Of course I said nothing about this to anyone. Since I recognized no deities, and even the notion of an “altered state of consciousness” was unavailable at the time, I was left with only one explanation: I had had a mental breakdown, ultimately explainable as a matter of chemical imbalances, overloaded circuits or identifiable psychological forces. There had been some sort of brief equipment failure, that was all, and I determined to pull myself together and put it behind me, going on to finish my formal education as a cellular immunologist and become a responsible, productive citizen.

It took an inexcusably long time for me to figure out that what had happened to me was part of a widespread category of human experience. Some surveys find that nearly half of Americans report having had a mystical experience. Historically, the range of people reporting such experiences is wide — including saints, shamans and Old Testament prophets as well as acknowledged nonbelievers like Virginia Woolf and the contemporary atheist writer Sam Harris. It is of course impossible to ascertain how much these experiences have in common. We may be comparing apples and asteroids.

On the religious end of the spectrum, people have tended to describe their experiences as encounters with familiar deities or spirits, while nonbelievers — like some quoted by William James — are likely to speak of a more generic “living Presence.” Others write of something more akin to my own experience, which was wordless and profoundly unsettling. When the early 20th-century Protestant theologian Rudolf Otto surveyed the works of (mostly Christian) mystics for clues as to the nature of the “Other” they had encountered, he concluded that it was “beyond all question something quite other than the ‘good.’ ” It was more like a “consuming fire,” he wrote, and “must be gravely disturbing to those persons who will recognize nothing in the divine nature but goodness, gentleness, love and a sort of confidential intimacy.”

Of course all such experiences can be seen as symptoms of one sort or another, and that is the way psychiatry has traditionally disposed of the mystically adept: The shaman was simply the local schizophrenic, Saint Teresa of Avila a clear hysteric. The Delphic oracles may have been inhaling intoxicants; all of the great Christian mystics showed clear signs of temporal lobe epilepsy. A recent paper from Harvard Medical School proposes that the revelations experienced by Abraham, Moses, Jesus and Paul can all be attributed to “primary or mood-disorder-associated psychotic disorders.” I suspect we would have more reports of uncanny experiences from ordinary, rational people if it were not for the fear of being judged insane or at least unstable.

An alternative to the insanity explanation would be that such experiences do represent some sort of encounter. It was my scientific training, oddly enough, that eventually nudged me to consider this possibility. Sometime in middle age, when I had become a writer and amateur historian, I decided that the insanity explanation may have been a cop-out, that I could have seen something that morning in Lone Pine.