WHAT DID they hope to get out of it? Two days after thieves made off with a spectacular haul of art masterpieces from the Isabella Stewart Gardner Museum in Boston, the biggest art theft in recent history, no plausible theory of the criminals' motive has been put forth. Money, of course, and yet the only consensus so far seems to be that money is what such a crime cannot possibly bring. The stolen works, including a Vermeer, several Rembrandts and a Chinese beaker from the second millennium B.C., are so well-known worldwide and have belonged to the museum for so long that they are, in a practical sense, priceless: no one could plausibly sell or resell them for anywhere near their real value, anywhere in the world. Only the totally uninformed could hope to pass them off as legitimately acquired works or works of lesser but still substantial value.

That leaves the field open for a whole range of more lurid, spy-novel scenarios: Was it ransom? A rich, crazed private collector? The "Asian connection" conspiracy theory, which whispers about prominent works showing up in Japan? Unlikely prospects all, but no stranger than the event. True, the art world (and underworld) have become vastly more fluid and fast-moving in the past decade, and art theft has been on the rise, mostly because of the dizzying, much-publicized ascent of art prices. But at the same time international computer networks and tracking mechanisms have become more sophisticated, and dealers are savier and more inclined to worry about acquiring "hot" goods.

The disposition of this case will help give a reading on the progress of these changes -- maybe to the surprise of those who carried off this theft with such evident ease and careful planning. "It's possible," says Constance Lowenthal of the New York-based International Foundation for Art Research, "that whoever it is woke up this morning with something less than elation." Hence, perhaps, a best-case scenario: ordinary thieves, drawn to the prospect of fantastic profits and now taken aback, maybe intimidated by the breadth and volume of outrage they have unleashed. Pricelessness is a concept not often encountered in the normal round of public business. And yet the emotional outpouring from Bostonians and others goes well beyond the monetary value of these works on some legitimate or illegitimate market. The Gardner Museum has stood far outside the whirlpool of art market changes in the '80s -- by the terms of its eccentric founder's will, the collection could be neither sold, added to nor rearranged -- and this adds in many minds to the general sense of affront and violation.

There has been some hand-wringing over the museum's admission that the stolen paintings were uninsured. But in a sense the whole matter of insurance is beside the point: what good is an insurance company's reimbursement when the object in question is, by definition, irreplaceable? The thieves may have hoped to exact some kind of ransom. But it makes no more sense to bargain with the kidnappers of paintings than with any other kind of hostage-takers. The museum yesterday announced a $1 million reward for information; as this implies, the real answer is detective work, and a happy ending is by no means implausible.