President Reagan has defined the problem with a characteristically ingratiating one-liner: "Sometimes our righthand doesn't know what our far right hand is doing."

Very deft -- but also too true to sound funny to serious members of his town's hyperactive diplomatic community. Desperate to report home some sort of early fix on the Reagan foreign-policy approach -- not just the grand design, but precise positions on particular issues of concern -- the diplomats seem fully as baffled as they were in the worst days of the Carter administration's disarray.

The confusion is easy enough to catalogue. On the grain embargo, the sale of AWACS to Saudi Arabia, on how to deal with the disarmament movement in Europe, on the nature and dimension of the Soviet threat -- on these matters (and more) an administration deeply committed to practice coherence has produced a cacophony of conflict.

The popular diagnosis is that the remedy is simply a matter of time -- that "the Reagan administration still hasn't figured out its foreign policy." But that describes only the symptom of what's wrong. The president's one-liner suggests he has a better understanding of the real malady: His administration, to paraphase him, doesn't know what it is doing because he hasn't yet decided who's supposed to be doing what.

What's really wrong, in short, is that the administration hasn't figured out how to figure out a foreign policy.

Now you can mark down the president's incapacitation after the shooting as an extenuating circumstance, up to a point. As he regains his strength, he will be able to apply a firmer hand. But the institutional and personality problems were plain to see before the president was wounded. It will take more than his full return to active duty to make them go away. What it will take is a hard, fresh look at what he has to work with and how to make it work. In the end, the president may well reshuffle the players on his first team.

But assuming that point hasn't been reached, the first priority for the president is to reexamine the foreign policy-making process and the procedures he himself presumably installed. What we were promised was "Cabinet government" and its most enthusiastic advocate was none other than Caspar W. Weinberger, now the president's secretary of defense.

"There is nothing built into the fabric of American government," Weinberger wrote grandly in The Washington Post right after the election, "that would prevent a system of Cabinet government from working and providing the president with the communicating and coordinating abililties necessary to ensure that the executive speaks with a single voice."

Nothing? Well, nothing save the natural porclivities of, say, Defense Secretary Weinberger in action -- publicly propounding foreign policy lines on arms sales to China, detente, the neutron bomb, arms control and the Soviet threat to Poland that were at least distinct from, if not at cross purposes with, evolving administration policy.

"The failures of the past," Weinberger wrote in the same article, were partly due to "the appointment of people who felt that developing and cultivating their own constituency was far more important than supporting the president who appointed them."

True enough -- not only in the distant past, but in the immediate past. The story of the AWACS-for-Saudi Arabia fiasco is the story of the military people at the Pentagon, with Weinberger as their point man, pushing relentlessly for quick consummation of the deal over the resistance of Secretary of State Hiag, and before the administration had any of its congressional ducks in a row.

Not that Haig hasn't done his share of shoving. The White House staff has also made it singular contribution to the very state of affairs that Weinberger to the very state of affairs that Weinberger last December was promising us that Cabinet government would prevent: "The American government has been ill-served by the constant quarreling and recriminations that seem such an inevitable part of the relationship of the departments with the president."

Perhaps "inevitable" was the operative word. "It is by no means easy for a president to orchestrate sensitively to his purposes the great bureaucracies," Walt Rostow testified befoe a Senate hearing a few years ago, out of his own experience as President Johnson's national security adviser. But he argued it could be done by a president appointing "strong, trusted" figures to the top jobs and taking "great pains to hold them together as a cohesive team."

Rostow added a second prerequisite: a large grant of authority to the secretary of state to play "a major role in orchestrating all the other instruments of foreign policy." Rostow's prescriptions roughly approximate Reagan's promises. Sooner rather than later, the president is going to have to decide whether his administration's performance even begins to measure up.