When a polished bronze version of Constantin Brâncuși's Bird in Space arrived in New York from Paris in 1926, American customs officers refused to admit it as art. Under the rules of the day, sculpture meant a carving or casting that reproduced a natural object, and the sleek, elongated form showed no recognisable bird. The shipment was instead taxed at 40 percent of its value under a category covering kitchen utensils and hospital supplies, a decision that set up one of modern art's most consequential legal battles.

The work had been bought by photographer Edward Steichen and shipped for an exhibition at the Brummer Gallery, organised by Marcel Duchamp, Brâncuși's friend and champion. Rather than pay, the artist sued the United States government. The case, heard in 1928, began as a narrow question about whether the object imitated a bird closely enough to escape duty, but it widened into a debate about who gets to decide what counts as sculpture at all.

Leading figures of the art world testified on Brâncuși's behalf, including Steichen, British sculptor Jacob Epstein and William Henry Fox, director of the Brooklyn Museum. Critic Frank Crowninshield argued that the work conveyed flight, grace and aspiration in the way a bird does, even without resembling one. The court ultimately ruled for Brâncuși, establishing that abstract work could qualify as fine art and enter the country duty-free, a precedent that reshaped how nonrepresentational art was treated under American law.

The sculpture at the centre of the dispute now belongs to the Seattle Art Museum. Brâncuși went on to make many versions of the bird in marble and bronze, each time stripping the form further toward a single arc, an approach that treated essence rather than likeness as the honest subject of sculpture. The courtroom victory gave that idea legal standing, and it remains a touchstone whenever the definition of art itself is contested.