Rules vs Principles in writing

Started reading a new book about writing screenplays: Story by Robert McKee.

He started the book off with an introduction that outline a number of cliches regarding writing in general, but the first paragraph of the intro I took a shine to. That was:

A rule says, "you must do it this way." A principle says, "This works. . . and has through all remembered time." The difference is crucial. Your work needn't be modeled after the "well-made" play; rather, it must be well made within the principles that shape our art. Anxious inexperienced writers obey rules. Rebellious, unschooled writers break rules. Artists master the form.

I like the idea of mastering the form, if I could only figure out what the hell it is....

Hopefully this book will tell me... Am I asking for the moon? No. Many others have learned. All it takes is someone to show the way.