Marcel Duchamp challenges our notion of art as something idealistic. Here is a quote about him from the Wikipedia:
Marcel Duchamp (1887 – 1968) was a French artist whose work is most often associated with the Dadaist and Surrealist movements. Considered by some to be one of the most important artists of the 20th century, Duchamp’s output influenced the development of post-World War I Western art. He advised modern art collectors, such as Peggy Guggenheim and other prominent figures, thereby helping to shape the tastes of Western art during this period.
Duchamp challenged conventional thought about artistic processes and art marketing, not so much by writing, but through subversive actions such as dubbing a urinal art and naming it Fountain. He produced relatively few artworks, while moving quickly through the avant-garde circles of his time.
The creative act is not performed by the artist alone; the spectator brings the work in contact with the external world by deciphering and interpreting its inner qualifications and thus adds his contribution to the creative act. – Marcel Duchamp