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.
Root causes are described in adjectives, not verbs, so keep asking "why" to get at them. Since most things are done or not done because someone decided to do them or not do them in a certain way, most root causes can be traced to specific people who have specific patterns of behavior. Of course, a normally reliable person can make the occasional error and if that's the case, then it can be forgiven, but when a problem is attributable to a person, you have to ask why they made the mistake--and you have to be as accurate in diagnosing a fault in a person as you would be if he or she were a piece of equipment.
A root cause discovery process might proceed like this:
The problem was due to bad programming.
Why was there bad programming?
Because Harry programmed it badly.
Why did Harry program it badly?
Because he wasn't well trained and because he was in a rush.
Why wasn't he well trained?
Did his manager know that he wasn't well trained and let him do the job anyway, or did he not know?
Consider how personal the questioning is. It doesn't stop at "Because Harry programmed it badly." You must go deeper in order to understand what about the people and/or the design led to the failure. This is difficult for both the diagnoser and the RPs, and it often results in people bringing up all kinds of irrelevant details. Be on your guard because people will often look to cover themselves by diving into the weeds.