"When I play, what I'm mainly doing is listening. Concentrating. When people talk in the back row I can often actually hear what they say."

Anton Kuerti is the best known pianist in Canada. He is serious about listening. "The greatness of technique," he says, "is finally in the ear."

He is so serious that he carries his own piano around with him in a van.

Now, it is nothing unusual for a concert pianist, coming to town for a recital, to be offered four or five instruments to choose from. Some virtuosos have their own pianos flown in. But a van?

"It's just a matter of time and money," says Kuerti, who performed here last night in the Beethoven piano sonata series at the Terrace Theater. "Some people think tuning a piano is all you have to do. But when Steinway put in Teflon bushings everything changed."

Plastic bushings make piano keys too stiff for subtle effects, he feels. Or else they're so loose they click with every note.

"My piano isn't that different. It's just standard, dead-on standard. Fifty grams' weight on each key. Standard hammer travel. The let-off is one-sixteenth inch. That's the distance the hammer backs off the string after it has struck it. If the let-off is a quarter-inch, you can't play pianissimo. It's like throwing a ball as close to the ceiling as you can: If you throw it from six inches away, you can be much more precise than if you throw it from three feet away."

Sometimes, on the day of a concert, Kuerti will spend four hours tinkering with his piano.

"The banging around in the van is nothing compared to what happens to the piano in a concert. Temperature is my one concern. In Canada you can go from 40-below to 80 in the hall, and that can do terrible damage. I keep the piano under a layer of foam rubber."

His piano is as sleek as a good mechanic's car.

Always, he listens. Always, he tries to extend the auditory range, dropping down into pianissimo, into the quiet end, bringing the audience with him, getting them to concentrate too, listening, as he barely touches the keys to create the tiniest blossom of sound in a desert of silence.

He has been known to talk back to audiences. He says he is not a grouch (remember George Szell telling an audience, "I'll give you exactly two minutes to clear your throats"; remember Beecham roaring at his tormentors; remember Stokowski having his musicians drift in one by one, coughing and talking, after the concert had begun), but simply wants the people to listen with him.

Once in St. John's, Newfoundland, he told a noisy audience of doctors that a few of them should go home, take two aspirins and go to bed. No one so much as cleared a throat for the rest of the evening. But he got a complaining letter afterward.

Kuerti, 45, who has been called "one of the top half-dozen pianists in the world today," is not as well known as he should be outside Canada. Perhaps it is because he goes his own way. A U.S. citizen whose parents fled Vienna and the Nazis in 1938, he moved to Toronto in 1965 to protest Vietnam. He spoke at peace rallies. He organized a demonstration at Aspen, Colo., stood on Defense Secretary Robert McNamara's front lawn and harangued him about napalm, a one-man Greek chorus, when the man came out to pick up his morning paper. Coincidence or not, two weeks later McNamara resigned.

Kuerti has never been invited back to Aspen. He has been told that things like this have hurt his career. He has been told that being a vegetarian and wearing no necktie hurt him too. "Who can tell?" he shrugs. "Gandhi says there are certain things you must do even if nothing comes of them."

He also refuses to be part of the standard concert circuit, with its inflated ticket prices and hustling impresarios. He'll play for $1,500 where he could easily demand $4,000. He plays in small towns as well as the cities. From here he takes his van to Rochester. Then India. Then Cuba. Since he won the Leventritt Award he has toured 25 countries, more than once repeating the feat that has given him a reputation as a Beethoven specialist: playing all 32 piano sonatas. He has recorded them for Columbia now, but the first time, in '75, when asked to do them for a Toronto recording firm, he only knew half of them and had to take a year out to learn the rest.

"I practice more than most pianists," he says. "I spend maybe 30 percent more time on it than most. But I have learned to enjoy it. I am always trying to make it touch me, move me. I hear it, the way I want it, in my ear, and I try to make it sound that way. I listen. I listen."