''You know, there's a lot of talk about authenticity these days, and I think it's nonsense,'' said Albert Fuller, the harpsichordist and director of the Helicon Ensemble. Mr. Fuller and his group have been presenting an early-music festival at Weill Recital Hall since Wednesday, with the final concerts scheduled for this weekend. ''The authenticity I'm interested in is our contemporary sense of the reality of the past, and what the music we have inherited has to do with us today. I believe that music is a window into the unconscious, and the miracle of it is that it lets us get close to what Beethoven, Bach and Mozart were thinking.
''This kind of psychic investigation is really a 20th-century phenomenon, something we would not have thought about in pre-Freudian times. And I think it's the whole point of the original-instrument movement. By hearing the music of these great geniuses performed with the timbres and in the styles that they heard, we get a clearer idea of how their mental mechanisms worked, and of what they wanted to share with other people. Of course we can play Beethoven on a modern, steel-strung violin, in a Russian performing style. But if we do that, we're laying on the music a lot of 20th-century associations: pogroms, revolutions, czars and Stalin. Beethoven didn't know anything about that.''
Mr. Fuller may take a more philosophical view of the early-instrument movement than some of his colleagues. But in a practical sense, their goals are the same. By researching the details of performance style (tempos, ways of phrasing and so on), and by returning to the gut-strung violins, natural-valve horns and wooden flutes of earlier times, they hope to make the music of the Baroque through early Romantic eras sound as fresh as it did when it was new.
Two decades ago, early-instrument performers were regarded as a bit eccentric by many mainstream concertgoers, and by musicians who saw the abandonment of technically more hardy and reliable modern instruments as a retrograde step. But the movement has grown quickly. Today, early instruments are the norm rather than the exception for Baroque music performances. And in the 1980's, incursions have been made into the Classic and early Romantic eras, with ventures as far into the 19th century as Berlioz and Schumann.
For fanciers of these antique instruments with their robust, gamy sounds, there are several concerts to choose from this weekend. Each of the Helicon Ensemble's remaining Weill Recital Hall programs offers music from the 17th through 19th centuries. Tonight at 8, the group is playing rarities by Bartolomeo Montalbano and Mario Uccellini, and better-known works by Couperin (the Concert Royal No. 4), Boccherini (a quintet) and Mozart (the Clarinet Quintet, with Lawrence McDonald as the soloist). Tomorrow evening at 8:30, the Helicon group will play two of Biber's ''Mystery'' Sonatas, a Handel Trio and the Beethoven Septet (Op. 20). The concerts are sold out but return tickets, at $12, may be available at the box office before the performances. Information: 247-7800.
The ''Schubertiade'' at the 92d Street Y, at Lexington Avenue, is not an early-instrument endeavor as such, but its programmers occasionally glance in that direction. Tomorrow evening's installment, at 8 o'clock, includes some of Schubert's early keyboard music, performed on the fortepiano by Malcolm Bilson. Tickets are $17.50. Information: 996-1100. A Handel Anniversary
John Eliot Gardiner, one of the most highly regarded British early-music conductors, will be in town with his English Baroque Soloists and Monteverdi Choir to mark the 250th anniversary of Handel's ''Israel in Egypt'' with a performance of that oratorio at Avery Fisher Hall on Sunday at 3 P.M. Tickets are $25. Information: 874-6770.
A recorder ensemble from Holland, the Amsterdam Loeki Stardust Quartet, will play Renaissance and Baroque music (the composers include Frescobaldi, Jenkins, Boismortier, Purcell, Bach and Vivaldi) at Corpus Christi Church, 529 West 121st Street, on Sunday at 4 P.M. Tickets are $10. Information: 666-0675. And the Oberlin Baroque Esemble is playing works of Couperin, Forqueray, Rameau and Dornel at the Frick Collection, 1 East 70th Street, on Sunday at 5 P.M. Admission is free. Information: 288-0700.
Of these, Mr. Fuller's Helicon Ensemble is offering the widest-ranging musical overview. ''The point,'' Mr. Fuller said, ''was to offer a grand tour through 200 years of music. Traditionally, a 'grand tour' was a journey through Italy, France and Germany. So my idea was to put something from each of those countries, and from the 17th through the 19th centuries, on each program. That way, if you can only attend one of the concerts, you still get a nice musical banquet.''
Depending how you look at it, the Helicon Ensemble is either a newcomer to the early-music scene or a fairly venerable troupe. This week's series marks the group's first public performances, but concertgoers who follow New York's early-instrument world will see many familiar faces in the ensemble. Among the players are the violinists Jaap Schroder, Stanley Ritchie, Linda Quan and Nancy Wilson; the cellists Myron Lutzke and Loretta O'Sullivan; the oboist Marc Schachman; the hornist William Purvis, and the bassist Michael Willens, all regulars in early (and modern) instrument chamber groups and orchestras that perform here often.
Many of these players were Mr. Fuller's students at the Juilliard School or at Aston Magna, the summer institute and touring ensemble he founded in Great Barrington, Mass., in 1972. And Mr. Ritchie and Mr. Schroder were among the Aston Magna faculty from the start. Mr. Fuller directed Aston Magna until 1983. That year, after a dispute about the organization's direction (Mr. Fuller wanted to forge ahead into the 19th century; other members of the board wanted to keep the focus on the Baroque and Classic repertories), the harpsichordist and Aston Magna parted ways.
The Helicon Foundation - named after the mountain where Mnemosyne, the Greek goddess of memory, conceived the Nine Muses - was established in 1984. Until now, the foundation has held private symposiums at which the players in the current ensemble presented their latest findings, with performances open only to the foundation's subscribers. After a few seasons of these, Mr. Fuller decided that it was time to go public again.
A Utopian Goal
''I admire musicology very much,'' Mr. Fuller said, ''but to me, musicology is not the flame. The flame is performance. Buckminster Fuller, who is no relation to me, said that the goal of knowledge is to let everybody on earth know everything there is to know. That's utopian, but you have to face in some direction if you're going to move down the path, and that strikes me as a pretty good goal.
''What we have done, by studying early instruments and early performance practice, is find out as much as we can about where we've come from and who we are. And the knowledge we can share with our audience is that the great composers of the past were human beings, more like us than not. However weird Beethoven might have been, we could have lunch with him, and it would be fun. We have to recognize that the fun, the joie de vivre, is also in the music, and has to be brought out.
''This is the most important thing our original-instrument investigations can make a point of: the brotherhood of human beings across time.