It's a phrase that pops up almost like a motto - to be decried or supported - in the half-dozen essays that make up ''Authenticity and Early Music'' (Oxford University Press), a new collection of the main papers delivered at a symposium on so-called musical ''authenticity'' held at Oberlin College in the spring of 1987. This is fascinating reading for anyone who has any opinions at all on the subject. And it's hard to imagine a musically aware person not having a least a modicum of opinion.

As was noted here last August in a column prompted by the debut of an American journal called Historical Performance, today - after decades of struggle by the early-music pioneers - it's actually fashionable to enjoy hearing 18th-century music played on reproductions of 18th-century instruments. Attendance at professional early-music events, in St. Louis as much as elsewhere, is far along on the course of an upswing. Recordings by early-music specialists of Baroque pieces have begun to outsell recordings by modern orchestras of the same pieces. And so on.

''But ironically,'' writes editor Nicholas Kenyon in his introduction to the collection of Oberlin papers, ''at the same time that the cause of period-performance styles appears to have been largely won in the musical world, increasingly loud questions have begun to be asked about the aims and the means of the 'authentic' approach from within its own ranks.''

Many, but doubtless not all, of those questions are contained in the volume's extended arguments by Will Crutchfield, Howard Mayer Brown, Robert P. Morgan, Philip Brett, Richard Taruskin and Gary Tomlinson. And a few more of them can be found in two other recently published books.

As befits a work of apparently objective scholarship, they tend to be raised only in passing by former Kansas City Star music critic Harry Haskell in his ''The Early Music Revival: A History'' (Thames and Hudson). But Raymond Leppard, the St. Louis Symphony's principal guest conductor and something of a specialist in the performance of Baroque opera, addresses a number of them head-on in his slim yet impassioned ''Authenticity in Music'' (Amadeus Press).

As often as not, these ''increasingly loud questions'' are followed by precious little in the way of solid answers. But that hardly diminishes their importance. Value in all music - the supposedly ''authentic'' kind and every other kind - is largely a speculative thing. Most of us never really find the definitive truth that we seek, yet in the very process of seeking it we find at least some aspect of it. In this as in many other controversies, to a certain extent the answers are contained in the questions.

For Leppard, the stickiest wicket is the failure of some in the ''authenticity'' camp to see the need for compromise. ''Absurdly bigoted'' is the way he describes the opinions of those who hold that the ''authentic message and content'' of early music can be revealed only when we approach ''the exact conditions'' of the ''music's first appearance.'' The truly authentic performance, he writes, is not ''a reproduction of what actually happened'' but rather ''a translation'' of the piece according to the needs of our own time.

Will Crutchfield, a New York Times music critic and the leadoff slugger in ''Authenticity and Early Music,'' is pretty much in agreement. But his prime target is what he feels is the gross misuse of the word ''authentic.'' All too often, he writes in a piece titled ''Fashion, Conviction and Performance Style in an Age of Revivals,'' the status of ''authentic'' is granted to music-making that is nothing more than an attempted re-creation - even if it's mechanical, even if it's dry as dust - of what is alleged to have been the music's first performance. Surely, he argues, there must be more to it than that. After tracing the evolution of the word through the listings in the Oxford English Dictionary, he concludes: ''Authenticity implies authority, and ultimately an author. The author of a performance - of a bow stroke, a crescendo, an impulse, a radiant act of absorption - is the performer, with whose condition we must be concerned if authenticity is what we're after.''

In contrast to Crutchfield, the University of Chicago's Howard Mayer Brown - in an essay titled ''Pedantry or Liberation?'' - argues that authenticity is the exact replication of a work in its original performance conditions, and he argues further that all true musical scholars must be committed ''to the ideal . . . that they are engaged in the positivistic task of discovering wie es eigentlich gewesen, 'how it really was.' '' Brown writes that performers, on the other hand, need be committed only to the task of making their performances vital, imaginative and convincing. But he adds to that thought an addendum spiced with a telling ''of course'': ''Personal commitment is a necessary virtue for performers. . . ., but it may be a luxury to which scholars ought not to aspire. Intelligent performers, of course, will inform themselves about the possibilities open to them, and the playing of the most intelligent will almost certainly these days be 'historically informed.' ''

The contribution of Robert P. Morgan, also of the University of Chicago, is titled ''Tradition, Anxiety and the Current Musical Scene''; it deals not so much with definitions of so-called ''authenticity'' as with the reasons for its importance in modern culture. Our society has become so pluralistic, so thoroughly inundated with easily accessible music in widely varying styles, Morgan writes, that we have in effect been cut off from the flow of tradition. ''We no longer have a culture of our own,'' he argues, and so ''by way of compensation we attempt to assimilate everyone else's.''

Then he makes an intriguing analogy to language: ''In linguistic terms, he have no native language; and as a consequence, we are forced to borrow foreign ones. But these latter, because they are not our own, have no permanent attachment to us and can be exchanged at will. This means we can no longer deal with the music of the past in our own terms. The necessary terms, to say nothing of a grammar to connect them, are simply not there. Deprived of a means of translation, we must try to speak the music in the original.'' To do that, Morgan writes, is obviously impossible, for the conditions of listening today are not the same as when the music was indeed ''spoken'' in the original. But the attempt, he says, is ''an entirely 'authentic' manifestation of our age.''

