This is a first novel about a romantic triangle. Two strikes. But Phillip Lopate is an experienced writer in poetry and nonfiction (his four previous books include the senstive "Being with Children"), and he approaches his time-worn subject with acute perceptions and mastery of style. The result is a psychological novel of considerable distinction.

Its virtues are hard to convey, unfortunately, in a plot summary. Nearly half of the novel deals with the question of whether Eric Eisner will make love to Marie Curtin, who is having an affair with his oldest and best friend, Jack Bogardes. They do so, finally, rather unsatisfactorily, and the remainder of the book traces the long-range resonances of this fleeting encounter in the psyches of the three people involved.

Eric and Jack are mid-'60s graduates of Columbia University - members of a generation considered more permissive about sexual conduct than those born earlier (and perhaps - the evidence is still incomplete - those born later). But that is on the surface, at least in this case; Jack is the victim of a superbly elaborate neurosis, Eric the proprietor of a highly developed sense of honor and justice, Marie a paragon of indecisiveness.

"The triad is a notoriously unstable form," a friend tells Eric, too late to do him any good. "In sexual matters it nearly always resolves itself into another duo." The question here is which duo. There are three possibilities, for Eric and Jack have enjoyed a friendship so long and intense that it practically amounts to platonic homo-sexuality (complete with platonic dialogues), and in the early stages of the triangle, Marie sometimes seems to be the odd person out.

In the end everyone loses everyone else; the triangle dissolves into three isolated lines, with Eric (the novel's narrator and most sympathetic character) deriving the only perceptible benefits from the episode - a wry, philosophical perspective on life: "I listen to people, not hearing everything, but listening to the part that wil show me how they are neurotic, tautological, self-deceptive. It makes an interesting if limited music. An yet I end up attending to their hysteria with an envious fascination that has at the bottom of it a suspicion that they are really closer to life, in their confusions, than I in my self-controlled path."

En route to this arid plateau, Eric undergoes a series of experiences (they might almost be called processes), which are described in meticulous detail and which are the novel's true subject. What Lopate is really writing about is how an unhappy experience becomes a kind of wisdom - the things that can be gained, if one is lucky, during the partial and temporary personality disintegration that follows an unfortunate love experience. The subject is more familiar in French literature - from Benjamin Constant to Andre Gide - than in contemporary American fiction, but the experience is universal among societies where sexual conduct is hedged by traditional taboos.

Those who have been through these experiences will recognize in Lopate one who has also been there and who has mapped the territory meticulously, precisely. Others may rejoice, if they are interested, in the existence of a book that conveys some part of the experience without the considerable emotional expense of acquiring it first-hand.

At one point, referring to an earlier loss, Eric puts almost the whole process in a few words: "that climactic wound of enlightenment that had not left off hurting me and energizing me for the year or so I had spent digesting it." But the details behind this brusque summary are intricate and subtle: deceptions (beginning with self-deceptions), hesitations, ambiguities, terrible weaknesses and tiny, catastrophic flaws in the way good people perceive and react to one another. A whole pseudo-psychological jargon has sprouted in our time to handle these phenomena, and usually it is useless because it is always tinged with the bias of the one using it. Lopate uses it, too, because it is part of the scenery, but he does not use it blindly. As a good novelist should, he presents the whole experience, raw and ragged at the edges.

Ultimately, all that one can say after an experience (or a book) like this is that humans are a poor, crippled, struggling lot, huddling together to ease their mutual weakness but unable to stop hurting one another in their huddling. The thought is painful (perhaps there should be something better than people to love - but what?), yet it must be confronted. Phillip Lopate has presented one such confrontation in splendidly crystallized form.