Pondering Art

What is art?  What makes one painting brilliant, and another one just so so?  In the photograph above, my evil twin and I are pondering the screen prints of Seattle "grunge artist" Art Chantry in a gallery at the Denver Art Museum. In another gallery at the museum, the paintings of Robert Motherwell are being featured.  To me, Motherwell's paintings look like squares of different sizes and colors on canvas, and yet they are considered priceless.  What do art experts see when they look at his paintings?  To me it is a mother of a mystery.