>What is right? How can a person know?

We watch, we experience, we learn, we read, we ask around, we interact.

Same as with any other thing. If I stole your wallet you'd immediately knew it wasn't right, wouldn't you?

>I don't think that principles are much more than oral tradition.

Well, I don't think oral tradition is not a good basis to base principles on. It implies historical experience, communal values, the test of time, and so on.

It might not be perfect, but then again, it need not be.