"I had another engagement," Nathaniel told me without explanation.
There's both madness and dignity in Nathaniel's appearance. He's got matted hair and grimy layers of clothing, including a sweat shirt tied around his waist to conceal a wooden club he carries in anticipation of someone attacking him on skid row. But he is meticulously neat to the extent possible, and he moves with polite bearing that suggests formality and respect.
He tuned his violin for several minutes, reacted somewhat nervously to a warm round of applause upon being introduced, and began to play.
I've heard better, more passionate playing from Nathaniel. He seemed tentative and at times turned away from the room, as if he was too uncomfortable with his own performance to face his audience. Performing here in this setting, with people focused on him rather than walking or driving past, must have been both thrilling and terrifying.
He began with something Beethoven-like, as he had planned, but seemed to lose the thread. Nathaniel is inspired and haunted by music and his history with it -- a history scrambled, interrupted, incomplete. More than once, he apologized to the audience.
I could only wonder at what was going through the mind of this man, who 30-plus years ago studied bass in New York City as a young phenom from Cleveland. I've talked to his friends of that time -- people who saw great promise unravel as Nathaniel's mind began playing tricks on him and the music he revered seemed ever more elusive.
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After switching to cello at Little Pedro's, Nathaniel was still a bit disorganized but much more comfortable, and there were moments when both his music and his expression -- head thrown back in pained delight -- were things of beauty.
Dr. Shaner later called it tragic that Nathaniel, or anyone who is chronically mentally ill, is left to live on dangerous streets. The civil rights concerns that led policymakers to empty the state's mental hospitals over the last three decades were well-intentioned, but the move helped create today's problem. Society has failed to deal with the stigma and destructive reality of mental illness.
"I'd like to seduce him into treatment, and he needs seduction," said Dr. Prchal. She said it wouldn't be easy -- Nathaniel needs medication as well as counseling on how to pull his life together. But there is greater success with those who -- like Nathaniel -- have no addiction problems, Prchal said.
"People with his IQ, tremendous ambition and achievement don't like to be ordered around. But this man has to be treated. I will try to meet with him and slowly start some dialogue about the possibility of treatment, and slowly also see if he would want to have his own room, maybe if only to store his instruments at first."
When I spoke to Nathaniel after his performance, he bowed his head and apologized.
"I was really, really bad in there," he said.
I encouraged him and handed him the tips that had been left. He said he wasn't worthy, but I folded the money into his hand and asked if he'd return the following Tuesday, an offer Pedro's owner Alexis Rivera is happy to extend.
"If they're interested in having me back," Nathaniel said, "I'll be here."
Reach the columnist at email@example.com and read previous columns at www.latimes.com/lopez.