Adam Roberts, Yellow Blue Tibia
The speaker in the Adam Roberts novel quoted above is one Soviet science fiction writer talking to another who has just admitted most of his books have just been rewrites, little more than Russian translations, of books by western writers. Yellow Blue Tibia is a humorous novel, and this passage is intended to be amusing. Yet it does contain a germ of truth. Writing, perhaps the most solitary of occupations, is, in at least three senses, "a collective activity."
Later Winegardner wrote one or more of those Godfather novels, using characters and plot lines developed by the late Mario Puzo. And so, while plagiarism may be wrong, writers do borrow from other writers all the time.
2. Wes Anderson quotes Austrian writer Stefan Zweig in the screenplay for his film The Grand Budapest Hotel, "It is an extremely common mistake: people think the writer's imagination is always at work, that he is constantly inventing an endless supply of incidents and episodes, that he simply dreams up his stories out of thin air. In point of fact, the opposite is true. Once the public knows you are a writer, they bring the characters and events to you -- and as long as you maintain your ability to look and carefully listen, these stories will continue to seek you out."
Perhaps some ideas do originate entirely in the author's mind, but usually ideas come from a variety of sources -- other writers (as mentioned above), experiences, newspapers, magazines, television, stories told by friends and acquaintances, etc. -- and are then mulled over in the writer's mind until they are ready to emerge in an entirely original form. Writers may need to be alone to write, but if they are too alone they will have nothing to write about.
3. Finally, writers need readers. I met Mark Winegardner at the Buckeye Book Fair held each November in Wooster, Ohio. Similar events are held throughout the country to introduce authors to readers in hopes some of those readers will buy their books. Authors and publishers depend on book events, including book signings, to sell copies. Not all writers can support themselves with their writing. Whether they can or not depends on readers. Their work, after all, is a collective activity.