This Article looks at a historical problem—the first use of case law by English royal justices in the thirteenth century—and makes it a starting point for thinking about the ways legal reasoning works in the modern common law. In the first Part of the Article, I show that, at its origin, the English justices’ use of decided cases as a source of law was inspired by the work civil and canon law scholars were doing with written authorities in the medieval universities. In an attempt to make the case that English law was on par with civil law and canon law, the justices and clerks of the royal courts began to treat cases as if they were the opinions of great jurists, to apply the same types of dialectical reasoning that were used in civil law discourse to those cases, and to work them into systems of authority. They used cases, as the modern common law does; but they used cases to create systems of the kind we usually associate with civil law. In the second Part of the Article, I turn to the modern common law and, using the methods of medieval case law as a mirror, show that the differences between civil law and common law reasoning are more perceived than real. American lawyers tend to view common law as flexible and creative, whereas they view civil law as ossified and hierarchical. This largely stems from the fact that common lawyers focus on the judicial opinion as the place where legal reasoning takes place. By integrating other texts, like the student outline and the restatement—which seek to create a harmonious system out of judicial opinions—into the picture of common law reasoning, I show that common law reasoning shares quite a bit in common with civil law reasoning.