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.
Robert McKee, Story: Style, Structure, Substance, and the Principles of Screenwriting (2010)