An essay by the University of California's Philip Brett offers readers a respite from the heat thus far generated; for the most part, it's a cool examination of the obligations of the would-be editor of early music.

The temperature rises again, however, in ''The Historian, the Performer and Authentic Meaning in Music'' by University of Pennsylvania musicologist Gary Tomlinson. Much of what Tomlinson has to say is based on his own involvement in the study of the philosophical context of an Italian fabula, or mythological play with music, by Angelo Poliziano probably written and first performed in Mantua in 1480. And with it he underscores this point: ''The authentic meaning of a musical work is not the meaning that its creators and first audience invested in it. It is instead the meaning that we, in the course of interpretative historical acts of various sorts, come to believe its creators and audience invested in it.''

The University of California's Richard Taruskin - in his ''The Pastness of the Present and the Presence of the Past'' (the cryptic title is a paraphrase of a line from a 1917 essay by T.S. Eliot) - similarly argues that so-called ''authenticity,'' or ''authentic meaning,'' has far more to do with the perspectives of performers and listeners than with the music itself. And, as does Morgan, he feels that the quest for ''authenticity'' is a symptom of the malaise of Western culture in the late 20th century. Then, more fiercely than any of the other writers, Taruskin denounces the positivist musicologists who take an authoritarian attitude toward ''authenticity,'' who think ''it is now possible to pass judgment on a performance one has not yet heard.'' But his main thesis is that '' 'historical' performance today is not really historical; that a thin veneer of historicism clothes a performance style that is completely of our own time, and is in fact the most modern style around; and that the historical hardware has won its wide acceptance and above all its commercial viability precisely by virtue of its novelty, not its antiquity.''

Making reference to an essay by Daniel Leech-Wilkinson that first appeared in 1984 in the English journal Early Music, Taruskin calls attention to ''the remarkable uniformity of approach'' that governs so-called ''authentic'' performances of music composed before 1800. Yet the same basic approach, he argues forcefully, is evidenced in performances that make no claim to ''authenticity.'' Clarity, lightness, speed and all the other virtues of period-instrument performances of early music are elements of the modern musical taste in general, he claims. That is why no one today attempts an ''authentic'' performance of Tchaikovsky: Such a thing, with scoops and slides and rubato all over the place, would simply not be to our taste. ''So why all the bloodshed and recriminations?'' he asks. ''Why not simply recognize our modern Bach for what he is, and stop the nonsense about authenticity?''

Not surprisingly, the combination of compound adjective and suggestive noun appears more than a few times in the pages of the inaugural issue of Historical Performance, a new journal published by the recently organized early-music advocacy group called Early Music America.

A rather dry example - but useful here, because it amounts almost to a definition - occurs at the start of a book review by Frederick Neumann. ''The early-music movement is closely identified with the search for 'authenticity' of performance,'' Neumann writes, and then - prudently - he reminds the movement's sympathizers that ''the concept of authenticity is obscure,'' that ''its very existence is questioned by a number of eminent musicians and scholars.''

A more forceful, less cautious use of the term is found at the end of a column by Early Music America president Benjamin S. Peck. After assuring readers that ''early music is coming of age'' and that ''early music has even begun producing 'superstars,' a sure sign of its growth toward commercial maturity,'' Peck concludes with this: ''The early-music movement now looks to be not a step back into the past, but rather a wave of the future, changing forever the way we approach our musical heritage.''

That's an effective statement, probably just the thing to get the early-music crowd - still feeling misunderstood and under-appreciated, to judge from some of the other pieces in Vol. 1, No. 1, of Historical Performance - to rally 'round the flag.

It's also a statement that is to a large extent valid.

Forever is a long, long time, of course, and ''th e way we approach our musical heritage'' is a difficult thing to measure. Unless doomsday is just around the corner, Peck is doubtless trafficking in hyperbole when he claims absolute permanency for whatever shifts in attitude have lately been brought about by the early-music movement.

In terms of a forseeable future, however, Peck's prediction seems on the mark.

It used to be that so-called early music - that is, music that not only pre-dates modern times but is performed in a manner more or less contemporary with its creation - was the province of antiquarians. Today it is also the province of the general public.

Finally, after decades of wishful thinking on the part of early-music pioneers, it's actually fashionable to enjoy hearing baroque music played on gut-strung fiddles and wooden flutes. The full houses and ecstatic ovations at last season's concerts here by Christopher Hogwood's London-based Academy of Ancient Music and by the Amsterdam-based trio of recorder player Frans Brueggen, cellist Anner Bylsma and harpsichordist Gustav Leonhardt typify how audiences worldwide are responding to this sort of thing. Record companies report that, given the choice between a 20th-century mainstream performance of a Bach concerto and an early-music treatment of the same work, the majority of customers in Europe, Japan and the United States are now opting for the latter.

