Note the point of principles - as the human mind seems to work - is not that they are true, beyond doubt, or irrevisible, but only that one cannot do without them to explain things.

And indeed, the main rational questions about principles are not how to prove them (since this can only be done from further principles) or how to defend them (their best defense is that they do logically imply what they are supposed to explain) but how to support them and how to test them.

Principles as a rule have some supporting evidence, that is not conclusive (since that would prove the principle and thus remove the need for its being a principle), and that may be weak or strong, and made weaker or stronger by further evidence.

Also, principles can be tested by inferring such logical consequences from them and other real or supposed knowledge one has as may be confronted with sense-experience to see whether these consequences are true or false.

Most principles human beings have adopted in history to explain their experiences or to underpin their desires have been based on wishful thinking, and even such principles as belong to science have been found to be fallible, and to need correction, supplementation or qualification with the growth of knowledge.

Principle about principles:

"The first principle is that you must not fool yourself—and you are the easiest person to fool. So you have to be very careful about that. After you've not fooled yourself, it's easy not to fool other scientists. You just have to be honest in a conventional way after that."
    (Feynman)