Art was imitation of nature, but in the 19th century, photography took over that function, and in the 20th, abstract art overturned the whole notion that art was about representation. And although art meant skill early on, conceptual artists elevated ideas over execution. So what is art? Does it have to be beautiful? Expressive? Original? Uplifting? Intellectual? Here’s how 27 artists, critics, and others answered the question, "What is art?"