THE GOOD TERRORIST. By Doris Lessing Knopf. 375 pp. $16.95.

DORIS LESSING's The Good Terrorist is a fine novel, a work of strong and scrupulous realism, absolutely contemporary. It patiently wins our assent at each step, achieving a logic so strong that its climax, a car bombing in a crowded London street, seems, when it comes, inevitable. What set of circumstances will produce a terrorist act? Lessing provides a convincing explanation, denying herself the glamour of alienation, the irony of idealism, the romance of obsession.

"The good terrorist" of the title is Alice Mellings, a woman of 36, member and guiding spirit of a fractious little clutch of self-declared revolutionaries in London who dream of attaining "seriousness" by linking up with the IRA, or with the Russians, whichever will have them. They seem to be motivated by no passion grander than spite or more dangerous than the thrill of ritualized tussles with the police. Alice makes a home for them. She dreams vaguely of a general destruction, after which the injuries people suffer at the hands of society will be no more. "A clean sweep, that was what was needed. And Alice saw a landscape that had been flattened, was bare and bleak, with perhaps a little wan ash blowing over it." But her real energies go toward an unconscious reenactment of middle-class life, the gentrification of a "squat," a house condemned by the authorities, vandalized by them so as to be uninhabitable (concrete poured into the toilets, wiring pulled from the walls) and inhabited anyway, first by a black youth and then by our revolutionaries, who give him to understand that he is not especially welcome there. Alice is devoted as any wife to a homosexual named Jasper who tolerates her love and takes her money. She cooks, cleans and shops, occasionally injured, occasionally gratified, as her efforts are scorned or acknowledged. As some four or five of the Communist Centre Union sit down to her kitchen table, she thinks, "It is like a family!"

This is a novel set in a society pervasively deceptive and self-deceived. These revolutionaries, having of course no regular employment, are clients of the Welfare State, and as such they flourish well enough in the existing order, especially when Alice brings her middle-class "authority" to bear on officials and police, winning help and concessions where the tired and poor are merely sent away. By dint of hard work, and grit, and theft, and a willingness to impose on the grudging trust of an unemployed carpenter -- he is constantly cheated of his labor -- she achieves such an aura of respectability that it extends its protection even to the "serious" revolutionaries in the squat next door, and their nocturnal trafficking in guns and explosives.

Postwar Britain is an interesting phenomenon: Disparities in mortality rates between social classes are greater there today than they were in the 1950s and a working-class youth has a relatively poorer chance now than he had then of attending a university. The Welfare State, so called, has proved to be a great engine of stasis, and The Good Terrorist shows us how it works. Quite simply, it responds with respect and a measured liberality to those it recognizes as middle-class. The poor find no help.

Alice's house, spared temporarily from its scheduled destruction by the authorities, seems like a refuge, but the social idealism of its tenants is merely another version of the ethic that prevails outside their doors. Every perishing soul that comes to them in due course perishes. Where they do not abet the workings of fate, they do not intervene either, and Alice more than once feels contempt for these "victims, born to be trampled over and cut down," an emotion she knows is shared by their bureaucratic Destinies.

The terrorist act with which the book ends is no more an outgrowth of ideology and alienation than it is of the dissociation that allows such victimization to occur, among professional alleviators of need on the one hand and committed enemies of social injustice on the other. Lessing gives us a convincing portrait of those famous forces of historical change that make things everlastingly the same.

THERE IS a very great absence of self-awareness among these revolutionaries. They are content to imagine evil as a thing outside themselves, readily identifiable, and liable to destruction at no moral cost to themselves. As the price of making the conflict of good and evil a historic rather than a psychic or spiritual drama, they seem to have lost the power to charm their own demons. Alice plunders her parents, rants at them, then forgets instantly that such things have happened, and never dreams that her parents might be seriously offended. Her worst impulses have a furious, autonomous energy, unrestrained by doubt or guilt or even a realistic sense of damage to her own interest. Others in her party speak in altered accents -- in British culture a considerable act of self-concealment. But in rage or weariness hidden voices emerge, allowing glimpses of urgent, unacknowledged selves, strangers in the household, of whom nothing is known.

Alice's mother, whose Old Left sympathies have begun to yield under her burden of sadness, says everything is mess and muddle and so it seems. Do alien interests contrive to put explosives in the hands of our little band, or is it chance? And does it matter? They see others like them whisked away to be trained in famous spy schools in Lithuania, and they meet Russian products of these same schools, trained to pass for Americans and almost convincing at it, though not quite. Behind the farce, is some stark intention taking shape? There are, Mrs. Mellings announces, reds under the bed. And so it seems. They are not, however, the problem any more than they are the solution. The world described in this novel is characterized by a profound disintegration of the moral self, the new Manicheanism that, taking the distinction between good and evil to be clean and easy confounds the two utterly. Lessing's realism achieves moral resonance simply by placing this dissociation in a world crowded with causal connection.