Common Law
Before statutes, regulations and the Constitution existed, people were governed by "common law"-legal customs that regulated behavior and business activities. Common law is the end result of a multitude of customs and general principles that began centuries ago in England and continues to develop today in judicial case law.
For example, a person who engages in reckless behavior and injures another person may be liable for damages under common law. Of course, there are statutes addressing reckless behavior while driving or setting out standards for safety on construction sites, but not all situations concerning liability for reckless behavior are covered in statutes. Thus, the common law principle that a person's reckless behavior may result in liability can be applied to a scenario the statutes do not address.