In the early days of aviation, the newly-invented aircraft had few, if any, instruments, and those they had were unreliable. Early aviators therefore followed the advice of horsemen, who had never had any instruments but their own bodies, and in particular their seat, from which to derive information about how well their horse was going. This meant “Flying by the seat of your trousers” or “pants” in US English meant using your own judgement about how your aircraft was flying.
In the early days of writing, authors had few guidelines except perhaps basic grammar. They wrote how they liked. There was no formula for success, so some succeeded and others didn’t, assuming we are to judge success by popularity.
The increasing popularity of literature as entertainment led to the serialisation of novels in monthly magazines, and authors were obliged to make each chapter interesting, but not necessarily to follow any structure. The crucial thing was never to miss a deadline for delivering an episode to the publisher. This meant paying attention to what was going on now, and letting future chapters take care of themselves.
Later, literature was subjected to analysis and structural forms were identified. Any story had to jump through a series of hoops in order to be considered good. You needed to know about hooks, character arcs, try / fail cycles, etc. You could go to college and be taught creative writing.
Now, some people consider that creativity cannot be taught. What you are learning on a course, they say, is mannerism. You are being asked to ape the modern masters of writing in the same way that the mannerist painters of the post-Renaissance era were taught to imitate the style of Titian or Raphael.
Such writers like to start a tale and see where it takes them, rather than planning out the whole story in advance and then writing it. Those who follow their own instincts are writing’s heirs to the old aviators who flew by the seat of their pants. They are vulgarly known as pantsters and their methods (or lack of method) is called pantsing.
And sometimes it works, and sometimes they come up with something that is highly original. And sometimes they get stuck and can’t even finish. A very few of them will make a major breakthrough. The majority will probably struggle to make money with any consistency, whereas those who follow the ‘rules’ will probably stand a better chance of a regular income. Even they can’t guarantee it, though.
