George Shultz's block-like body, which seems built from material with an atomic weight approximating uranium's, is this day encased in a brown suit which, at the State Department, suggests self-confidence. He is wearing his Marine's tie, which may explain the confidence.

He has the emotional equivalent of a marathoner's metabolism: The motor of his emotions ticks very languidly. Not so his intellect. He has the charisma of a drowsy clam, but becomes a clam wide-awake and on tiptoe when the topic is economics. A former professor at America's most formidable university (Chicago), he is one of this century's four most intellectual secretaries of state (the others: Hughes, Acheson, Kissinger). Fortunately, he has more economic sophistication than any predecessor.

The fact that many American bankers are lousy at running banks is threatening America's diplomacy as well as its economy. Foreign policy is a hostage to bankers' irresponsible commercial credit decisions. Mexico provides only the most recent example. It cannot repay on schedule $80 billion of debt. The nine largest U.S. banks have lent Mexico the equivalent of 40 percent of their capital and reserves. The U.S. government must be nervous when Mexico must suddenly reduce its standard of living in order to pay U.S. bankers.

With 30 million persons unemployed in the West, protectionist pressures are growing. Shultz says, correctly, that the current generation of world leaders is committed to free trade. But, as Shultz knows, that commitment grew before economic growth slowed, before the growth of the political temptation to export unemployment by subsidizing exports.

Shultz works not in the capacious office used by recent secretaries, but in a smaller one, where a map of Beirut is a residue of last week's agenda. He believes Israel employed excessive force when dealing with the PLO but, more importantly, he believes Israel is a strategic asset for the United States in the eastern Mediterranean region, in part because of Israel's demonstrated willingness to use force.

(Israeli force, about which U.S. officials complained, was indispensable to the U.S. objective. Philip Habib could charm the birds out of the trees, but only force could get the PLO out of Beirut.)

The PLO, Shultz believes, is not apt to be a factor in foreseeable negotiations. Are negotiations foreseeable? Shultz has a Talleyrand-like knack for using language to conceal his thoughts, but he hardly bothers to conceal his conviction that any settlement must involve "some" withdrawal by Israel from territories occupied in 1967.

Before Begin's era, that territory-for-peace formula, implying adjustments of the 1967 borders to enhance Israel's security, was the aspiration of a majority of Israelis. For the United States to advance that position today might require U.S. pressure on Israel. Hence, Shultz must control the U.S. temptation to seem tough by showing hostility toward Israel.

Concerning what matters most -- the Soviet Union -- Shultz, who opposed ratification of SALT II, seems free of the fatal inclination to make arms control the centerpiece of U.S.-Soviet relations. But, he notes, the serious negotiations in Geneva concerning Reagan's proposal refutes the idea that substantial reductions of force levels can not be the basis of a serious negotiating position. Shultz is too polite to note that the refuted idea has been fostered by the State Department. He notes that persons who have said the President will be content to let throw-weight limits wait for a second-stage agreement have misrepresented the President. Again, Shultz is too polite to note the source of the misrepresentations.

There is today a curious convergence between detentists and some anti-detentists: Both think the Soviet system can be transformed by U.S. actions. Detentists speak of a restraining web of agreements. Some anti-detentists ascribe Soviet dangerousness to the Soviet's ideological elan, and talk of puncturing that elan with sharp rhetoric. But the Soviet system is not as malleable as the detentists think, or as rickety as some anti-detentists think.

Both groups express an American inclination to believe that U.S. policy can bring about the end of the arduous and dangerous competitive relationship. Shultz seems impervious to that impulse. He is among those who believe that the fundamental task of U.S. statecraft is to limit Soviet options, not change the Soviet system.

Hence, he is intellectually equipped to do the most important thing a secretary can do: He can talk sense to Americans about what the Soviet system is, and why there is no currently foreseeable end to the arduous, dangerous days. The course we are condemned to run requires a marathoner's temperament.