When I was in my teens, I did not get why my [now late] uncle often asked pointed questions especially of people he had just met. By my middle-twenties, I had learned he was “probing” for what he might find useful in a short story or a novel – including from friends of mine. I remember too my mom more than once also warned her older brother: “I don’t want to see this in a f-cking book.”
I recall that here because this pops us now and then as a renewed “debate.” I have touched upon it before. It seems a good time now, though, to re-address it here – in full:

Director/screenwriter Christopher Nolan declares there that “Write what you know” is not just “the least useful maxim” for writers. He adds, “It’s nonsense.” He also asserts unequivocally: “You should write whatever you want.”
Another:

The slap at English professors aside, that writer is quoted there as calling it a “totally false adage” in “bad” books on writing.
Due to the disdain from such figures, we likely also get memes and social media writing “advice” like this:

And why is it “bad writing advice?” Here’s why, this one asserts:
Writing is not about *you* learning something! (Exclamation mark used deliberately.) Readers don’t give a damn about what you may have “learned.” It is about you writing to entertain/inform those *readers*.
Is that really the goal for those genres? Isn’t it actually about using such outlets to explore our earthly/human situation merely from another perspective? To do that, for example, don’t we put the “Zerg” characters on “Planet X”… yet, in the end, we are still dealing with what matters to us as human beings?
You can only write “diverse” characters and cultures if you *know* them and about them. The best way to do that is to have enough life experiences around and with such people that you can write them. If you try to just make them up out of thin air, from no real-life guide(s), you are going to get yourself into literary trouble – big time.
I have no idea what that second sentence actually even means. That said, while this is obviously uncomfortable reading for many writers/authors, actually having experiences is really a major basis for writing fiction. For think about this: If you do little “interesting” in real-life, and so never experience much of anything about which you might write, are you going to be able to write anything that does not come across as “one-dimensional” at best?
Interestingly, though, after all of that telling us how “bad” such advice is, they end here…
So “write what you know?”
If you are confused, you are no more confused than that person above seems to be given where they ended up based on where they started.
Curiously, none of those above cite that “bad” advice’s source by name. Do they even know who it may have been? Its original source is possibly Ernest Hemingway, who wrote this in a 1928 letter to his publisher:

Thus it seems there that a perhaps (since much “misquoted”) “maxim” and “adage” was born nearly a century ago.
Vital, too, its context. Hemingway had been a newspaper reporter (so traveled a great deal) who moved into writing fiction. He wrote that letter having just had his first big success with his 1926 (first published full) novel, The Sun Also Rises…

…In it, he sources its fictionalized main characters from some real people he got to know in Spain and France especially in the summer of 1925. (It may also be worth noting that not all of them were thrilled when they found out what he had done; “Robert Cohn’s” real-life “source” was so angered he never spoke to Hemingway again.) So it is easy to understand Hemingway making that statement above: he had fictionalized some of his experiences and people he had known and that had “worked” for him as an author. Importantly, he also makes it plain in that letter that he feels he does NOT know enough yet to write, for example, a new novel he is contemplating that may be sort of a “modern Tom Jones.”
Separately, a site called Inc. tells us Hemingway at some time also wrote this:
Write about what you know and write truly and tell them all where they can place it… Books should be about the people you know, that you love and hate, not about the people you study about.
Note in the first sentence Hemingway opened with “Write about what you know” and what follows is, again, pretty important in the context. I am no Hemingway expert, but I have never seen an original “Write what you know” in so many words in an authoritative source coming from something he wrote. “Write what you know ABOUT” or “Write ABOUT what you know” are softer statements, which are, again, hard to dismiss if one is being honest.
You can, of course, suggests Nolan, “Write whatever you want.” But is that actually sound advice? After all, should you write about “what you don’t know about?”
I found several of the comments to that Nolan Instagram post quote worth zeroing in on here. First:

And here is that “pointed to” comment:

Which reads, again, a lot like that last bit of “advice” above from that “Memes for Writers” Instagrammer… which basically means, well, uh, “write what you know [about].”
Second, this next commenter responds this way to the first Nolan commenter above:

That is I think worth highlighting here because it reveals what I detect is the major underlying reason many writers are quick to reject the “write what you know [about]” suggestion. It is rooted in personal insecurity and the fear that they actually don’t know very much, or that if they do try to follow it that they will “run out of ideas.” Helpfully, though, this is also Hemingway on “running out of ideas,” in A Moveable Feast:
But sometimes when I was starting a new story and I could not get it going, I would sit in front of the fire and squeeze the peel of the little oranges into the edge of the flame and watch the sputter of blue that they made. I would stand and look out over the roofs of Paris and think, ‘Do not worry. You have always written before and you will write now. All you have to do is write one true sentence. Write the truest sentence that you know.’ So finally I would write one true sentence, and then go on from there.
That may also be why writing fiction is one of those crafts in which we tend to get better with age: life experience is certainly helpful in writing “true” sentences.

In response to that @advicetowriters tweet above quoting that author’s “rejection” of the “maxim,” I wrote this – rather lightheartedly – in 2019, in “Hemingway-style.” Given the topic, it is worth re-posting. I do so also because I suppose it forms my bottom line opinion on the “write what you know [about]” “maxim”/“adage”:
I didn’t know who that author quoted there is as I’d never heard of him before seeing that tweet. I googled him and he’s big. But in sci-fi. Assuming that quote is accurate – and it is the internet, so you have to be careful – his first book was based on his Vietnam War experiences. And he also wrote a novel based on Hemingway. But DON’T write what you know? Huh? It’s like me not knowing Jefferson wrote “All men are created equal…” Okay, maybe not that bad, but you get my point. If you won’t write what you know, sure as hell don’t write what you don’t know. So, yeh, of course he said he believes that, but you and I and half of the world also know that if most sci-fi guys wrote only about sci-fi stuff most really knew lots about it would probably all be storylines sourced from their personal adventures reheating coffee in the microwave or something. He takes a shot too at English (male?) profs’ morals and I don’t care about English profs’ secret morals either way, but are we supposed to think sci-fi male writers are all Clooney and Pitt when so many fall back on writing ludicrous fantasy women and how many of those guys don’t even know any real women to write about anyway. I’ll stop. Not my genre. I’m sure some are great writers. Me, sure, who am I to talk? I know I wasn’t in Europe in the 1790s, but people are people and love and all of that stuff is perpetual. It’s just the clothes and other stuff that’s different. Paris and London were both much the same then as now. Smaller, yeh, but much the same. I know lots about that era, so I feel I can write that stuff because I feel I do know it. I wouldn’t try to write about life in Kamchatka or in Siam in 1790 because I’ve got no clue. Good writing sense from Hemingway there if you ask me. That tweet to me is the worst example of confusing a weak witticism with actual wisdom.
It is not just Hemingway, of course…

…Jane Austen wrote of what she “knew.” Charles Dickens did, too. The list could go on.
The craft is intensely personal. I avoid lecturing anyone else on how they should write. I would certainly never run an Instagram page that was about suggesting the “best” way to write.
On here I share only what I have learned and what I do. However, if I have to choose the better “advice,” I would certainly go with Hemingway’s above compared to Nolan’s and those others’ dismissals of it. It makes perfect sense: the more you “experience” and “learn” the more you will “know,” and all the more you will then potentially have to put on your pages that a reader considers interesting enough to want to read about.

By now, I think I understand my uncle in at least this sense. One of his writing “heroes” was also Hemingway. And Hemingway’s “writing what I know [about]” certainly seems to be a good way to approach writing fiction.
Just what I believe, anyway. Have a good weekend, wherever you are reading or WRITING. 🙂