The museum didn't discover the forgeries. It was simply a matter of the guy who made them - a Mexican artist and now museum employee named Brigido Lara - owning up to his handiwork. He spilled the beans some months ago to a pair of writers who visited Veracruz, where Lara works, on assignment from Connoisseur magazine. There is still no real proof that Lara himself fabricated the pieces, according to Harry Parker, director of the Dallas Museum of Art which, along with the Metropolitan Museum of Art in New York, also seems to be in possession of some Lara originals. But several of the pieces in St. Louis and Dallas that Lara identified as his own have already been proven to be fakes, and the others are definitely under suspicion. In any case, the full story - or as much of it as Lara has chosen to tell - will appear in the June issue of Connoisseur; apparently officials at the St. Louis museum felt it would be better if they broke the news themselves.

The news broke on Thursday, and it fell to me to produce a side-bar on the history of art forgery to supplement a reporter's account of the current scandal. It was an assignment I accepted eagerly, both because I like pre-Columbian art and because the concept of art forgery is one that long has fascinated me.

As often happens when critics write about history, the side-bar evolved into a speculative essay, and in its concluding paragraphs it opened what must have struck some readers as a can of worms. I don't want to say that I'm going to sink my teeth into it here - the metaphor isn't very tasteful. But now seems as good a time as any to dig deeper into the matter.

The question I raised about the Lara forgeries was this: Although the objects' value as historical artifacts is obviously lessened by their exposure as fakes, does that mean that their aesthetic value also - and necessarily - is decreased?

The question was phrased another way more than a quarter-century ago in an article by Aline B. Saarinen in the New York Times Book Review section.

''If a fake is so expert that even after the most thorough and trustworthy examination its authenticity is still open to doubt,'' Saarinen asked, ''is it or is it not as satisfactory a work of art as if it were unequivocally genuine?''

Saarinen's riddle is quoted at the head of ''Art and Authenticity,'' a chapter from philosopher Nelson Goodman's 1968 book ''Languages of Art'' that is included in Denis Dutton's 1983 anthology ''The Forger's Art.'' All 11 of the writers who contribute essays to ''The Forger's Art'' address the question in one way or another. And 10 of them conclude that, no, for various reasons the fake is not as aesthetically satisfactory as the original.

The one who takes exception is Monroe C. Beardsley. He is addressing the issue of copies when he writes: ''I reject the idea that there can be two indistinguishable paintings very different in value and very different in meaning. Since the meaning of a painting must be a function of its pictorial properties, I hold, with Goodman, that their meanings must be the same.''

But elsewhere he deals with one-of-a-kind fakes of the sort that are in the St. Louis Art Museum's collection.

''I deny that the practice of discriminating among Rembrandts, genuine or fake, is an aesthetic activity,'' he writes, ''. . . because to me the question is, what kind of aesthetic experience is it possible to obtain from looking at the picture?'' A later statement is even more bolstered by logic: ''Forged works, to put the point somewhat differently, cannot be dismissed on the ground that they possess no aesthetic qualities, because if they did not possess such qualities, there would be no such things as successful forgeries, and we know that there are.''

Beardsley sides with Goodman in the so-called empiricist, or phenomenological, approach to art, and I go along with both of them. There are many criteria by which we place ''value'' on works of art; the one that counts the most has to be the one connected with our aesthetic experience of the work.

Musical compositions are often misattributed. They are seldom forged, however, since - except for their manuscripts - the compositions are not in themselves physical objects but rather patterns of sounds indicated by sets of instructions. But a singular example of ''fake'' music, I think, proves the point.

The violinist Fritz Kreisler was just beginning his career around the turn of the century; he was talented but unknown, and he felt the old-fashioned little salon pieces he liked to write would be dismissed by critics if he attached his own name to them. So he claimed they were by Vivaldi, Porpora, Dittersdorf, Martini, Couperin and other 18th-century composers at the time still not much subjected to the scrutiny of musical scholarship. Audiences loved them, and so did the critics.

It was not until 1935 that Kreisler revealed his hoax. The only critic who made a stink about it was the London Sunday Times' Ernest Newman, to whom Kreisler tactfully responded that a critic ought not feel his prestige has been tarnished just because that which he's already deemed worthy turns out to be written by some other composer. ''The name changes, the value remains,'' Kreisler told Newman. The New York Times' critic Olin Downes agreed. ''Mr. Kreisler has added to the gaiety of nations and the violinist's repertoire,'' Downes wrote. ''Shall we begrudge him that? Should the man who kissed the wrong girl in the dark condemn the practice of kissing?''

It may be that the St. Louis Art Museum curators ''kissed the wrong girl'' when they chose for the cover illustration of their elaborate 1980 catalogue of the Morton D. May collection a picture of a polychromed buff earthenware female figure, dated A.D. 400-700 and attributed to the Remojadas culture that flourished in Veracruz during Mexico's Middle Classic period. The jury is still out on this one, but the object is among the pieces Brigido Lara says he created in the 1950s and '60s.

The figure stands about 2 1/2 feet tall. Her arms are extended and her hands are held palms upward. She wears a slightly irritated expression on her face. Her shoulders seem to be shrugging. It's as though she's saying: ''OK, I'm a fake. So what?''

Actually, there's quite a bit at stake here if the terra cotta figure is indeed a phony.

For one thing, it and some of the other suspect pieces are considered prime examples of pre-Columbian art, and thus the entire set of standards by which that art has lately been judged will be called into question.

For another, if the figure is a fake it no longer tells us something about the culture that until recently we thought produced it. It perhaps tells us something about Lara's culture and about his vision of the pre-Columbians. But it no longer carries a message from ancient times; it no longer serves the communicative function that is inherent in all authentic artifacts.

For yet another - and this is the one that doubtless will most bother private collectors of pre-Columbian art - if the figure is a fake it no longer has much market value. People pay lots of money for pre-Columbian art, not just because they like the way it looks but also because its relative scarcity automatically gives it a high price tag. In souvenir shops in New Mexico and Arizona, terra cotta figures in pre-Columbian style can be had for less than $10 apiece; some of them look a great deal like objects in museums, but the stuff in the museums is considerably more expensive, largely because it's thought to be genuine. Everyone must be wondering: Now that some of the items in the St. Louis Art Museum's collection are known to be forgeries, how much are they really worth?

Goodman and Beardsley would argue - and I agree - that from a purely aesthetic point of view they're worth just as much now as they were before Lara's fakery was made public. Beauty is in the eye of the beholder, and the eye beholds as much - or as little - beauty in these pieces as it did a month ago. As Kreisler said, ''The name changes, the value remains.''