MIKE PULLS HIMSELF UP onto the top of the pillar and steps over to the belay.

"Mike, what if the pillar suddenly collapsed?" I ask.

"Won't," he says. "It's been here forever."

"But what if it did?"

"Buck, you'd catch me."

Mike is the most optimistic person I know. He is sanguine, imperturbable.

"What if you lost your ice ax?" Mike asked one day in 1979. "Could you still climb the couloir?"

Mike and I always assumed we'd come home. That was the principle of what-if. But after we had kids, we began to wonder what if... we didn't?

We were training for McKinley by climbing the snowy chutes on either side of the Diamond. So we tried it without our axes, scraping little holds in the snow with our woolen mittens. What if you lost your ice ax and crampons? I asked. Could you still get up the couloir? We kicked tiny steps with our heavy leather boots and gouged mitten holes and climbed it. But what if you were descending? We practiced glissading with nothing to stop the death slide but a sharp rock in our hands.

Thus began our private game of what-if. What-if was meant to make us more resourceful, more capable of surviving desperate situations. And it did—for a time.

We swap leads and Mike moves out onto a sheet of gray rock split by a pencil-thin crack.

"But, Mike, what if I couldn't and you were killed? Was it worth it?" I'm baiting him and he knows it.

"Yup. Right up until the moment I die ... then it's completely not worth it."

"That's not an answer and you know it."

The crack has closed off and Mike is holding on by his fingertips. He uses this predicament not to respond.

Our game of what-if was good fun for more than a decade, but it changed after we had kids. Before, we always assumed we'd come home. That was the principle of what-if. What if this or that happened—how would you get yourself out of the fix? But after kids, we both began to wonder what if ... we didn't? What if we were killed? By a grizzly, by a river, by a collapsing pillar of stone. It's a natural thing to ask once you start thinking about someone besides yourself. We may be leaving on expeditions to Canada soon, but we are dads now, not Huck and Tom. Justin and Addi are three years old; Teal and Carlie and Kevin will turn one this summer. Our game of what-if has evolved into the fundamental conundrum of our lives: Is it morally possible to be a serious adventurer and a father?

For my part, I hide behind the hackneyed and sophistic excuse that it's who I am. That if I were to quit adventuring, I wouldn't be Mark Jenkins—which I know is bullshit. People change all the time and don't lose their identity. They often become someone better. I just don't have the willpower.

Mike does. He's been trying to reform himself for years. He's weaning himself off adventure like a heavy drinker weans himself off Scotch. Slowly, with frequent relapses. He has promised Diana—and himself—he will do only one big trip every two years, but I think this expedition to Baffin will be his last. Inside, I know Mike believes serious adventure, expeditioning, is incompatible with being a father—you are imperiling not simply your own life but the lives of your children, which is immoral. So he will have to give it up.

To calm his existential qualms, Mike has taken to putting more and more effort into planning the logistics of an expedition. This upcoming trip to Canada is a case in point. He's spent weeks testing gear, studying maps, developing contingencies. He told me he thinks he can bring the risk down to something acceptable. I told him he's in denial. Risk is integral to adventure. A freak accident, an unanticipatable rockslide, an avalanche. No risk, no adventure. He knows this, but he's torn between being the man he is and the man he believes he should be.