What is original?

After watching Part 1 of Everything is a Remix, my class and I had an interesting discussion about influence and appropriation in art. The examples it uses start with music: Led Zeppelin and hip hop, both of which the class had some knowledge of. When you ask the same questions about the visual arts, it becomes hard to define originality.

An art teacher who was visiting the class brought up the work of Marcel Duchamp and the Mona Lisa postcards he defaced. The more you know about it, the more original the concept becomes.