Stuart Pivar, a 94-year-old chemical engineer who lives in New York, has been collecting art and antiques since he was a child. He estimates he has picked up about 300 pieces over the years, including a portrait of himself by his friend Andy Warhol and paintings by Jean-Michel Basquiat, Jackson Pollock and Edgar Degas.

Pivar is also convinced that he owns an unsung masterpiece by Vincent van Gogh, a large landscape titled “Auvers, 1890” that is signed “Vincent” on the back.

But a much more important voice does not agree: the Van Gogh Museum in Amsterdam, whose judgments carry incredible weight because it has the largest collection of works by the 19th-century Post-Impressionist. Its curators and researchers study every aspect of the Dutch artist’s life and work.

When the museum sent Pivar a 15-page letter in 2021 explaining why it did not deem the painting he had spent a few thousand dollars on at auction a van Gogh, he responded by suing for $300 million in U.S. District Court. The museum’s failure to recognize the painting was “negligence,” he argued in court papers, and had reduced its value to almost nothing.

The cost of fighting lawsuits and responding to an influx of inquiries during the coronavirus pandemic — when hundreds of people believed they had found an original van Gogh at an auction or in a dusty attic or under a grandfather’s bed — has made the museum increasingly resistant to authentication requests. Without its imprimatur, however, large auction houses like Christie’s or Sotheby’s are unlikely to sell something attributed to van Gogh.