Until Duchamp, criticism was aesthetically based. The critic talked about a painting's subject, the way the artist handled color, drawing, composition and the like. With Readymades, the object's appearance and beauty were no longer the issue -- indeed, they were irrelevant. What mattered was the idea behind the work -- the point the artist was trying to make. So art criticism moved from the realm of visual experience to that of philosophy. The writer no longer had to base his critical observations on a close scrutiny of the work of art. He could simply riff.

--from article on the crisis of writing on art from Wall Street Journal