What is art?

I recently had a thought about what makes something art. Now I I know that there is a type of art that says anything that is outside of nature; that is man-made and serves a function is art. So yes paintings would fall under this category; but then again so would furniture. Perhaps a better way to view art is something man made that not only has an affect on you, but also stirs the imagination. A chair can have an affect on you ( when you sit in it it makes you feel good), but most of the time when your sitting in a chair you don't find your self thinking as a result of sitting in the chair. Yet you may find yourself thinking from listening to beautiful music or seeing an instillation piece like Megan's. So maybe it would be better if we considered the definition of art as being; something man made that not only has an affect on you, but also stirs the imagination.