I was just leaving BART’s Montgomery Station during a morning rush hour when, seemingly out of nowhere, a shock wave struck me in the forehead with the force and precision of an attack drone. I staggered to my knees, releasing my grip on the trekking pole I was using for support. Instinctively I covered my head with my jacket while blows rained down on me. “Help!” I shouted. “Stop!” As the beating continued, all I heard was the sound of my pole splintering, the ring of turnstiles, and the click of heels as pedestrians passed my hunched body.

It may have been just half a minute, but it was long enough for me to ask myself, in desperation and despair, if anyone cared enough to stop the beating they were witnessing. I heard no call in response to my cries, no one shouting, “Stop!”

And then it was over. With infinite care, hands on both shoulders lifted my face from the floor and brought me gently to rest on my back. Three women gazed down at me as I lay dumbstruck, my mind unable to grasp what had just happened. Then, tears welling in my eyes, I realized these were indeed “the sisters of mercy.” I couldn’t stop expressing my gratitude for their coming to my aid. “But of course,” they said, as if anyone would have done the same. But of course, not everyone did.

I was taken by ambulance to San Francisco General Hospital’s trauma center. Run through a CAT scan to check for damage to my brain and skull, I lay strapped to the platform, fearing the worst.

“So what did you find?” I finally asked the silent, empty examination room. From behind a window, the technician called out, “You have a brain!”

I do indeed, and it’s more or less functioning again. But the shock will not so soon be dispelled.

Amid a maelstrom of conflicting emotions, I struggle to reconcile three very different expressions of human behavior — the random violence of an individual (now in custody and charged with three felonies) with no clear motive for attacking a stranger, using my trekking pole as a deadly weapon; three compassionate women who came to my aid; and a procession of caring, competent professionals who affirmed that some parts of our broken health system still work well.

But there’s a fourth expression of human behavior I find more troubling. What about all those passengers who must have seen the beating as they passed me and my assailant on their way to work?

Three years ago my daughter and I witnessed two men beating a woman on a deserted street corner. I suddenly recalled not having intervened 40 years earlier when I had seen a similar beating and how haunted I had felt in succeeding decades by my failure to act. Now, without hesitation I called out in a strong, clear voice, “Stop!” We chased the men to their getaway car. Two years later, I met the woman and realized how much richer my life had become for having stepped forward in that moment.

What kind of society are we becoming when, for whatever reasons, we choose to turn away as we witness violence being done to others? And how will we feel when we ourselves are attacked and onlookers choose to look the other way?

Mark Sommer, 71, is an author living in Emeryville and Trinidad (Humboldt County).