In the corner of the gallery, there's a large grey lump of fuzz, a compacted dust bunny the size of a collie. A small sign lists the medium as "Vacuum Cleaner Lint." Price: $7000.

It's hard to imagine someone willing to pay for a huge ball of lint to put in their living room.  Perhaps you feel a bit repelled,  and laugh, "That's not art!"

Modern art is characterized by its challenges to all kind of accepted norms, artistic and otherwise. Pushing boundaries has become a mode of distinction in the artworld. The Impressionists (1874) were first to arouse general indignation by painting everyday subjects in a loose and spontaneous style rather than conventional historical art according to academic principles. They were considered lazy and boorish for flouting society's accepted rules of art, substituting more personal visions of color and light.

The development of photography freed art from the task of realism, and artists like Monet opened the door for expressions that went far beyond Impressionism, expanding boundaries and new conceptions of art itself.  One path led through Picasso's cubism (1908) to the non-objective abstraction of Kandinski (1923) and Pollack (1950), in which the painting is not meant to represent anything at all in the "real" world, but to evoke moods and ideas in a harmonic dimension of its own, like music. Art for artís sake.

Freudís revelations of the power of the unconscious self and Einstein's revolutionary ideas about the nature of physical reality spread into all facets of art, inspiring surrealists like Dali and Magritte (1935) to paint improbable worlds of pure imagination. At each new juncture, many viewers rebelled, declaring, "It's not art!"

Marcel Duchamp's sculpture, "Bicycle Wheel" (1913), combined two common objects, a wooden stool and a bike wheel, displayed as an original sculpture.  His "ready-made" art was derided and he was pronounced a charlatan. Yet somehow, it could not be dismissed, raising uncomfortable questions about the supposed separation between the realms of art and everyday life.

Eventually, much of this new art was incorporated in our cultural memory, no longer controversial, but accepted, even celebrated. This, in turn, led contemporary artists to go further in their "boundary-pushing." Sometimes, the pushing became an end in itself, crossing every line, breaking every taboo, dragging art through the mud just to see what survived. Art was exhibited that was purposefully ugly, badly made, incomprehensible, phony and stupid, yet collectors paid big money for it.  Conceptual artists came along, claiming that even the creation of an artwork is irrelevant, just the idea of the artwork is sufficient. Painting was declared dead.

All of which raises the age-old philosophical question, what is art? There were times when the answer seemed obvious. A dictionary defines art as "skilled creative work to produce something beautiful, significant and original." It seems to be a universal human urge, prevalent in all societies going back to cave paintings and beyond.

Art plays a social function as well as a personal one, embodying our mutual agreements about what's real and valued, and sometimes promoting a certain social order. Art is akin to religion and science in its attempt to make sense of the apparent chaos of life and gain some control over it.

Perhaps art is best described as an ever-evolving process of communication. In simplest terms, a work of art is conceived and executed by an artist, then presented to an audience who will interact with it, grasp its meaning, or not, and perhaps feel enriched, fascinated, awakened, persuaded or entertained by it. The echoes of this art appreciation reflect back into the artist's world, informing the next creative cycle.

The "Mona Lisa" by Leonardo da Vinci (1505) is probably the best-known painting in the world. Whether they appreciate it or not, no one questions whether it is art. So if the sign on the door says "Art Gallery," you'll adopt a special frame of mind as you open it. Whatever your reaction to the giant dust bunny in the corner, remember-- it's been said before.

Dan Cooper is a landscape painter and creator of "E-Painting of the Week," a free email service. His work may be seen online at www.dancooperart.com He lives in Northern California.