The Thanatos Syndrome<i> by Walker Percy (Farrar, Straus & Giroux: $17.95; 372 pp.) </i>

“Thanatos” is the Greek word for death, and in his newest novel, which might be described as a philosophical thriller laced with Southern Discomfort, Walker Percy proposes that tenderness leads to the death ovens.

He proposes this in the person of a disarranged Catholic priest who lives on a fire-watch platform in the Louisiana scrubland and periodically comes down to preach. But with Percy, who is ingenious and paradoxical, truth increases as the square of disarrangement.

To make a motto of it: Beware of everything in American life that is sleek and plausible and seeks to smooth over human prickliness in the name of human convenience.

The America in which “The Thanatos Syndrome” is set is a few years ahead of our own time. The abortion debate seems to have been left behind. Humane arguments for the rights of the strongly alive over the weakly alive have prospered to the point where the Supreme Court has ruled that an infant is not a human being before 18 months. Qualitarian Centers--in extreme and carefully counseled cases and with the relatives’ consent--arrange the elimination of the hopelessly ill, old and the badly handicapped young.

And Father Simon Smith, spiritual descendant of that other high-up liver, St. Simon Stylites, fulminates in disjointed phrases. As a young man, he studied in Nazi Germany and found himself dangerously attracted to the plausibility of a socially engineered Greater Good that argued the need to cut a few corners.

Humanity is nothing but corners, though, and once you begin cutting, where do you stop, short of Dachau? An excessive question, no doubt, and Father Smith grows excessively unhinged. His sermons are against the well-intentioned arrangements of his comfortably purblind countrymen. He tells his congregation that they are good people, but lethal.

“Look at you,” he says. “Not a sinner in sight. More people have been killed in this century by tenderhearted souls than by cruel barbarians in all the other centuries put together.” He pauses. “My brothers, let me tell you where tenderness leads . . . to the gas chambers! On with the jets!”

Father Simon’s warnings that the bottom-line arrangements of modern society are a step away from mass murder, provide “The Thanatos Syndrome” with wonderfully jittery background music.

In the foreground, the theme is fleshed out by a comfortably shambling Southern Bourbon, Dr. Tom More. Another saintly reference, of course; this one, to Henry VIII’s counselor who overcame his worldly nature to serve his conscience instead of his master.

Virtue, in Percy’s books, is more likely to come from old privilege displaced than from new privilege asserted. More is the straggly offshoot of aristocratic forebears in the Louisiana parish of Feliciana. He is a psychiatrist whose business has suffered from his old-fashioned reliance on pure analysis rather than the newer chemical treatments.

The anti-chemical bias is a kind of conversion. It dates from a term in prison where More was sent for a morally complicated scheme to sell amphetamines to drowsy truckers.

When the book opens, he is out of jail and has resumed his practice. And he finds his old patients altered in strange ways.

They seem calmer, for one thing. His inhibited women patients have put on weight and become languorously, aggressively sexual. A black caretaker, with whom he’d had prickly if amiable dealings, suddenly turns bland. People’s memories for facts and figures have become amazingly sharp; his wife has suddenly acquired an extraordinary talent for computing probabilities in bridge. On the other hand, people have begun to speak in oddly curtailed, two- and three-word sentences.

The puzzle set for the ruminative More becomes the book’s puzzle. With the help of Lucy, a distant cousin who is also a doctor, and in love with him, he turns detective. Their investigation, conducted by computers and by snooping along the bayous and marshes of the parish, is utterly engrossing.

They discover that the local water treatment plant has been rigged to add a sodium derivative to the water supply. The rigging has been done by an ambitious local doctor and a shady, high-flying entrepreneur. The purpose is to suppress irritability, antisocial behavior and other qualities that complicate human nature.

As the doctor, Bob Comeaux, explains after More finds him out, what they have done is no different from fluoridation. Instead of decay, what is being eliminated is all the troublesome activities of the superego, all the qualities of aggression and self-doubt that interfere with human happiness. He recites all kinds of statistics: Crime is down; productivity is up; the local football team’s performance has vastly improved.

On the other hand, people are relapsing into contented animals. Contented, and sometimes vicious. In an unnecessarily melodramatic twist, Percy makes the entrepreneur the ringleader of a group of pedophiles who run a boarding school and assault the sodium-doped children.

“The Thanatos Syndrome” does, finally, turn plot-heavy, though for the most part, the ruminations of More, his growing catalogue of suspicious evidence, and his arousal from rumination to action are recounted with graceful suspense.

If the climax is laborious and rather creaky, it is, I think, because the author has begun to take less interest in his characters and more in the lesson they are propounding.

More--earthy, slightly eccentric and, finally, just a little too good at scoring for humanity and routing the villainous technocrats--acts as a human filter for Father Simon’s manic signaling. His adventures decode the signals.

There are no gas chambers, but the misplaced Utopianism of the water-dopers ends in the dehumanization of both the manipulators and their subjects. Percy’s warning against those political, social and economic forces that seek to make life easier by making it uniform is not new. Aldous Huxley was at work on it 50 years ago. But Percy has brought it engagingly up to date.