Anyone who has studied fine art in an academic environment is familiar with that question. It is the question that has been asked repeatedly, in different ways, throughout the 20th and 21st centuries, and is still asked in fine art classrooms worldwide. The bulk of modern and postmodern art has been devoted to asking that question, in one way or another, usually by removing some convention that had been assumed to be one of art's defining characteristics to that point. By removing different conventions - pictorialism, originality, corporeality et cetera - modern and postmodern artists have produced minimalism, pop art, conceptual art et cetera.
The question What is art? begs a definition where none is needed; an education in art theory is not necessary for a man to know the difference between art and non-art - merely the sense that God gave a caveman. And like the question What is truth?, no man interested in defending the revelation of the Christian religion, or the worth of everything informed by it through the ages, would ever need to ask it.
To anyone who still thinks, after a century of puerile experiment, that this question is worth asking, I ask the following: Would you want to eat in a restaurant where the chefs were seriously interested in the question What is food??