He takes mincing steps, his cane weaving, often missing the ground. He has double vision, and is almost blind in one eye. Yet when we’re driving to his doctor’s office, my father can recite the turns I should take. The streets in the few square miles around his house are still imprinted in his head despite advanced Parkinson's Disease.

My seven-year-old son draws a map of Vietnam—his birth country—complete with illustrations of a compass and a ship. He includes very personal labels, based on a trip we took last December. Under "Hannoi," he inserts "us + me"; under "Da nang" he puts "didn't go to"; and with "ho chi men city," he writes "I was born here" and "next time."

I wonder if geography has far more to do with mapping inner worlds than the physical landscapes we walk through. But the land matters, too; it’s not just how we form it in our heads. Watching my child figure out how to map what he sees and feels—and my father’s struggles to hold on to it—I’ve come to realize how much the physical world shapes our spirits as well.

But here's the difficult question, one I hate to face: What happens to your inner map once the brain unravels?

My dad is holding on, barely. But if you think of the brain as a map, you can't avoid the erasure marks of Parkinson's or stroke or, more profoundly, Alzheimer's. Whole internal landscapes wash away; villages disappear.

My knowledge of geography is thoroughly embedded in personal experience: my old wood-block map puzzle of the United States; the many times on family car trips that we crisscrossed the flat agricultural lands of the Central Valley to the Sierra Nevada mountains; the bike trails I've traversed repeatedly in Maine's Acadia National Park.

My son's growing brain means his inner cartographer is constantly drawing and notating, too, and the result is rich and beautiful. Meanwhile, my dad's brain is disintegrating, and I keep wondering, how will he know where he's going? Doesn't he need to know?

Recently, geography has preoccupied me in unexpected ways. This is my second year of taking a Vietnamese language class at the college level, and a few weeks ago, my fellow students and I translated a narrative titled "Dia ly Viet Nam"—“The Geography of Vietnam.”

Even recounted in the dry sentences of a language textbook, the subject has all sorts of resonances. There are sections on "Nui va cao nguyen" (mountains and highlands) and "Song" (rivers). Each section contains densely packed paragraphs of Vietnamese, detailing facts like the length in kilometers of various rivers, including "song Cuu Long" (the Mekong).

In class and at home, I pored over a map of Vietnam, split between learning the facts of the landscape and a different way of expressing those facts.

Language itself is a kind of map, a way of capturing meaning and culture. I started studying Vietnamese because of my son, but the other day it occurred to me I may be trying to rescue my father, too, compensating for his losses. A China specialist, he now reports losing whole swaths of Mandarin. "I'll never get it back," he tells me with a strange, tight smile.

It's a peek into the void I'm not ready for. Of course I can't rescue him. It's myself and the fantasy of endless possibilities I want to salvage.

Meanwhile, unknown to me until I attended a second-grade breakfast at his school, my son had drawn his own map of Vietnam. My husband and I were speechless when we first saw it. I felt teary. My boy had worked so hard without any prodding from us.

This brought home how personal the sense of location is. After one visit to Vietnam a year ago, my son can recreate that geography and label the places that matter to him more vividly than the landscape of where he lives in Massachusetts.

I don't mean he cares more for his birth country, although I think he'd say that he does. He's also rooted in our corner of Cambridge, the streets of our neighborhood that we often walk to school, the shops a few blocks from our house. It's possible to live many places in your mind and heart, and I see his map of Vietnam as so much more than a geographically correct drawing.

I want to follow him, but I can't; he's already moving away from us, into his young fabulous life. I don't want to follow my father, and that scares me. My new map of the world has dragons and demons on the borders, and includes the possibility of falling right off the edge.

The funny thing is, in the past month I've felt an odd burden lift as well. I can see that my internal topography is changing all the time. Even a year ago, I would have fought this realization; I would have clung to all the familiar defenses. I know this world, don't I? It is solid, it makes sense, or I will make it make sense. I have my map.

But just as a map of a country like Vietnam isn't static—a literal landscape that was bombed—we can't take the bedrock of any of us for granted. My father no longer does. I know he has been terrified as his boundaries shrink; his inability to move from a chair or his bed has trapped him in space. But his Parkinson’s has yet to trap his mind.

It’s speeded his thoughts up, he says, in an almost hallucinogenic stream. This could be grounds for more terror, yet my dad is a former professor and writer. What worries him most is his inability to express himself. He’s been angry at “Him” up there, doing this to a man of words: “Job should never have given the bastard the satisfaction.”

He accepts his river of thoughts, though, the network of railroad tracks. They're metaphors for his constant internal monologue: flowing water, rattling trains through the night. During his few good hours, he scribbles tiny lines of poetry, and these poems are now the closest he gets to a map.

At the doctor’s office, when he was trying to explain something to me, he stopped, glazing for a moment. “Damn,” my father finally whispered. “I can see that thought, pulling away from me like a railroad car. I’m waving to it.”

We are creatures in the world. We are running and tracking and planning and trying to catch our breaths. We gaze upwards at the stars. We try to write it down. Eventually, we can't.

Already my memory of Vietnamese is spotty; words I know seem to spark out of existence. But it must be so much worse for my dad. He is making his last map, I think, and the roads are disappearing under his feet.