The book that first set me on my way was “Watership Down,” by Richard Adams. I was nine years old when I read it. Basking in its afterglow, I plotted an epic novel about a small group of fugitive otters—one of whom was clairvoyant—who get driven from their home by the ravages of building work, and swim up the River Severn to its source, in Wales, where they establish an egalitarian community called Ottertopia.

As any child author can testify, you can’t begin until you’ve got the map right. So I traced the course of the River Severn from my dad’s road atlas onto Sellotaped-together sheets of A4. Along the looping river, I drew woods, hills, and marshes in the style of the maps in “The Lord of the Rings”: blobs with sticks for trees, bumps for hills, and tufts for marshes. What about toponyms, though? Should I use existing human names, or make up Otterese words for places like Worcester or Upton-upon-Severn? Would otters have words for motorways or factories or bridges? Why would they? Why wouldn’t they? Never mind, I’ll sort that out later. I spent hours on that map, plotting the otters’ progress with a dotted red line and enjoying how nonchalant I’d be at school the day after my unprecedented Booker Prize victory. I’m sure I managed at least half a page of the novel before I got distracted.

Not long after, I borrowed Ursula Le Guin’s “Earthsea” books from the hallowed Great Malvern Library. My literary début was now going to be set in a vast, planet-size fantasy archipelago. (Even that word is shifty and enchanted, sometimes pronounced “archie-pelago” and sometimes “arky-pelago,” even by the same person.) Wizards, epic voyages, underground labyrinths, talking dragons, languages, rings of power, cosmopolitan ports, primitive societies toward the edges . . . . I could just feel how amazing this book was going to be! All I had to do to get started was draw the map. This time, I asked my mum for an A1 sheet of thick cartridge paper, mounted with masking tape onto one of her heavy artist’s drawing boards. I ran my fingertips over the pristine expanses of parchment-thick paper, drooling over its infinity of possible archipelagos. My job was simply to summon one up to the surface with my Berol felt-tip pens.

I spent days on my fantasy islands, some as vast as Australia, others as small as Rockall. Who lives here? Peaceful goatherds or raiders and pirates? Traders or wizards? Halflings, humans, orcs, or elves, or what? As I had half-realized with the otters, I found that you can’t name a place without thinking about the language and world view of the people doing the naming. My map finally finished, I got as far as writing the third or fourth page of Volume 1 before getting bogged down, but, really, it was the map that was the novel, the exercise in world-building, the bit that showed what I didn’t have the stamina or technique to pull off, not yet.

These early maps were also what we now call a displacement activity. As long as I was busy dreaming of topography, I didn’t have to get my hands dirty with the mechanics of plot and character. Nor could I fail to produce my masterpiece if I hadn’t actually begun. While none of the novels I’ve published as a writer contain maps, my notebooks are littered with them. Scenes (or suites of scenes) need spaces to happen in. What those spaces look like, and what is in them, can determine how the action unfolds. They can even give you ideas for what unfolds. This is why mapmaking and stage-sketching can be necessary aspects of writing.

If I’m describing a character’s ascent of a mountain, I need to know what he or she will find on the way up. Most of this information won’t get into the text, at least not directly, but I need to know. So either I use a real mountain that I’ve climbed often enough to keep in my memory, or I go hunting for one in the right area on Google Earth, or I draw my own. It’s very sketchy, but that’s O.K.: you can work with sketchy. What you can’t work with is a blank. My mountain and its trail were changed substantially by the time my novel “The Thousand Autumns of Jacob de Zoet” was published, in 2010, but the scribbled, only-kind-of-a-map—done in a cafe in Skibbereen while my car was being serviced, I remember—was enough to catalyze an exchange in my imagination between the mountainside I already had and the mountainside I needed. Much artistic creation involves this Ping-Pong exchange: not between Nothing and Something, but between Something O.K. and Something Better.

Later in the book, a woman plots her escape from a mountaintop monastery of baby-making, baby-sacrificing, soul-extracting crypto-Shinto monks. (Long story.) Again, this series of scenes would have been impossible to “visualize” if I didn’t know the layout of the buildings. So I dredged through my memory for temples and shrines I’d visited in remote Japan, transplanted an amalgam of these on to a hidden castle I once climbed up to in Okayama Prefecture, and came up with a sketch. It won’t be winning any awards for draftsmanship, and it ended up being scaffolding for something more precise (which I can’t now find), but this first pass at a picture-map of my monastery let me work out where everything was in relation to everything else.

Also in my notebooks are maps of key locations in books whose plot subsequently changed course, leaving these places unvisited and silted up, like an oxbow lake. My novel “Cloud Atlas” has a section set on the Big Island of Hawaii, in a future where technology has regressed back to first-millennium levels of sophistication. Only a small republic of technocrats, who call themselves Prescients, still keeps the flame of twenty-second-century science alive. Originally, I had intended to narrate this part of the novel from a Prescient point of view, and settled upon an island in the Aleutian chain as their home. Using a method I pioneered when at work on my otter masterpiece, I transposed a map of Prescience—the Prescients’ small port city—onto to a real island, somewhere off Alaska.

My map of the port, to my forty-eight-year-old eyes, looks like a settlement built by the Romans in their rowdy province of Britannia. After designing Prescience, however, and seeing it from a God’s-eye view, it struck me that the semi-barbarian Hawaiians were a more interesting bunch to spend a hundred pages with, so I moved the action a couple of thousand miles south and bought myself a map of Hawaii. Writing involves these changes of tack, but my imaginary cartography didn’t feel wasted. The maps were research, and, even when research doesn’t appear in the finished book, it’s still present in passing references, in what we sense the characters know, in the absence of avoidances and in an authority of tone reminiscent of those gifted and respected teachers who never had to raise their voices.

Another map-fiction relationship is one in which the map itself becomes the blueprint for the fiction, so that, once you’ve worked out your map, the section of the story it depicts is pretty much plotted out. I suppose this is another sense in which my boyhood impulse to Start with the Map wasn’t so misguided. In one chapter of my novel “Black Swan Green,” the thirteen-year-old protagonist, Jason, is required to race across a row of back gardens in order to become a member of a shadowy village gang. To avoid repetition, the gardens needed to be different in character—some manicured, others neglected, one full of gnomes. I was also aiming at a “Rear Window” effect, with Jason eavesdropping on or glimpsing the lives of the residents. On nearby pages, the numbered houses have their own sections, listing the names, ages, social class, and personalities of their residents. But it’s the illustrative map that serves as the chapter’s organizing principle, and its structure.

As a middle-class kid growing up in a village in Worcestershire, I was taken on trips to nearby Hereford Cathedral, where a famous medieval map of the world is housed—the Mappa Mundi. I still like to go back when I can, not only to mingle with my past and future selves but also to study the map with my freshly older mind. It’s a magnificent farrago of best guesses, classical locations, Biblical myth, and not a lot of sea. As a navigational tool, the Mappa Mundi would clearly be a dead loss. As a map of the medieval mind, however, it has few peers. I wonder if that isn’t the point about maps of fictitious places, too? They are maps of minds. You lose yourself in them and find if not factual truth, then other kinds. You meditate upon them. You meet yourself in them. You co-opt them and set stories of your own there, or fragments of stories, at least. Fictitious maps give form to a thing—the imagination—that has no form. They are mysteries and answers to those mysteries.