At the park, I let her out, and while I get my poles out of the back of the car, she sniffs all around – I call it “checking her peemail” – to see who’s preceded us. Then a questioning look to determine which way we’re going today, and we’re off. Watching her enthusiasm, I’m reminded that this is one of those times that are as close to perfect as we’re ever going to get in this lifetime.

Over nine decades I’ve had a bunch of different breeds – terriers, hound, German shepherd, Springer, Sheltie – and I’ve finally settled on a favorite. It’s terriers. If they ever really sleep, I’m not aware of it. Instead, they doze, with every sense alert for whatever’s coming next. Bred to catch and kill vermin, they’re almost impossible to dissuade from the chase. And when the lethal moment has passed and their human takes to his easy chair, they’re transformed into snuggly little lap-warmers.

Once I get the car closed up at the park and my hiking poles settled on my wrists, I turn toward the trail of the day, and she’s off ahead of me.

She came to me when she was about six months old, and she’d been raised right. Still, she was a puppy and full of reckless impulses. So I kept her on a leash the first two times we visited the park. We both hated it, and her impulses often pulled me off balance. So on our third visit, I stopped and said, “Look. You’ve got a license, a rabies tag, an identification tag with my phone number, and a chip. You’re probably almost impossible to lose. So today we’re going to have a little test. I’m going to take the leash off and see how you behave.” I unclipped her, straightened up, and she was off like a shot, straight up the trail toward Montreal. My heart sank; she was gone. But then I saw her swinging to the right, and in a few seconds she came zooming past my legs and began a second orbit. I clipped the leash into a loop and draped it over my shoulder.

A few weeks after that, she and I joined the Public Broadcasting television crew and the author Tom Ryan and his dog, Samwise, on a hike of Pine Mountain in New Hampshire. She was still all over the place. But at one point Tom said, “You know what? That puppy checks you out about every five seconds.” He was right; she does. Still does, even though she’s now in adulthood. It’s a mutual thing: she seems to need the security of my presence, but also seems to be checking to make sure I’m all right, especially after I’ve fallen, as I sometimes have.

So off we go down the trail. I marvel at the happy bounce in her step. Pure terrier: tail in the air and wagging, comically bowlegged, eyes and ears straight ahead. Self-confident, self-important little twit. But then she stops suddenly. Somebody coming, and with a big dog, too. She lets me catch up to her and circles around my feet, just in case. Then she checks to see if the stranger is bearing treats.

This is not a bit of an idealized, simple boy-and-his-dog picture. She’s in her prime; I’m old. My balance is augmented by a pair of high-tech, adjustable hiking poles, my legs by prostheses and a supporting metal bar, my eyes by lenses crafted in Zurich (which I fancy help me read German better than I used to), and my ears by expensive electronic aids adjustable for different settings. If I hear a bird song that previously I couldn’t, I simply whip out my cell phone, hold it up toward the song, and get an instant identification. More techie than traditional.

If I fall or jerk suddenly, my watch says, “You appear to have fallen. Do you need help?” If I don’t answer, I get help whether I want it or not. Also a little character beside me asking the same question. Sometimes even under me, which is lovely far moral support, but kind of hindering my recovery and resumption of verticality.

She, however, is pristine, unencumbered by any medical intervention ­– except, of course, for a childhood spaying and the monthly pill she takes to ward off fleas and ticks. I wonder if she knows, as she trots and sniffs and wags her way along the trail, that this is as good as it gets, that it can’t get any better. I hope she knows.