What Sam Shepard specializes in is the ill-made play. He's a genius at it, an originator whose creative impulse owes more to the fractures of life than to the strictures of drama. Often critically dismissed because he doesn't write by the rules: long considered willful, intractable, amok - for all that, the truth is that Sam Shepard is the single most important American playwright since Eugene O'Neill. In my book, he has written two major plays, "The Tooth of Crime" and "Buried Child," and now a masterwork, "A Lie of the Mind," which opened last week in New York at the Promenade.

If Shepard has, in fact, raised the ill-made American play to a status of dramatic art, critics - as Richard Gilman points out in his introduction to "Seven Plays" by Shepard - have a tough time with him. (Critics may be classified here both as journalists and academics, the regular aisle-sitters and the armchair analysts.) Shepard, 42, began writing in 1964 and has written 31 plays. Shakespeare, who died at 52, is generally credited with 36; O'Neill, who died at 65, with 45; Tennessee William s, dead at 71, with 24, not counting innumerable one-acts. Harold Pinter, 55, has written more than 20; Edward Albee, 57, 23; Pinter is still at it. Albee, who hasn't written in four years, is in decline.

It is not, of course, quantity that counts, although quantity at least indicates continuation of dramatic experience. In Shepard's case, that continuation seethes with sustained power. If you add to that sustainment a startling imagination - which leaps from a nonintellectualized approach - you have a maverick master dramatist. You have Sam Shepard, the long-haired, drugged-out 1960s rocker transformed into a rawhide Western hero pining for a lost America.

From the beginning - for me, that is, with a play called "Icarus' Mother," which the Theater Company of Boston staged in 1966 - Shepard seemed uncommonly talented. In the range of Shepard's later work, this early play, with its peculiar sense of nondirection, its collagelike effect, would establish the inimitable Shepard tone, a tone that would come to sound like rock 'n' roll blare with the lyric (meaning) trailing off as a loud but unclear whisper (no exact meaning, only suggestibility). Unlik e other playwrights who began improvisationally (O'Neill's early "sea plays," Albee's vaguely surreal one-acters), Shepard began without format, without preconceived notions of what theater should be. In a sense, he was a man expressing loosely eloquent dreams, scaling them like pebbles across water. His inspiration, he has suggested, came from The Who, the Rolling Stones, Credence Clearwater, Janis Joplin, Jimi Hendrix, all in tune with the scattered cacophony of Beckett and "Waiting for Godot." This was later refined into a statement that is pure Shepard and was published in 1977 in "Contemporary Dramatists": "I'm pulled toward images that shine in the middle of junk. Like cracked headlights shining on a deer's eyes, I've been influenced by Jackson Pollock, Little Richard, Cajun fiddles and the Southwest."

Shepard became a playwright out of desperation. He wanted to be a rock 'n' roll star. Even now with his playwriting career publicly upstaged by his silver-screen sainthood as the Gary Cooper of the 1980s (in "Days of Heaven," "Raggedy Man," "Frances," "Tne Right Stuff," "Country" and "Fool for Love"), Shepard's comments about himself or his art are almost taciturn. But here, I think, are two of the most revealing: "A lot of people think playwrights are some special brand of intellectual fru itcake with special answers to special problems that confront the world . . . that's a crock." And, "I consider theater and writing to be a home where I bring the adventures of my life and sort them out, making sense or non-sense out of mysterious impre ssions . . . language is a veil hiding demons and angels which the characters are always out of touch with. Their quest in the play is the same as ours in life - to find those forces, to meet them face to face and end the mystery."

In the early plays ("Icarus' Mother," "Cowboys," "La Turista"), Shepard's adventures were tantalizing but difficult to grasp. Yet they effectively mirrored the inchoate images of Shepard's time, his stumbling through a confused (and confusing) world and, presumably, our stumbling along with him. Whatever else the plays did or didn't do, they became - like much of contemporary painting (Magritte to Pollock) and music (from the Beatles to Stockhausen) - the dramatic equivalent of what they were about. As Shepard developed, he continued with this conceit, not necessarily refining his technique (which bothers a lot of critics) but deepening his impressions. Shepard went astray in the mythically oversized "Operation Sidewinder" (produced at Lincol n Center in 1970) in which he imagined a snake slithering through a dim fable, raping a woman and eventually being revealed as a White House computer with arcane powers. "Sidewinder" was sloppy, a mistake, it failed to connect on any level, and Shepard w as later to blame the production.

Then, in 1973, came "The Tooth of Crime," a brilliant, hard-headed, unforgettable play that takes the form of a deadly "game," a countdown, between two rock stars, victim and victor, a has-been and his up-and-coming nemesis, Hoss and Crow. Here, Shepard, stretching himself beyond the fragmented give-and-take of his early work, began to announce the outlines of a theme, then, in an invented language, to sing riffs around it. Within both the milieu and metaphor of the rock world, "The Tooth of Crime" tells us what the 20th century has become: a Babylon of hype where heroes are changed as readily as shoes. And it does all this still without resorting to the conventional forms of drama: clearly arranged images; pattern of thought; defined, psychologically motivated characters caught in a struggle from which something is to be learned. Instead, Shepard simply shows things as they are through the slant of his perception and experience. His attitude is that it's too late for lessons or cautio n, there is only the quirkiness of ongoing life. The competition between Hoss and Crow and the theme of loss ("We were warriors once") behind them (and us) was then picked up by Shepard, riffed again, penetrated and developed in the so-called "family" pl ays Shepard went on to write, "Curse of the Starving Class," "Buried Child," "True West," "Fool for Love," "A Lie of the Mind." Off to one side would come experiments like "Tongues" and "Savage / Love."

In "A Lie of the Mind" Sam Shepard rages - dramatically and comically - through the bleak houses of two American families, charging against their dreams and their depression, their unchangeable past and their anxious future. While "Lie of the Mind" is quintessential Shepard - deliberately ill-made, or made by his own absurdist rules, often indulgent scene by scene, repetitive in its final moments, eruptive, clotted, subliminal and steadily astonishing - it also signals another development in Shepard's playwriting: the importance of realistic plot. What Shepard is telling us - with a series of memorable images tied to expressive language, then knotted somewhat needlessly on down- home music provided live by the Red Clay Ramblers - is profound : We are all trapped in the masks that our past and present force us to wear, all suffocating behind bonds and pressures that have nothing to do with our true, individual identities. We live lies of the mind; some of us escape, some don't. This perceptio n may have been struck from Freud, Pirandello, Genet and Beckett, but Shepard's expression of it is all his own. The play is a confrontation (scorching and hilarious) between two families, between a dead father and his children, between brothers, between three husbands and three wives, including one husband who batters his wife into brain damage. Pulsing in the background is the central theme of social and political betrayal, the erosion of the American West and, with it, the promise of the country.

Don Shewey's recent paperback "Sam Shepard," a good, readable life-and- career study put together without the playwright's cooperation, closes with a Shepard quote Shewey borrows either from an unspecified interview or a program note to a Shepard play. "I'm driven by a deep dissatisfaction. What you accomplish in your work always falls short of the possibilities you know are sneaking around. The work never gets easier. It gets harder and more provocative. And as it gets harder you are continually reminded there is more to accomplish. It's like digging for gold. And when you find the vein, you know there's a lot more where that came from."

As American playwrights go, Sam Shepard is the gold rush.