This week, if you believe Dr Kathleen Weil-Garris Brandt, the world recovered a great treasure. The circumstances were delightfully cinematic. For years the gloomy lobby of a Fifth Avenue mansion has contained a small stone sculpture of a cupid with a quiver in the shape of a lion's paw. The object was in plain view but so familiar that it was, effectively, invisible. One day, though, as Dr Brandt was passing, the lobby had been illuminated for some special event. Peering through the glass, she saw the sculpture brightly lit for the first time. One can imagine the scene as an Annunciation, a beam of golden light bearing down on the sacred object. Dr Brandt's heart beat a little faster. The contrapposto of the back, the tightness of the curls, the modelling of the lion's paw... surely this was no humble piece of garden statuary. After further research, and with much trepidation, she advanced the theory that the work was actually by Michelangelo. New York had lost a modest Italian ornament and gained a priceless work of art - a brilliant trade by anybody's standards.

Dr Brandt has found support among other art historians. Our own Dr Nicholas Penny, of the National Gallery, weighed in on her side, telling the New York Times that "The more one looks at it, the more it grows on one, not only as a remarkable work of art but something that makes sense as a work of Michelangelo."

What exactly does it mean, though, to "make sense" as a work of Michelangelo? It is an important question, as Dr Penny has good reason to know, because the last art-world fuss over the authenticity of a Michelangelo came much closer to home. It arose over the reattribution - the promotion, in effect - of the National Gallery's Entombment of Christ. The case against was put by an enraged Professor James Beck: "To make the Entombment a Michelangelo", he said, "diminishes the creativity of Michelangelo. It means placing a third-rate work at his doorstep. That is a violation of his integrity." (To have 20th-century academics presume what your integrity consists of might be considered a larger violation still, but let that pass.)

Integrity is important to us, and has been for centuries. The quality has another name, too, and one rather more pertinent to these questions of artistic provenance - "authenticity". And here we arrive at a paradox. In its larger moral sense, authenticity (as applied to people or their behaviour) contains some notion of intransigence or resistance to the smoothing impulse of society. We detect authenticity in humans by those features in them which don't conform to pattern. In art history, however, quite the opposite is true. Authenticity that "makes sense" depends on an essentially artistic finish if it cannot come up with incontrovertible paperwork. In the absence of a signature (and, what's more, a signature which has several more signatures to vouch for its authenticity), we rely on something more like plausibility or coherence.

In Sincerity and Authenticity, Lionel Trilling wrote that "the work of art is itself authentic by reason of its entire self-definition: it is understood to exist wholly by the laws of its own being, which include the right to embody painful, ignoble, or socially unacceptable subject- matters. Similarly the artist seeks his personal authenticity in his entire autonomousness." This may not be how Michelangelo thought of the matter (Trilling is speaking specifically of a 19-century habit of mind), but it is, to some degree, how we now think of Michelangelo, in his isolated and heroic creativity. And as he isn't around to clarify exactly what it is to be authentically Michelangelesque, we must do the work for him. Dead people don't have the luxury of "autonomousness".

The result can be decidedly odd - the careful creation of a new being, incapable of error or even the labour of false starts. It has happened to other artists, too. In the past 90 years, we have lost more than half the Rembrandts that were in existence at the beginning of the century; not through some inconceivable carelessness on the part of curators, but because the Rembrandt Research Project has doggedly hacked away at what it considers to be false attributions. They have relied mostly on stylistic considerations (just as Dr Brandt did with her Michelangelo), effectively removing "lesser works" from the corpus. As they proceed, Rembrandt becomes a greater and greater artist and, in some respects at least, less and less humanly interesting.

Clearly the procedure works both ways - if a work of art is elevated to the corpus, it is honour- bound to live up to its new estate. This sometimes takes diligence, as with the world's fresh scrutiny of the New York cupid. "The more one looks at it," Dr Penny said, "the more it grows on one." But would anyone have looked so hard or seen so much if that new and glamorous authenticity had never been advanced?