AS SURE AS every action breeds an equal but opposite reaction, any major new scientific paradigm is certain to provoke its mirror image among those whose religious faith requires a secure theological mooring. That's to say, among Catholics. It's a matter of the way we were brought up. Instead of being encouraged to tough out the knottier problems of faith like the kids down the street at the Old-Time Pentecostal Sunday School (whose motto was, "Darned tootin', we believe in Noah's Ark!"), we young Catholics were expected to reason our way to God, and this required being on friendly terms with Science.

But accommodation to the scientific age was a work of duty, not love. Catholics simply didn't want to be caught applying the torch to more heretics, like Bruno and Galileo, who were destined to have the last laugh. Semiotics, however, is another matter; it is a "science" as gloriously a priori as Aquinas' Summa. In it the old shadow-battles of nominalism versus realism can be staged again, and virtually any meaning can be adduced from any grounds by suitable manipulation of signifieds and signifiers. Theology redux.

So delighted is Walker Percy with his discovery of semiotics that he has given over the trade of novel-writing and taken up raw philosophy--without, however, abandoning a novelist's empirical wisdom, his sense that his books must be written for an audience. (For man lives not by semes alone.) Cannily, Percy has disguised his semio-theological tract, Lost in the Cosmos, as a self- help book--"The Last Self-Help Book," no less, according to his millennialist sub-title--and he takes care to dress his semiotic paradigms with exempla from lowbrow TV programs. No mandarin, he! Percy grooves on Love Boat, Fantasy Island, and (quite obsessively) Donahue (Percy is the author, after all, of The Moviegoer), and then he retires to his study and thinks deep thoughts about what he's viewed. To wit, that we are a lost, doomed people, alienated from our true Selves (there are diagrams showing the exact nature of the displacement) and given to booze, cheap sex, and ill-judged wardrobes.

But (you may protest) you're not like that. You're only a little alienated, some of the time, and anyhow you never watch Donahue or Love Boat. Percy isn't listening to your protests. He's got your number, and whether you like it or not he's going to tell you who you are and what you've got to believe, beguiling you all the while with chummy multiple-choice questionnaires and outlines for failed novels that he leaves, like an absconding god, for you to finish. His constant direct address to the second- person singular makes the book an oppressive experience for any you not in substantial accord with his neo-Thomistic logic-chopping and Southern-Gentlemanly condescensions. The Pope Himself could take lessons in infallibility from Percy. Some examples of Percy as a Defender of the Faith:

"For every Mother Teresa, there seem to be 1,800 nutty American nuns, female Clint Eastwoods who have it in for men and are out to get the Pope."

Sounds as if Percy watched All in the Family from time to time, too.

"Although Galileo fell out with the Church, he would hardly have taken so much trouble studying Jupiter and dropping objects from towers if the reality and value and order of things had not first been conferred by belief in the Incarnation." How's that for co-opting the opposition!

Actually, Percy is often amusing, and some of his cases of conscience (or "thought experiments") are effectively provocative, though usually one senses that he is playing with a stacked deck. But the book as a whole is so slipshod in its reasoning and cavalier, even whimsical, in its treatment of history, that anyone with two semesters of Western Civ under his or her belt will surely feel short-changed by Percy's purviews. In one portentously titled sub-chapter ("A Short History of the Demoniac Spirit of the Erotic and the Violent in the Christian Era, in the Transition from the Christian Era, and Finally in a Purely Technological Era"), Percy's succession of erotic ur-texts goes like this: St. Paul, St. Augustine, Dante, Chaucer, Don Giovanni, Fanny Hill (whose "sinister Charm" is anachronistically hidden by layers of "Victorian clothes"), thence to Joyce Kilmer, Betty Grable, and Stalin--all in a single page. Talk about montage!

"But," I wanted to object, "you've left out Shakespeare." And Cosi fan tutte has a much saner fix on its subject than Don Giovanni. And I don't see any evident connection between Joyce Kilmer's diminished view of the erotic and "an increase in violence with the rise in technology: 20,000,000 dead." As a History of the World, this is on a par with the Mel Brooks' movie of the same name. Lost in the Cosmos is neither good philosophy nor a good read nor yet a book likely to help any Self I Know of, including its author. CAPTION: Picture, Walker Percy; Copyright (c) 1983 Jerry Bauer