As a judge, jay applies common law rules. these rules develop from

The rules developed from decisions of the courts in legal disputes
Common law is used to handles the cases that had already been previously handled by the court.
If,  the case had never been handled and would influence the well being of large number of people, the case would be forwarded to the Supreme court.

A- slaverythe civil war was fought to prevent the expansion of slavery, not the abolotion of it