This is usually a set of beliefs which justifies their actions, and advantages them.
It is generally a collection of morals and theorems which they have come to accept over time. They usually contain many contradictions, but the human mind is a complex instrument which can contain many contradictions and argue them quite well when self-interest is involved.
In fact the human brain has evolved largely to justify behavior (not actually solve problems as many believe), so most people are very good at justifying their belief set.
See