IN CHAPTER eight of Thomas Mann's lofty metaphysical novel "Doctor Faustus," which is about a fictional German composer named Adrian Leverku hn, one of the book's more lofty outpourings in the search for eternal truth begins this way:

"What did he talk about? Well, the man was capable of spending a whole hour on the question: Why did Beethoven not write a third movement to the Piano Sonata Opus 111?"

And then Mann's narrator goes on at what seems to be interminable length to explore the implications of this question about Beethoven's 32nd -- and last -- piano sonata, without (as is typical of such pontificators) ever answering the actual question.

Still, even though this bit of philosophizing doesn't seem to make any particular sense -- which is precisely Mann's point -- the question itself has a ring of validity. That is because this sonata from Beethoven's desolate old age, as well as the two similar sonatas that come just before it, are as much mighty spiritual experiences as they are magnificent vehicles for the piano.

This is no longer the proclamatory, defiant Beethoven of the Fifth Symphony or "Fidelio." It is no longer a man challenging life, but one who is looking back at life and, at times, almost caressing it.

It is introspective music in which the unexpected pauses and harmonic ambiguities can be as eloquent as the passionate melodic lines. These works have an emotional and intellectual density unexcelled anywhere in keyboard music.

The last sonatas will be the subject of an extraordinary event at the Kennedy Center Concert Hall tomorrow night. Rudolf Serkin, by common consent the most eminent Beethoven pianist today, will play these three works in sequence. It will be a feat of physical strength, of which Serkin continues to have plenty, but more significantly, it will be a feat of concentration and emotional stamina. Many major pianists avoid these works altogether, because they are such tests of one's interpretive powers. But Serkin has always been bold, and never more so than in tackling these profound expressions from the aging Beethoven just three months short of Serkin's own 80th birthday.

The last sonatas are unconventional, but rather similar, in form. Each is dominated by a lengthy final movement, to which the earlier movements seem to be sharply contrasted preludes. Two of those finales, both of which seem cosmic in scale, are in the theme and variation format--in the 30th sonata, Op. 109, and in the 32nd. One has to hark back to Bach's Goldberg Variations to find keyboard works in this form that are on such an elevated plane. The format of the 31st sonata harks to classical models as well, with a majestic fugue.

That nostalgic, but dry-eyed, sense of caressing life is evoked from the very first E major notes of the 30th sonata, which is scheduled to begin Serkin's recital. It is a dreamy rhapsodic melody upon which Beethoven promptly juxtaposes some of the most rigorous rhetorical complications. He seems to be posing the yin and yang question of peace versus turmoil, which is at the root of all three works and which also finds resolution in those sublime final movements. There follows a short prestissimo movement that passes like a gush of air leading to the prayerful opening theme of the finale. It has six variations of a prayerful theme that are of heart-wrenching intensity. They finally lead back to the prayerful theme, knit in a web with those almost endless trills in the high register with which Beethoven in his later years would so touchingly evoke the mystery of life, and of death.

If the 30th sonata is primarily a devotional work, the 31st, in A flat major, is a passionate affirmation of life. It opens with a sinuous movement that is probably the most conventionally direct one among these works--a quietly joyful piece. Then there is a reflective slow movement that leads to an arioso joined without a break to the massive fugue. It opens with a full workout for the subject. That is interrupted by a repeat of the arioso that leads in turn to the fugue again, but this time inverted. The sense of affirmative momentum keeps swelling and finally explodes into a final fortissimo A flat major chord that reaches over five octaves of the keyboard. These last two movements are so interlinked that they are in effect one movement, throwing into doubt the validity of the suggestion of the charater in "Dr. Faustus" that it is odd that the last sonata has only two movements.

The 32nd, in C minor, is more compressed, though, less discursive. In the thundering first movement Beethoven is wrestling more strenuously, in a real rush, than anywhere else in the sonatas with that yin and yang question about the nature of life.

Then comes the theme and variations, an ethereal song of love and commitment that is Beethoven's farewell to the piano sonata. The quiet ecstasy of this movement is just about beyond compare in keyboard music. It is in that most elemental of keys, C major, and consists of a theme of the most elemental simplicity that is gradually put through a far-reaching set of variations and ends in cathartic affirmation.

The last three sonatas sum up three of the fundamental emotional themes of Beethoven's lifetime. To hear all of them played together in one evening by a Serkin is the chance of a listener's lifetime.