Critics trying to deal with the idea of musical interpretation might keep in the back of their minds what Isaac Babel said, that ''if the world could write by itself, it would write like Tolstoy.'' Something similar drew me admiringly to Paul Lewis's evening of piano music at the 92nd Street Y on Saturday.

Listening to Mozart and Schubert was like listening to Mozart and Schubert, not like observing a man deciding how Mozart and Schubert should be played. Seemingly eliminated was the performer as middleman, as the wholesaler who takes goods from the composer's warehouse and dresses them up for public consumption. Mr. Lewis is more ''factory to you,'' as the old cut-rate advertisements used to say.

Mozart's C minor Fantasy (K. 475) and A minor Rondo and Schubert's Sonata in G (D. 894) seemed to breathe on their own, without anybody's help. You would be right in pointing out that all this ''natural musicianship'' is really the result of small decisions by the hundreds, but that is something I don't want to think about it. Leave me with the illusion that Schubert's ever-beautiful sonata on Saturday was the world writing about itself.

Ligeti's ''Musica Ricercata,'' placed between the two Mozart pieces, plays a game of addition. The first of its 11 movements uses a single pitch, the succeeding sections each adding another of the chromatic scale's 12 tones to the mix. The earlier movements are interesting for the tests they pose to a composer's maneuverability, given his narrow leeway. At the same time he is having fun with old-style Lisztian rhetoric and the waltz-time styles of Chopin and Schubert.

Later on, Ligeti's Hungarian side appears in dancing upside-down dotted rhythms (short-long instead of long-short). Like any good comedian Mr. Lewis played his material straight.

The Mozart rondo is almost Schubertian in its wistful theme. The variations that follow seem to occur on the spur of the movement, unfolding like a string of choruses improvised by a jazz performer. The Schubert sonata wanders where it will, like sleepwalking. The Andante is self-induced hypnosis, rocking back and forth between minor and major. Schubert is in no hurry, and neither was Mr. Lewis. I particularly admired his stubborn constancy of tempo at times when the temptation to indulge the thrill of the moment is always strong.

PHOTO: Paul Lewis performing at the 92nd Street Y on Saturday night.(PHOTOGRAPH BY NICHOLAS ROBERTS FOR THE NEW YORK TIMES)