The old adage of the creative writing teacher, “Write what you know,” has always sounded like a counsel of despair. If every writer only wrote what he knew, there would be no books in the library but memoirs; the whole premise of the novel is that it is possible to know things you never experienced. But change the phrase slightly, to “write what you can imagine,” and it becomes a good rule of thumb for fiction. The only reason to write a novel, and certainly the only reason to read one, is if the novelist imagines his creation so intimately that it becomes, paradoxically,more real than reality. The best fictional characters, from Elizabeth Bennet to Moses Herzog, have the uncanny ability to seem closer to us than our actual acquaintances. Austen and Bellow have smuggled them, through sheer imaginative cunning, across reality’s border.

The peculiar and striking badness of “Terrorist” (Knopf, 310 pp., $24.95), the latest novel from the busy desk of John Updike, stems from its complete failure of this kind of sympathetic imagination. Obviously, Mr. Updike cannot know firsthand the world he writes about in “Terrorist.” The inner-city high school that breeds despair in Ahmad, the 18-year-old protagonist, and the shabby second-story mosque that offers him a desperate hope, are equally far from the suburban, bourgeois, post-Christian milieu of Mr. Updike’s best fiction.

Such distances have never stopped Mr. Updike from mounting his fictional expeditions. If Rabbit Angstrom’s world is the one he knows, he has imagined many others, from the medieval Danish court of “Gertrude and Claudius” to the postapocalytpic America of “Toward the End of Time.” But in “Terrorist,” the expedition never even gets going; Ahmad is stranded on the far side of the border, not a person but a clumsy impersonation. As a result, “Terrorist,” which so deliberately sets itself to address big, headline-making themes – religious fanaticism, political violence – reads like just that: a grab for a major subject, motivated by a desire for relevance rather than genuine curiosity.

To see how Mr. Updike’s failure of imagination reveals itself at the level of the concrete detail, take the scene where Ahmad visits a black church in his decaying New Jersey town of New Prospect. Ahmad, the son of a promiscuous Irish-American mother and an absent Egyptian father, is supposed to have taken to Islam at the age of 11, mainly as a way of connecting to his missing parent. He stalks through the halls of Central High, a mostly African-American school in a city rather like Newark, fuming with disgust for its fallenness: “Devils, Ahmad thinks. These devils seek to take away my God”are the novel’s (ever so subtle) opening lines. But Ahmad’s fierce puritanism is challenged by the flirtations of Joryleen, his beautiful classmate, who invites him to come listen to her sing in a gospel choir. Despite the double blasphemy involved in going to a Christian church out of lust for a woman, Ahmad can’t resist, and finds himself in a pew on Sunday morning. This is how Mr. Updike introduces the sermon that he hears: “Then a long prayer is offered by the Christian imam.”

Now, exactly whose consciousness, whose view of the world, is supposed to account for the designation of a minister as a “Christian imam”? Ahmad is not a recent immigrant from Saudi Arabia; he is a born-and-bred American, raised by a Christian mother, who has presumably heard of ministers and priests, and maybe even seen a few. The very fact that he converted to Islam would, if anything, make him hypersensitive to the distinction between an imam and a pastor. He would surely not need the clumsy comparison “Christian imam” to identify the man giving the sermon in a Protestant church.

No, the only reason why Mr. Updike employs this phrase is to remind us, with a crude nudge, that Ahmad is a Muslim. A pious Muslim, Mr. Updike implies, is a kind of alien beamed down to Earth, who can only comprehend the local customs through literal-minded analogies with his home planet. But what this really means is that Mr. Updike himself cannot imagine what it is like to be a pious Muslim. It is to cover this failure that Mr. Updike supplies such large quantities of ostentatious book-learning about Islam: “There has been a recent, rather amusing controversy over the scholarly dicta of a German specialist in ancient Middle Eastern tongues, one Christoph Luxenberg,” and so on. That this is supposed to be said by Shaikh Rashid, the terroristic, fanatically devout imam, to his pupil Ahmad, only heightens its absurdity: No one talks like this, and certainly these people in this situation would not talk like this. (Note, too, the mustache-twirling tone of “rather amusing controversy,” Mr. Updike’s stab at making Shaikh Rashid sound icily villainous.)

This is only one example of the tonal awkwardness that besets “Terrorist” on virtually every page. Like a clumsy mimic whose impressions all sound the same, Mr. Updike’s characters all speak the same lifeless prose. “He feels that such a relativistic approach trivializes religion,” says Ahmad of his imam; “Ah, Paradise; one can hardly wait,” says the imam himself. One thuggish character is called Tylenol, Mr. Updike’s ludicrous idea of black American’s name.

Unsurprisingly, the parts of “Terrorist” that seem plausible are those in which Ahmad remains offstage. Then Mr. Updike can turn his attention to a much more familiar subject, the lusts and disappointments of the aging male.This time out, the male in question is Jack Levy, Ahmad’s depressive guidance counselor, who embarks on an affair with Ahmad’s bohemian mother, Terry. The affair returns Mr. Updike to familiar territory, and if the reader feels he has seen this kind of aestheticized sexuality before – “the freckled skin of her upper chest looks a bit crepey … in contrast to the soap-white strip this side of the bra’s edge” – that is because it is what Mr. Updike does best.

Yet “Terrorist” is inevitably drawn back to Ahmad’s story, and to the conspiracy that turns him from a confused teenager into the titular monster. Exactly how this conspiracy plays out would be unfair to reveal, since the last 30 pages of the novel are devoted to its suspenseful unfolding. Suffice it to say the culmination of “Terrorist” feels as arbitrary and underimagined as everything that has gone before. It is hard to care whether Ahmad will blow himself up, when he has never really been alive in the first place.

akirsch@nysun.com