'Principle' in the Bible

What, then, is there to boast about? That has been eliminated. On what principle? On that of actions? No, but on the principle of faith.

So I find this to be a principle: when I want to do what is good, evil is right there with me.

but I see in my body a different principle waging war with the Law in my mind and making me a prisoner of the law of sin that exists in my body.