There are two different sources of law:

1. Positive law: those laws which are approved and promote by the Congress and Senate thanks to the representation of the people in the chambers.
2. Natural law: basic principles which are inherited in the rule of law in each nation such as "good faith", "moral", "equity" etc... these concepts are not necessarily being expressed in the law itself but they are inside them. It is like their "soul"

We can say that Natural law comes from equity or "equilibrium". Whenever you want to approve a law you tend to reach equal footing.