Whistler never quite overcame. Legacy Whistler was inspired by and incorporated many sources in his art, including the work of Rembrandt, Velázquez, and ancient Greek sculpture to develop his own highly influential and individual style. He was adept in many media, with over 500 paintings, as well as etchings, pastels, watercolors, drawings, and lithographs. Whistler was a leader in the Aesthetic Movement, promoting, writing, and lecturing on the "art for art's sake" philosophy. With his pupils, he advocated simple design, economy of means, the avoidance of over-labored technique, and the tonal harmony of the final result. Whistler has been the subject of many major museum exhibitions, studies, and publications. Like the Impressionists, he employed nature as an artistic resource. Whistler insisted that it was the artist's obligation to interpret what he saw, not be a slave to reality, and to "bring forth from chaos glorious harmony". During his life, he influenced two generations of artists, in Europe and in the United States. Whistler had significant contact and exchanged ideas and ideals with Realist, Impressionist, and Symbolist painters. The artist Walter Sickert was his pupil, and the writer Oscar Wilde was his friend. His Tonalism had a profound effect on many American artists, including John Singer Sargent, William Merritt Chase, Henry Salem Hubbell and Willis Seaver Adams (whom he befriended in Venice). Another significant influence was upon Arthur Frank Mathews, whom Whistler met in Paris in the late 1890s. Mathews took Whistler's Tonalism to San Francisco, spawning a broad use of that technique among turn-of-the-century California artists. As American critic Charles Caffin wrote in 1907: He did better than attract a few followers and imitators; he influenced the whole world of art. Consciously or unconsciously, his presence is felt in countless studios; his genius permeates modern artistic thought. During a fourteen-month stay in Venice in 1879 and 1880, Whistler created a series of etchings and pastels that not only reinvigorated his finances (this was after he'd declared bankruptcy following the Ruskin trial), but also re-energized the way in which artists and photographers interpreted the city—focusing on the back alleys, side canals, entrance ways, and architectural patterns—and capturing the city's unique atmospherics. In 1940 Whistler was commemorated on a United States postage stamp when the U.S. Post Office issued a set of 35 stamps commemorating America's famous authors, poets, educators, scientists, composers, artists, and inventors: the Famous Americans Series. The Gilbert and