The Rule
Creativity Requires Input
An author feeds his creativity with real world experience. If it's important to write what you know, you'd better know a lot of things.
"Once again, we have an author who, while he's sitting there, he's looking around at things directly in his field of vision and those happen to be the things that are the solution..."
Counter-Example
"Where do you get your ideas?" Ray Bradbury very famously had an office jam-packed with flotsam and jetsam that he claimed to use for inspiration.
Ray Bradbury also traveled to Ireland as part of writing the screenplay for "Moby Dick" and years later wrote a dozen short stories and four one-act plays based on his six months there. His trips to Mexico also greatly influenced his work.
Commentary
An old saw of writing used to be that authors had to have many jobs and visit many places—to experience things broadly, in other words, in order to write broadly. This was doubtless overplayed in the 20th century, with Hemmingway out there on battlefronts and the like, but still, it's never been easier to go someplace new and do something different, and it will keep your work from feeling insular.