Three-hour arguments about the solutions to poverty, the responsibilities of capitalism, the true causes of crime and the relationship of income to citizen morality are not hard to find in this town. On the contrary. But to find one in which all sides are represented, with unflagging originality and wit for all, is something else. That would seem to require a whole new spectrum of political theorists; but one old Fabian Socialist rascal can also do it alone, as is being proved again at Arena Stage, where George Bernard Shaw's "Major Barbara" is playing. It seems an apt time for "Major Barbara" -- but then, what time is not startlingly apt for that argument between the munitions millionaire and his daughter, the Salvation Army officer, over whether morality is the first human concern or a luxury for the materially satisfied? The Arena production, directed by Martin Fried, is a remarkably clear one, because it has the minor characters providing comic continuity while the central ones make the cerebral arguments -- clever as these also are -- with persuasive dignity. Biff McGuire, as Andrew Undershaft, and Christine Estabrook, as Barbara, are both so eminently reasonable, in addition to being attractively charged with their respective missions, that the difficulty is to avoid siding with both simultaneously. And Robert W. Westenberg, as Barbara's fiance, the professor of Greek and S.A. drummer, is charming at representing on stage this audience predicament. The humorous background comes in two styles: drawing-room comedy among the rich and low comedy among the poor. Mikel Lambert, Kevin Donovan and Charles Janasz are exact in their set roles as the imperiously dimwitted family. But the roles at the mission require something more, and Richard Bauer, as Snobby Price, the expert derelict who feeds the souls of the charitable, is not just funny but unnerving. When June Hansen goes entirely for the humor, in playing Barbara's superior officer who disillusions her by taking her father's money, she seems only an amusing hypocrite; and the greater point, that she understands and operates in Undershaft's world, as any such executive would have to regardless of motivation, is lost. The settings, by Tony Straiges, are simple by Arena's recent standards, but when an enormous cannon dominates the stage, it certainly speaks for itself. Perhaps, however, the actors could stop fooling with it, as if any minute now they're going to blow up one another or all of us. Three hours of Shavian repartee is quite enough excitement as it is. MAJOR BARBARA -- At the Arena through November 22.