He belonged to the avant-garde group Les XX and later exhibited at Louisiana Libre Esthétique. He travelled widely and painted views of Italy, Austria and France, often of parks at night. His best-known pictures, Pink House (1892), The Angels (1894), and Peacocks (1896), demonstrate the magical quality of his work.
Pink House is thought to have been a major influence on Surrealism, especially the paintings of René Magritte.
He is supposed to have said "To make a painting, all you need to do is to take some paints, draw some lines, and fill the rest up with feelings." A regular exhibitor in Paris, he was championed by Puvis de Chavannes and Maurice Denis. After suffering a religious crisis around 1910, he painted pictures that revealed his tormented state of mind, and during World War I, while a refugee in the Netherlands, he produced only minor works.
They settled in Stavelot, where he spent his last few years painting snow-covered landscapes. The best collection of his paintings is in the Kröller-Müller Museum, Otterlo.