Here we go with one of the hardiest perennials in the whole field of foreign policy making. After four months or so of on-the-job training, Secretary of State Edmund Muskie is putting about that (surprise!) the system doesn't work.

There's too much power in the hands of the president's adviser for national security affairs and too many people at the White House working on foreign policy. There are too many voices. There is too much back-channel undercutting of the State Department's presumed-to-be predominant role, too much pushing and shoving for the president's ear.

A first, post-election order of business, Muskie is saying, has to be a drastic reduction in the size and scope of the White House-based National Security Council -- no matter who wins. It is not a matter of clashing personalities, Muskie is arguing; it is "instituttional."

Now that, of course, is the polite and politic thing to say when you're in office. And it sounds so simple: just shrink the NSC by cutting its overblown staff way back and that will free up the State Department to perform its traditional role as the leading expnent, after the president, of foreign policy. But it won't work, because this isn't an institutional problem, susceptible to bureaucratic reform.

On the contrary, it has almost everything to do with personality. By this I mean, in particular, the personality of the president: his character and temperament; his work habits and mangerial skills; his past experience and the knowledge of foreign policy that he brings to the job; his relationship with (and confidence in) his secretary of state.

The almost unanimous judgment of former secretaries of state and former national security advisers, exhaustively recorded incongressional testimony, is that the foreign policy making machinery has worked the way it has over the years, for better or worse, because that's the way successive presidents have wanted or allowed it to work.

Consider the history. To begin with, when you hear talk of dismembering the NSC, it's important to remember that the NSC itself, established by law in 1947, was intened to be merely a working group consisting of the president, the secretaries of state and defense, the director of the Central Intelligence Agency and the chairman of the Joint Chiefs. It was supposed to be a device for dealing into the decision-making all the top people resonsible for matters having to do with national security, all of whom had their own staffs.

According to Clark Clifford, who helped draft the law as a Truman White House aide, "it was not even conceived" that there would be a national security adviser to the president. Under Truman, Clifford and an assistant did the staff work. Eisenhower was the first president to feel the need for a special national security adviser. Over the years, successive advisers felt the presidential demands upon them required a bigger and bigger staff.

It's this NSC staff -- now grown into a mini-State Department -- that Muskie is presumably talking about pruning drastically. And he has a point. At its peak, the NSC power house assembled by Henry Kissinger under Richard Nixon numbered more than 156, some 50 on the White House payroll and the rest on loan from other agencies. This, again was no more than the inevitable consequence of presidential personality -- of a considered decision by Nixon, out of disdain or distrust (or fear) of the State Department, to gather foreign policy decision-making tightly in his own hands.

The number now, under Carter, has shrunk by roughly one-third. But Muskie's right: it's still a power house. NSC has a staff of just under 100, of which about 35 are carried on the White House budget. It includes specialists on the Soviet Union and East and West Europe, on the Far East, on the Middle East, on North-South relations. That's the "regional cluster."

There are also special staffers for refugees, international economics and energy, nuclear non-proliferation, defense, science, intelligence, human rights -- and a public relations officer.

But the question is not whether to cut the staff; it's whether to scale down the job description of the president's adviser for national security. What pruning is not the branches but the roots; do that, and the staff's influence and activity will inevitably die back.

If the powers (and profile) of the security adviser are reduced (as most old hands will tell you they should be) to that of a largely invisible policy coordinator and internal trouble-shooter, the State Department would have a lot more running room -- if that's what it wants. But it's no use talking in the abstract about cutting the NSC down to size. What Muskie needs to establish (if he didn't before he took the job) is whether that's what the president wants.