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

source: Robert McKee, Story, 3 tags: Rules, Art, Writing