Why have the bitterest recent controversies over censorship and the arts been provoked by photographers?
Robert Mapplethorpe and Andres Serrano, for example, probably never would have become national emblems for depravity had their medium been paint and canvas rather than camera and lens.
Similarly, a recent New York Times Magazine profile of photographer Sally Mann, whose intimate portraits of her three young children are suffused with references to pre-adolescent sexuality, noted that federal prosecutors had warned the artist she risked prosecution on child pornography charges.
The Times profile also reported that Jack Sturges, another contemporary photographer whose work involves images of prepubescent girls, had his equipment and materials confiscated by the FBI.
It's hard to remember the last time a painting caused such a stir -- perhaps Edouard Manet's Le Dejeuner sur L'Herbe? Photography, it appears, possesses a unique capacity to disturb the status quo.
The peculiar power of photography lies in its illusion of truthfulness. The invention of photography made possible, for the first time in history, an imitation of reality produced wholly by mechanical means.
Unlike painting or sculpture, for example, the photographic image is formed by a lens and fixed on film or paper by a photochemical reaction. The resulting representation thus conveys the impression of possessing a reality independent of human hand or eye.
This illusion of objectivity lends photography a psychological veracity that obliterates the distinction between "image" and "reality" in the viewer's mind.
We unconsciously accept the "truthfulness" of the camera's image as if it were some purely natural phenomenon, like the reflection in a mirror.
It is photography's unique ability to compel belief in the truth of what is represented that has resulted in its status as the most embattled of modern arts. (Under the term photography I include its twin progeny, movies and television.)
A line from the English poet John Keats hints at the ethical dilemma every photograph presents us with. In "Ode On a Grecian Urn" he writes, "Beauty is truth, truth beauty -- that is all Ye know on earth, and all Ye need to know."
Keats' identification of the good with what is beautiful and true preceded the invention of photography. It would have been impossible today, given all that we have learned about the deceptions of Hollywood and the advertising industry.
And yet because photography, like music, operates mostly on the level of the unconscious, there is a part of us which still wishes to believe that what we perceive as beautiful is also true, and vice versa.
When photography exploits our weakness for its images in the service of selling soap flakes or political candidates, perhaps its agency can be dismissed as relatively harmless.
But when its subject matter touches on the deepest human concerns -- our sexuality, for example, or our religious beliefs -- the tension aroused between what we know to be good and what we perceive as "beautiful" and "true" can become almost unbearable.
Some of Ms. Mann's portraits, for example, appear to portray children who have been abused physically or sexually (the photographer admits she occasionally stage-manages sittings to produce such effects). Others seem to imply an uneasy coexistence between childhood innocence and incipient sexuality.
Many people find photographs of nude or semi-nude children vaguely disturbing, possibly because of a sense that what is presented as good, beautiful and true also carries with it an odor of the forbidden. Critics have charged Ms. Mann's pictures express a dangerous moral ambiguity that could incite unstable individuals to reprehensible acts.
Similarly, Andres Serrano's notorious "Piss Christ," a photographic depiction of the Christian savior immersed in a jar of urine, aroused so much outrage precisely because the medium itself conferred on the image a specious impression of truthfulness that many viewers regarded as blasphemy.
It may be that photography's psychological tyranny over our perception of reality will eventually fade. But after 150-odd years of incessant bombardment by images, it shows no sign of weakening yet.
More likely the future will bring new developments that amplify its power. For example, it may one day be possible to combine digital electronics with laser holography to create the equivalent of three-dimensional photographs. Such images would be visually indistinguishable from objects in the real world.
In any case, our difficulty in coming to terms with the graven illusions we create seems far from over.
Glenn McNatt writes editorials for The Baltimore Sun.