What is ART?

I took a visit to The Art Institute of Chicago this morning. After enjoying my favourite Seurat and all the Impressionist paintings, I turned into Modern Art. I came across with this Piet Mondrian .
This was what the lady next to me said,
'I don't understand why people called this art, It is nothing. You can do it, I can do it. Why is it here? Why do we call it art?'

what she just said, smashed my brain completely! I pretended that I heard nothing, and kept enjoying myself with this Mondrian's ART. I remember the first time when I learned about Mondrian was that introduction to fine art history in my year 1 undergraduate. I can't remember much on what Dr. Muir had told us. But I do remember the first time when I saw the original painting of Piet Mondrian in Tate Modern. I was amazed.

I am disappointed at that Lady's comment. For me, art is art no matter it is TECHNICALLY amazing like those Renaissance art or it is as simple as just lines and squares. what is important is its historical contribution. I don't understand how could people use a 21st century perspective to understand something back to the 20th or even 19th century. This is so not fair to those artists. If it's not art, then we could say John Cage 4'33'' is not music either. 'you can do it, i can do it.'

well, this is just me. you don't have to agree.

In Lozenge Composition, Mondrian reoriented a square support to produce a dynamic relationship between the composition and the diagonals of the edges. The fifth of sixteen diamond-shaped works, this deceptively simple painting reveals an exacting attention to subtle relations between lines, shapes, and colors. Mondrian hoped that his art would point the way to a utopian future in which the principles of universal harmony would be embodied in all facets of life and art. This was the goal of the De Stijl movement, first formulated in Holland around 1916-17 by Mondrian and a small group of like-minded artists and architects. from The Art Institute of Chicago