This adage can mean many things, for “knowing” is left undefined. I think when Mark Twain (the purported source) said it, he meant that writing about one’s hometown might take on a richness hard to achieve in a setting one’s never experienced. Yet writing can be wooden, whether or not we know a subject inside and out. I can’t imagine Twain being concerned about authoritative expertise, or that he’d have minded anything outlandish, straight out of the imagination, as long as one did it well.
What helps us “do it well”? Jung wrote that anything that comes up—imaginatively, in sleep or waking—is from the deep mind. Characters that arise in our novels might seem utterly unfamiliar to our waking mind, yet Jung would say it’s one of our inner selves, from somewhere in the psyche, making itself known. When I carve out a scene that feels real to the reader, I’ve taken a deep dive into that place—a soul place?—from which creativity comes. I’ve put myself fully into the characters’ lives. I’m “knowing” them. That’s how they get flesh, with consistent personality and dialogue.
There are many ironies around this phrase, “what you know.” Memoir writing is a great example. If you’ve ever tried to read a memoir by someone who’s not crafted it with deep reflection and honesty, you might have experienced the unpleasant sensation of thoughts that are so undigested, they’re not even raw, but something else. Did the person writing about their own life really “write what they know”? We can have perceptions so false about the past that a good science fiction writer might be coming closer to putting down something real. Yet the memoirist who keeps at it finds, even sometimes by fictionalizing what isn’t quite remembered, that the real thing is coming into focus, along with a lot of homeostasis.
I was asked once to teach a high school astronomy class with only a few days to start preparing and little background. I propose that the vital strengths called for were access to intellectual curiosity and the internet. I may not have been able to expand upon any question asked, on the spot, but are we being asked to be sages on the stage, or should we be guiding exploration, keeping alive the passion for finding out? One of the most scintillating aspects of writing my fantasy books with travel to medieval times was the research.
There are some great aspects to setting a book where one is living or where one has lived before. It can give the writing a particular flavor, due to the details. With added benefit, one can imagine oneself halfway in the story while going about one’s mundane life, stepping off a curb and writing lines in one’s head, living with one foot in fantasy. But, with care, I can put you in medieval Wales, smelling the scents, hearing the sounds, feeling the rain. I say, “Write what you want to know.” And go deep with it, knowing on many levels, even what you fantasize.