Hanging in New York City’s Frick Museum is a wonderful painting called The Polish Rider. It bears the signature of Rem­brandt, but some art experts say it’s a fraud. If they’re proved right, the painting will be virtually worthless in the art market. Yet for many art lovers, it will still be a wonderful picture.

Would the reaction be the same if the artwork were one of Andy Warhol’s famous reproductions of a Brillo box?

That question sends Gurstein, author of The Repeal of Reticence (1996), on an inquiry into the history of aesthetics and the debate over the differences between art and imitation. She begins with philosopher Immanuel Kant’s distinction in Critique of Judgment (1790) between aesthetic judgment and taste. Aesthetic judgment involves the appreciation of objects that are inherently beautiful, while taste involves the appreciation of objects in relation to ourselves. A cookie, for example, has no inherent beauty, but we can appreciate the delightful encounter of ingredients and taste buds.

The explication of taste led art historians and others to the question of forgeries. Hans Tietze, for example, argued in 1936 that a painting is more than its physical attributes: It is also “the expression of a personality, of an epoch, of a nation, and of a race.” A forgery might appear beautiful to the untrained eye, but the connoisseur will detect its defects.

In Languages of Art (1968), Nelson Goodman took a more radical tack. He argued that there is no such thing as the disinterested appreciation of beauty. What happens if we are confronted with both a Rembrandt and a perfect copy of it, Goodman asked? Just knowing that one is a forgery shapes our perception of it. What we know always shapes what we see.

That argument was quickly “pushed to its further extreme,” says Gurstein. While Good­man held that prior knowledge shapes how we perceive a work of art, “in today’s art world, prior knowledge is everything; it determines whether an object qualifies as art or not.” How do we know that Warhol’s Brillo boxes are art? Because he (and the art cognoscenti) said so. Today, museums are full of such works—Marcel Duchamp’s famous Fountain (1917) is a urinal, Damien Hirst’s more recently controversial This Little Piggy Went to the Market, This Little Piggy Stayed Home (1996) is a bisected pig floating in formaldehyde.

There’s a paradox in all of this, Gurstein notes: “In the quarrel over forgeries, those who love beauty for its own sake are sophisticated aesthetes and those who care more for the work’s pedigree than its aesthetic qualities are philistines.” But in today’s culture wars over sensational contemporary art such as Hirst’s, “those who expect to find beauty are now dismissed as philistine, while those who appreciate objects without aesthetic attributes and for reasons that have nothing to do with beauty turn out to be sophisticated, art-world insiders.”

The decline of beauty as an ideal has many other causes besides the changes in aesthetic theory, Gurstein allows. Yet the “longing for aesthetic experience” has not declined, as the crowds jamming exhibits of Vermeer, Matisse, and other old and new masters show. The question, says Gurstein, is whether today’s artists will arouse the same ardor a hundred years from now or whether their objects will, “with the passage of time, drift back into the realm of the commonplace from which they are momentarily lifted.”