WHEN ASKED "Why do you write?" a professional novelist has no difficulty in finding his first answer, usually regarded as cynical and venal: "I write for money." This sounds terrible because the only novelists who get into the news are the very wealthy, and the term "money" seems to mean the Marxian Mehrwert , excess profits squandered on yachts and villas and alimony while the Third World starves. "Money" to me means merely the stuff I pay the rent with. Subsistence is the first law of life, and novelists are, unlike real poets, not exempt from obeying it.

But if my concern were only subsistence I would practice a far easier trade than confecting fiction for the discerning reader. Writing is excruciatingly hard, and the more you write the harder it gets. Yeats' words -- "The fascination of what's difficult" -- are pertinent to my kind of novelist. Masochism comes into it too. Also, and this is hard to articulate, there is a profound personal need to say something -- not directly, like a mafioso or politician (I see now those examples are ill-chosen) but deviously, laterally, wrapping the message in so many layers of paper art that, with luck, the reader may fail to get the message and take in only the art.

My new novel is very long, and I evidently had a need to express myself at some length -- a need which came before even the conception of subject-matter. I will write a novel of some quarter-million words: this is analogous to expressing the desire to compose a sonnet or a symphony. Shape and size are, in the initial creative stirring, enough. For an American to write a novel of such length is not unusual: a big nation needs big books, not the 50-page fripperies which pass for books in decaying France. But for a British writer the situation is very different. We are used to turning out slim fiction of 80,000 words or so, chiefly because time and income are about equally ballanced in such modest enterprises: what or who, in the absence of campus sinecures, will subvent a British Anna Karenina or From Here to Eternity (a fair description of the length of James Jones' novel)?

The subject-matter of my Earthly Powers was, I now see, rumblings in the unconscious and sending only one message, in that gestatory phase, to the conscious: make it long, it needs length. To state, now and briefly, what the subject-matter is is difficult. I have been told that any novel should be capable of summary on the back of a postage stamp and even be epitomized in the title. Pride and Prejudice is about pride and prejudice, and Humboldt's gift is the theme of Humboldt's Gift. My original title for my new novel will be retained in Europe, but it has been considered too much of a mouthful for laconic America. It was, is, The Prince of the Power of the Air, which comes out of Thomas Hobbes' Leviathan and may be considered highly, Europeanly, ambiguous. The air is full of angels and their prince is good. The air is full of devils and their prince evil. The novel is about the difficulty of deciding what is good and what is evil.

The narrator is an aging homosexual writer modeled on William Somerset Maugham. His sister married an Italian musician whose brother was a priest. This priest rose to be Cardinal Archbishop of Milan and eventually, in 1958, Pope Gregory XVII. He can, if the reader wishes, be identified with Pope John XXIII. The homosexual writer narrator is asked by the Vatican to confirm that he witnessed a miracle performed by Pope Gregory, when he was still only Monsignor Campanati, in a Chicago hospital in 1929. The narrator, whose fictional trade leads him to confuse the imagined with the actual, is forced to look back over his long life, assemble the undoubted historical and biographical and separate them from what is fabled by the daughters of memory. He sees the miracle clearly; a small boy dying of meningitis is restored to life by the laying on of hands and a simple prayer.

But what happened to this boy in later life? Surely God, in subverting the processes of nature, had some special intention for him? In discovering this special intention we come face to face with the great mystery of good and evil. Perhaps it is too easy to think in terms of a perpetual war going on between God and the Devil: the universe is not sustained by so simplistic a dichotomy. Perhaps God, if he exists, is beyond good and evil and is merely an ultimate power to whom human morality is of no interest. He is on nobody's side. The novel, tracing as it does the memories of a life which has seen plenty of public, as well as private, evil, presents not only the big moral problem but also the problem of free will. How free are we? The homosexual narrator believes we are not free at all. He never willed his homosexuality, and yet the Church tells him that homosexuality is sinful. Is one then born into a state of sin which can only be redeemed by putting off action -- in the instance of the homosexual, the action of genuine love between man and man?

I'm not theologian. All I can do as a novelist is present situations which perhaps theologians can explain, though I doubt it. What I've tried to do is to invent characters typical of our age -- British, American, Italian, German, black, white, brown -- and show them in moral delemmas damnably hard to resolve. At the same time I try to show the 20th-century moral situation from the end of the Great War onwards. My Italian priest-bishop-pope believes that God is good, that God made man in his own image, that therefore man is good. Even the Jew-killing SS officer he captures and tries to bring back to the light is, according to this theology, good. It is the devil who causes evil: he gets into us and we have to exorcise him. But suppose a good act, a miraculous saving of life, produces great evil? This is not a question my pope character has to consider, since he is dead when it is asked about his own miracle, and he is on the way to becoming a saint. But what is a saint? There are saints in the story whom the world would call sinners.

I have, I see, said nothing about the novel. Certainly the above lame summary is no substitute for reading it. For me it is rather like trying to summarize the content of Beethoven's Eroica Symphony. A novel is a novel, a work of craft or art, a structure, a big meal that can't be turned into a vitamin capsule. I'm not advising anyone to read Earthly Powers: that is rather the job of its publishers. But I wrote it to be read and I should be glad if people would read it and perhaps even buy it. Why do you write? To make money. No, seriously, why do you write? Now, I think, you know the serious answer.