Something Old, Something New

In class, we discussed Aristotle, and had to question the application of his theories to modern day art. A large point that questioned was a question that can be applied to many older theories. Back then,art consisted mainly of literature, music, performance art, sculpture and paintings; usually of historical or mythological scenes. Today we have so many extensions of that. We have graphic design, fashion design, web design, and dozens of other new fields of art. Many of these new categories don't fit into the old definitions of art. My main question was about art's functionality. Back then, there was no such thing as commercail advertising. The most you would get was a sign in a marketplace with prices for products, and even that was a stretch. Art was meant to be purely aestetically pleasing, to enjoy and be enjoyed, which is why it was so heavily criticized by people like Plato. Now we can look at a broadway poster and appreciate it for it's aestetic value, but it's main purpose is to sell us something. Can art, by the old rules, have a function other than being aesthetically pleasing, or must we tweak the old rules to fit new fields such as graphic design for advertising?