Brancusi discovered essences in a fragment

Constantin Brancusi did not consider his sculpture abstract. "[T]hat which [people] call abstract is the most realistic," he said, "because what is real is not the exterior form but the idea, the essence of things."

His marble carving "Sleeping Muse I" (1909-10) is featured in an exhibition of his work at New York's Guggenheim Museum, along with a recently discovered later variation on the idea called "Sleeping Muse III/IV." "Sleeping Muse I" was an early manifestation of a form that preoccupied the Romanian-born sculptor off and on throughout his career.

It is difficult to describe the tranquil self-containment - the "essence" - of this immaculately crafted ovoid head, suffused with a minimal quietness, without resorting to such words as "reductive" or "simplified." Yet such words misrepresent the process and intent of its direct carving. "Simplicity," Brancusi said, "is not an end in itself."

Rebelling against Rodin, Brancusi did not supply skilled artisans with clay models to be translated into marble. He carved the marble himself. Carving might seem to be a subtractive process, but it involves intense discovery nevertheless. What is taken away matters far less than what is found and left. Brancusi's forms are egglike, and express such essences as newborn innocence, beginning, musing, and peaceful rest.

They seem strikingly complete rather than reduced. In the tradition of classical sculpture as it has often come down to us, "the fragment" has long been an acceptable sculptural idiom. Rodin collected such fragments. The portrait bust, though no more than head and shoulders, was not even seen as a fragment. Brancusi developed the fragment as a complete sculptural essence, a concentration on only what matters.

Art scholar Alexandra Parigoris explores this in the exhibition catalog, using the phrase "partial figure." She aptly quotes German poet Rainer Maria Rilke musing on archaic fragments: "No-one knows their intention ... no history casts a shadow over their naked clarity - they are. That is all."

• "Constantin Brancusi: The Essence of Things" is at the Guggenheim Museum in New York until Sept. 19.