Scotland is often said to use the civil law system, but it has a unique system that combines elements of an uncodified civil law dating back to the Corpus Juris Civilis with an element of its own common law long predating the Treaty of Union with England in 1707 (see Legal institutions of Scotland in the High Middle Ages), founded on the customary laws of the tribes residing there. Historically, Scottish common law differed in that the use of "precedent" was subject to the courts' seeking to discover the principle that justifies a law rather than searching for an example as a "precedent", and principles of natural justice and fairness have always played a role in Scots Law. From the 19th century, the Scottish approach to precedent developed into a "stare decisis" akin to that already established in England thereby reflecting a narrower, more modern approach to the application of case law in subsequent instances. This is not to say that the substantive rules of the common laws of both countries are the same, but in many matters (particularly those of UK-wide interest), they are similar.