First "modern" artist?

I think it depends how you are meaning "modern". Clement Greenberg says Manet is the beginning of the Avant Garde, because of his clear rejection of previous romanticism, and reflecting life rather than ideals. I've also heard Cezanne or Kandinsky and the Blue Rider group, because of their willingness to throw away form. Picasso seems the strongest in challenging the aesthetic and what the requirements of "art" are. Matisse and the Fauves were also rejecting standards and opening the palatte. I also like Duchamp for truly asking people to question "what is art? why do i think something is art?" I think the previous answerer (tallthingirl) is correct. Most Art Historians say the beginning of the Impressionism movement, that started with Manet's exhibition, is the beginning of the Modern Art movement. Although I'd personally say that Picasso's cubism, Duchamp's urinal and Rothko's color blocks firmly established it as a movement and not just a fad.