India, March 10 -- I was hiking the Grand Canyon last fall when I found myself clinging to a boulder as two alarming thoughts flashed across my mind. The first was that the trail was so narrow, the drop-off so steep, I could easily slip and plunge several thousand feet. The second was more bizarre: that some nasty imp in my psyche might decide that jumping was a good idea.

I shared those intrusive thoughts with my trail guide. "The French have a phrase for that," he said. "It translates to 'the call of the void.' " Apparently the feeling is not uncommon, he said, though he had never experienced it himself.

Neither had I, at least not before my forties. But then again, I had never been to the Grand Canyon; I had avoided visiting becaus...