Under a full moon in an empty desert, a woman sleeps on the sand beside her mandolin and water jug. A lion has come to sniff her. It doesn't attack. Everything is completely still.

Henri Rousseau was self-taught. He worked for years as a toll collector in Paris — friends nicknamed him "Le Douanier", the customs officer — and painted in his spare time. Critics mocked his flat, childlike style. He was entirely serious.

He offered The Sleeping Gypsy to the mayor of his home town, Laval. The town declined. It vanished for years and was rediscovered in the 1920s, when the Surrealists, who loved its dreamlike calm, championed it. Today it's one of MoMA's treasures — proof that sometimes the outsider gets there first.