One could make much the same point about Pinter, too. That is not, however, to say that Mr. Mamet is heavily influenced by him or, indeed, by anyone else. Life is his tutor, and he has learned his own lessons from it, as far as both language and ideas about language are concerned. Nor do his interests stop there, whatever his critics may think, however deficient they may find him intellectually. It was Edward Albee, I believe, who said he showed little evidence of a mind as fine as his ear. The theatrical scholar Ruby Cohn has rather similarly called Mr. Mamet ''virtually bare of ideas.'' The answer to that is that ideas aren't any less ideas for being lived out rather than stated. Actually, if I've one criticism of ''American Buffalo,'' it is that its ideas are sometimes too explicit, too obvious.

''The play is about the American ethic of business,'' Mr. Mamet himself once said. ''About how we excuse all sorts of great and small betrayals and ethical compromises called business.'' In fact, that could be the epigraph to several of his plays, from ''The Water Engine,'' in which corrupt money-men suppress a machine that costs nothing to run, to ''Glengarry Glen Ross,'' now at the National Theater in London. This is probably Mr. Mamet's most scathing piece to date, a comprehensive study of the real-estate shark pool, where the only law is swallow or be swallowed. Beside its razor-teethed salesmen, the characters of ''American Buffalo,'' though on or over the criminal fringe, seem strangely scrupulous. At any rate, they have to convince themselves that burglary is ''business'' and not a reason for ''shame.'' Indeed, Mr. Mamet's implied point is that, if crooks can identify with businessmen then businessmen are probably crooks. As he has also said, ''There's really no difference between the lumpen proletariat and stockbrokers or corporate lawyers who are the lackeys of business. Part of the American myth is that a difference exists, that at a certain point vicious behavior becomes laudible.''

But Mr. Mamet isn't simply a latter-day Dreiser, yearning to trim the capitalist jungle. What really interests him in ''American Buffalo'' are the shadowy, elusive frontiers between business and friendship, greed and altruism and, dare one say, evil and good. Donny, who owns the junk shop, plans to demonstrate his faith in his ex-junkie gofer, Bobby, by sending him to rob a coin-collector. But Teach, who needs money, persuades him that this is bad business and substitutes himself as burglar-in- chief. Everything then proceeds to go awry; and, as it does so, Teach gradually sucks Donny into his own embattled, paranoid world-view. All relationships are really ''business'' relationships. Even Bobby is repaying Donny's kindness with betrayal. There's no trust, no friendship: ''We all live like the cavemen.''

It's a despairing analysis; and it is, I think, his despair that transforms Teach from a two-dimensional hood into one of the richest characters in the contemporary repertory. Behind the anger, the bravado, the violence, the solemn rhetoric, there's an over-age infant dimly aware he can never claw his way out of the urban trashcan. This insecurity, this unease is much signaled by Mr. Pacino at the Booth. Black-eyed, mushroom- faced, he paces the junk shop, looping his hands, jabbing with his fingers, nervously stroking his hair, absently touching an old tennis racquet here, a bowling pin there. So restless is he that one is tempted to repeat the advice Peter Ustinov once gave a young Method actor: ''Don't just do something - stand there!'' For one begins to wonder if Mr. Pacino's obsession with busy detail doesn't explain his central failure, which is to embody that desperation, to communicate the feeling of hanging by the fingernails over that ash-filled abyss. One hears the voice of raging disillusion, one sees the gestures, but one doesn't quite believe their source is the heart and stomach.

Yet the play is strong enough to survive both that and Mr. Brown's disgracefully offhand treatment of its moral climax. Is Teach's glum assessment of the human condition correct or not? Just as it begins to seem so, the truth of Bobby's supposed treachery emerges. He did not, as Teach believes, double-cross his partners-in- crime by stealing a valuable nickel behind their backs: he bought it, ''for Donny.'' This is the line that James Hayden's Bobby manages to drop, and the reason it matters is that it represents the point at which an apparently unstoppable tide suddenly yet naturally reverses itself. The world isn't, after all, exclusively populated by cavemen and cannibals. There is loyalty and friendship, even down there, in the lower depths.