Principles are ways of successfully dealing with reality to get what you want out of life.
Ray Dalio, one of the world’s most successful investors and entrepreneurs, cites principles as his key to success.
People's personalities are pretty well formed before they come to you, and they've been leaving their fingerprints all over the place since childhood; anyone is fairly knowable if you do your homework. You have to get at their values, abilities, and skills: Do they have a track record of excellence in what you're expecting them to do? Have they done the thing you want them to do successfully at least three times? If not, you're making a lower-probability bet, so you want to have really good reasons for doing so. That doesn't mean you should never allow yourself or others to do anything new; of course you should. But do it with appropriate caution and with guardrails. That is, have an experienced person oversee the inexperienced person, yourself included (if you fit that description).