1. Magic is natural.
2. Harm none -not even yourself- through its use.
3. Magic requires effort. You will recieve what you put into it.
4. Magic is not usually instantaneous. Spells require time to be effective.
5. Magic should not be performed for pay.
6. Magic should never be used in jest or to inflate your ego.
7. Magic can be worked for your own gain, but only if it harms
8. Magic is a divine act
9. Magic can be used for defense but should never be used for attack
10. Magic is knowledge -not only of its way and laws- but also of its effectiveness. Do not believe magic works-know it!
11. Magic is love. All magic should be performed out of love. The moment anger or hatred tinges your magic you have crossed the border into a dangerous world. One that will ultimately consume you.
-From Scott Cunningham's Encyclopedia of Magical Herbs
In my opinion, #11 is the most powerful principal out of all of them.