Civics textbooks should be recalled by their publishers for revision. They teach that there are three branches of government: legislative, executive, judicial. But today there is a fourth. Call it the Greenspan branch. It is enchanting to have a branch of government named after a saxophonist.

Nature was prodigal when distributing talent in the Washington Heights section of New York City about four decades ago. Alan Greenspan and Henry Kissinger were (if you can imagine this) teen-agers there. When Greenspan was an irrepressible young blade, he played saxophone and clarinet in a traveling dance band that included a saxophonist, Len Garment, who was to be counsel to President Nixon just before Greenspan was chairman of President Ford's Council of Economic Advisers.

At the Julliard School of Music, Greenspan studied jazz and baroque music. The combination prepared him for economic reasoning, which looks ornate but is primarily improvisation.

After service to three presidents, he has learned the awful truth that no good deed goes unpunished. He has been made chairman of the commission assigned to recommend cures for the Social Security crisis. This musical economist is supposed to make it easier for the political branches of government to do their jobs.

Increasingly, American government resembles baseball's designated hitter, who can do only one thing. The designated hitter is only an aesthetic calamity in baseball: other members of the team can do other things. But it is a systemic calamity when the political system can do only one thing--extend benefits--and cannot provide requisite revenues. The result is a ratchet effect in the political system: all movement is in one direction. Government gives; it cannot take back. It is plagued by the mismatch between its hair-trigger spending mechanisms and its arthritic approach to raising revenues.

By now, virtually 100 percent of all congress men and senators recognize the danger to -- and the danger from -- the Social Security system. But there are many blends of measures -- all painful -- for dealing with it. And perhaps only, say, 30 percent of Congress can be rallied behind any one blend.

So unless Ronald Reagan and Tip O'Neill can come to terms, the conflict over Social Security reform will continue to resemble a gunfight in which whoever draws first gets shot. He will be shot at by everyone too timid to take the lead in choosing among painful alternatives.

The political branches want Greenspan's commission to make proposals -- and get riddled with bullets. Then the political branches can -- if this seems the least unsafe option -- hide behind the shredded remains of the commission's recommendations, bowing with well-advertised reluctance and distaste to the commission's "expertise."

But Greenspan's commission is different from many "expertise" groups that governments have used to solve problems -- such as the British commission assigned to refute rumors that Hitler did not die in the bunker, or the Warren Commission charged with establishing the facts about the Kennedy assassination, or the group asked to evaluate basing modes for the MX. Those groups were asked to render technical judgments about empirical evidence. Greenspan's commission is inescapably involved in making equity judgments of fundamental importance.

Any recommendations the commission makes about raising revenues or moderating benefits will involve answers to questions such as: who should pay how much to whom? How should the competing values of social generosity and economic efficiency be accommodated? How are such virtues as thrift and foresight to be encouraged? These are political questions because politics is about how we ought to live; it is about the authoritative assignment of social values.

The political branches have protested, with more vigor than sincerity, about the courts' "usurping" power and responsibilities. But the political branches have all too often been only too eager to see judges take custody of issues (such as race relations, capital punishment or abortion) that properly belong on the agendas of the political branches. Now those branches may further dilute their responsibilities by making frequent use of commissions like Greenspan's. (Of course, the political branches never wanted a Greenspan commission to tell them how to raise benefits.)

Greenspan has the gift -- rare among saxophonists -- of saying with calm clarity what he feels with intensity. So one sometimes wishes there were more Greenspans to deploy against those problems before which politicians take only evasive action. But the existence of Greenspan is, for the political branches, a temptation to the bad habit of displacing their responsibilities. Those branches believe, as Oscar Wilde did, that the way to cope with temptation is by surrendering to it.