A principle is a kind of rule, belief, or idea that guides you. You can also say a good, ethical person has a lot of principles.
In general, a principle is some kind of basic truth that helps you with your life. "Be fair" is a principle that guides (or should guide) most people and businesses. A politician who tries to do the right thing rather than win votes is acting on principle. A person who has principles is a good, decent person. On the other hand, if you say someone has no principles, that means they're dishonest, corrupt, or evil.
Principle as a final cause
Final cause is the end, or goal, which guides one to take the necessary actions to obtain it.
For that there needs to be an intelligence capable of conceiving the end and realizing that certain actions must be taken to achieve the goal.
Science does not recognize the finality of the natural causes as a guiding principle of investigation.
It is also understood therefore that the principle guides the action as a norm or rule of behavior, which produces two types of principles.
Principle as moral law
It represents a set of values that orientate and rule the conduct of a concrete society. The law establishes an obligation in the individual's conscience that belongs to the cultural field in which such values are accepted. It supposes the liberty of the individual as cause, that acts without external coercion, through a process of socialization.