Built from mistakes. Stories can be made of beautiful mistakes. Jazz can emerge out of riffs that are happy accidents. Do you have an ear for it? When your paint runs can see the signal to imagination that it sends?
There are leaden people who can paint a picture into nothing; they can write a story into dullness; and they can edit a film to make it unwatchable.
Knowing when is something that extends through all the creative arts, and often not knowing when to stop comes hand in hand with not knowing how to start, or what to start. Some people don’t start.
Writers write is a simple self evident truth.