How much the popularity of early music has to do with genuine enlightenment on the part of listeners and how much it has to do with mere trendiness is hard to say. In any case, the public's appetite for early-music performances shows no signs of abating. More and more listeners are attuning themselves to the sounds of antique instruments; in response, more and more performers are learning to play those instruments. In this country, at least, the early-music market has in effect just been opened up, so it's safe to say that it will be some time before supply begins to exceed demand. Indeed, the ''wave of the future'' churned up by the early-music movement has some distance to go before it breaks.

The 54-page first edition of Historical Performance includes a purely musicological article on the instrumentation of Monteverdi's 1607 opera ''Orfeo,'' three highly detailed book reviews, some reports on regional early-music activity (in Ohio, in Boston and in the Washington, D.C., area), a few sets of remarks from various Early Music America committee chairmen, a package of notes on happenings in the early-music world both here and abroad, a long listing of workshops and festivals scheduled for the summer months and a slew of advertisements placed by instrument makers, music publishers, artists' representatives and schools that offer early-music degree programs.

But the really juicy stuff is contained in the several essays that are, like the word ''movement'' itself, somewhat hortatory in nature.

I've already mentioned the editorial in which Peck, with rhetoric appropriate to a pep rally, gives his vision of the future of early music.

Perhaps for the sake of balancing the Early Music America president's optimism, there is also a lucid and slightly inflammatory explanation by New York-based baroque oboist Stephen Hammer of why record companies are paying lots of attention - and lots of money - to early-music groups in England while practically ignoring early-music performers (''early musicians'' is the awkward label by which they often identify themselves) on this side of the Atlantic.

Even more provocative is the column in which Benjamin S. Dunham warns the early-music faithful that while ''as take-over specialists, we are becoming spectacularly successful,'' ''as evangelists, we may be missing the boat.'' ''The 'fast' money in the music field is going into commissioning programs and the support of contemporary music,'' Dunham says; if the early-music people want to get in on the action, he suggests, they have to do more than provide their audiences - and their potential corporate sponsors - with something more than the equivalent of 17th-18th-century pops concerts.

And Thomas Binkley, head of the early music department at Indiana University, attempts to put the whole early-music phenomenon into perspective with an account of pre-20th-century manifestations of what he calls ''historically informed'' performance practice.

Binkley stretches his point beyond its endurance when he argues that a 1585 performance of ''Oedipus Rex'' in Venice - presented not only with thoroughly up-to-date music and costumes but also with a thoroughly up-to-date Italian text - exemplifies ''historical awareness'' just because the show's producers chose for their source material an ancient Greek tragedy.

He's on target, though - albeit rather beside the point - when he says that all musicians are involved ''to some extent'' with historical practice, simply by virtue of the incontrovertible fact that their instruments and their repertoires have histories. Immediately before that statement comes this sentence: ''In the last few years the intense interest in historical musical practice has caused a brief shiver of resentment in another quarter.'' For occupants of that quarter - for veteran orchestral string players who are suddenly being told that the way they've always played Bach and Mozart is no longer the correct way - the remark will likely be taken not so much as ironic understatement but as fightin' words.

Oddly, nowhere in Historical Performance's debut number is there speculation about why interest in ''historically informed'' performance of the 17th- and 18th-century repertoire has surged to the point where the early-music movement has almost become an early-music revolution.

Maybe the writers assume that everyone knows why. Maybe they feel that the success of the movement was somehow pre-destined. Maybe they just haven't thought about it much.

Anyway, for those who feel the matter is worth discussing, here are my thoughts:

First of all, early-music practitioners in recent years have gained remarkably in technical proficiency.

Their forebears were typically much better scholars than they were musicians, and the imbalance of talent was such that even in the 1950s and '60s audiences justifiably equated performance ''authenticity'' with generally lousy playing. In 1973, when Christopher Hogwood set precedent by forming his Academy of Ancient Music, he made darned sure that the free-lancers he engaged were as competent as those in London's modern symphony orchestras. In order to be competitive, Hogwood's imitators have had no choice but to follow suit; as a result, early music today - so long as it's handled by professionals - sounds a lot better than it used to.

Secondly, the public has long been ready for a change.

The main reason why attendance at symphony concerts has lately been dropping is the audience's mounting frustration with hearing the same old pieces played year after year in the same old way. New works for orchestra in the 1960s and '70s made more enemies than friends, and even the deliberately ''accessible'' compositions of the present decade tend to have a rough go of it. Still, people enjoy novelty; they need novelty. The safest way to get it - especially for people with conservative tastes - is through fresh, innovative and sometimes radically daring early-music performances of pieces that used to belong to the standard repertoire.

It is the two factors in tandem - the rise in early-music performance standards coupled with the audience's dissatisfaction with music of its own era - that has given the early-music movement its momentum. One without the other would not have been enough. Probably the world's finest early-music playing would still be largely ignored if listeners today were not in the market for something new, and probably the world's most novelty-starved consumers of music would reject early-music performances that were not up to snuff.