Perhaps it's because I've spent most of my adult life writing and discussing writing with fellow writers, but external impetus is usually taken as self-evident. The impression I was under was that this discussion was about what form the internal conversion from impetus to creation takes, but now you're making it sound as though it's really about where writers get their ideas. There's a reason writers get impatient with that question; it implies there's some kind of magical formula we know that a non-writer doesn't. There is no magic formula. There is a natural inclination to interpret input in a way that's conducive to writing, and there is training that hones that inclination (or, to a certain degree, creates that inclination).

One of my closest friends is a brilliant visual artist. Her creative process is much the same as mine; her chosen medium of creation is different, based partly on natural inclinatiion.