Thousands of people took in the bronzed, glacial grandeur of Philip Glass’s “Akhnaten” at the Metropolitan Opera on Tuesday evening. A couple of miles downtown, a far smaller group heard a very different Glass — lively and sly; intimate and changeable — as Maki Namekawa gave the American premiere of his new piano sonata , his first venture into that genre, at the Morgan Library & Museum.

Mr. Glass may not care about his legacy, but he’s hardly unaware of the dynamics that shape a composer’s reputation. “If I’m to be remembered for anything,” he is quoted as saying in the program notes at the Morgan, “it might be for the piano music, because people can play it.”

As his career took off, he was increasingly asked to play solo concerts. So, simply enough, he began to write piano pieces to perform at them. For a while, he kept these mainly for his own use; if people wanted to hear them, he reasoned, they’d have to hire him. And, as he observes, they were eminently playable; while a good pianist, Mr. Glass was never a virtuoso.

These works — like the first book of études, the five parts of “Metamorphosis,” “ Mad Rush ” — have since been published and widely recorded, and they are what Mr. Glass means when he says that his piano music is what’s most likely to be remembered among his massive output. But gradually, and especially over the past decade, his piano compositions moved beyond the level of what he could plausibly perform, their technical demands perhaps culminating — at least for the moment — in the new sonata.