"I'll bet there are more composers in Princeton now than there were in Europe in the entire middle of the 18th century," declares John Harbison, a composer of 46, whose new string quartet will have its world premiere here tonight in a concert at the Corcoran Gallery.

Harbison's assertion of a population explosion at the compositional level is, of course, part hype, a little out of character for so soft-spoken and ruminative a figure. After all, Princeton (which is where Harbison grew up as the son of an eminent history professor) is not that richly endowed. Nor was the 18th century quite that hard up.

But his point about the profusion of notes per se is well taken. The concert music scene in this era is extremely fluid, and nothing is clearly either In or Out. Some composers clutch catechismally at the dogmas of serial development or to the latest in synthesized sounds, while others -- like Harbison, who has tried a variety of styles -- see melody once again as a new kind of adventure.

Harbison, who is also a violist and a conductor, is making substantial contributions to this diverse scene both as a composer and as a professional observer of other composers' work. He is currently composer-in-residence with Andre' Previn at the Los Angeles Philharmonic.

His latest enterprise reflects what is new in music: Quartets have not been the regular events in the 20th century that they were for the 200 years before. But Harbison's deep interest in chamber music for traditional instrumentation also reflects a renewed interest in concepts of the past. "The thing I've always been interested in," he says, "is trying to work with something that looked back and that is assumed to be hopeless."

It has not been an entire disappointment that people actually like what he has been doing recently. There was, in fact, something startling about the fact that in four months this year his dark, deeply personal work, Piano Quintet, composed in 1981 for the Santa Fe Chamber Music Festival, got performances by three separate groups in Washington. That didn't happen to anybody else's piano quintet -- not Brahms', not Dvora'k's. It is a work inspired by inner turmoil -- in this case, a tragedy that struck his two younger siblings -- and the lyric intensity that resulted strikes powerful chords in many listeners.

The other Harbison work best known in Washington, his Piano Concerto, which took first place in the Kennedy Center's 1980 Friedheim Awards, is in a similar idiom, if not the same mood.

The composer comes to this musical language through a very basic principle: A desire to communicate. It is not something, Harbison argues, that an artist simply decides to do. "It is a mystery, the power of the drive to write concert music -- as well as the proliferation of it, given the relative opportunity and visibility. I understand very much when a talented teen-ager becomes a rock guitarist. Because he feels that it gives him something. It may be that writing concert music is just a very, very powerful urge, kind of unquenchable. It's like writing lyric poetry. I mean, people do it in spite of the fact that no one makes a living from it, and your best audience is maybe about 200 people."

Harbison has composed with distinction in the most esoteric musical languages. He discussed a chamber work called "Containment," written at age 24, a work as full of obscure total chromaticism and advanced development as one could ask.

"At the time, I thought this was the best thing I had written," he observes. "And I thought, well, I can keep on doing it. But a friend convinced me that the best thing I could do was to leave it behind. It was a dead end.

"The next two or three pieces were not so good, but they were different. And that was what was important."

Describing the new composers, Harbison adds, "Rather than just trying to bite off a bit of territory, there is a real feeling that a lot of these new people want to make a Big Statement, even if they possibly fail. That's the thing I have found that has been helpful to me -- that is, a willingness to really screw up. I have written a few pieces where I didn't know where it was particularly going. Where I was not sure what they were going to turn out to be. And I've certainly written some pieces that didn't work at all. But I think you have to shake the sense of being safe . . . Today's composers don't seem to feel they are going to be ostracized if they write a piece which is a bomb.

"I think it's quite easy to write a single striking and individual piece. But what is hard to write is a number of good pieces which develop the range of ability within a composer. In the '50s and '60s we would exhort the composer in the direction of the individual statement. Then we condemned the composer to stay digging away at that uncharted bit of uncharted territory, where really the air is too thin and the walls are too narrow."

In the best composers, this approach can lead to considerable expressive freshness -- as in Harbison's most eloquent work.

The Cleveland Quartet, which commissioned the Harbison, will play it in a concert starting at 8 tonight at the Corcoran. It will be repeated tomorrow night at 8:30 p.m. in the University of Maryland's Center of Adult Education.