Sounds like common law—one of the biggest, oldest "codebases" there is.

To quote Wikipedia:

> Common law is deeply rooted in stare decisis ("to stand by things decided"), where courts follow precedents established by previous decisions.[5] When a similar case has been resolved, courts typically align their reasoning with the precedent set in that decision.[5] However, in a "case of first impression" with no precedent or clear legislative guidance, judges are empowered to resolve the issue and establish new precedent.