Principle, principal and interest

When we stand for something, we usually claim that it is for general principle, not for specific personal interest. What is the relation between principle and interest?

When we buy a bond, we pay roughly the amount of principal. In the future, we will get paid interests periodically. We pay for principal now so we get paid for interests in the future.

The same relation holds between principle and interest. There are many different principles. We stand for principles that will gain us interests in the future. When our interests change, the principles we stand for change as well.