In firing his most experienced Cabinet members, the president has made clear that he will no longer tolerate independent opinions - or, in fact, any views that might prove "abrasive" (the president's word) to the White House staff. He will henceforth steer the ship of state by the beacons and bearings of political expediency. No other meaning can be given to the interposition between himself and his Cabinet of Hamilton Jordan - a young man admittedly uniterested in the "issues" whose only apparent qualification is a flair for campaign strategy.

But if the control of our domestic policy is now narrowly concentrated in the palace faithful, the conduct of our foreign policy is dangerously diffused. Secretary of State Cyrus Vance is no longer the undisputed shaper and interpreter of our relations with other nations. A central and sensitive sector of our foreign policy - the Arab-Israeli conflict - has been turned over to Robert Strauss, a man previously unexposed either to the practices of foreign policy or to the esoteric problems of the Middle East. He is not, he proclaims, a "subordinate" of the secretary of state but rather his "partner."

Secretary Vance's writ is threatened even more broadly by the burgeoning authority of the national security adviser, Zbigniew Brzezinski. Similarly independent of Vance, he acts as a co-equal secretary over the whole spectrum of our foreign policy. He deals with foreign ambassadors, undertakes diplomatic missions, negotiates with chiefs of government, makes speeches, appears on television and gives press interviews. (Brzezinski even has a press officer of his own.)

Who, then, enunciates our foreign policy? The secretary of state, or the national security adviser who speaks froma quite different philosophy? During years of schism in the Roman Catholic Church there were two popes - for a while even three. Who, then, interpreted the will of God?

A coherent foreign policy requires that only one voice speak for the president. Clarity cannot be achieved merely by good will or by that classical answer to sloppy bureaucratic structures, "coordination." No two officials will ever give the same emphasis to delicately shaded questions of foreign policy. In the final analysis, a two-headed foreign policy is quite as grotesque as a two-headed human being.

Not only does the prevailing ambiguity confuse foreign governments, it upsets the Congress. Though the national security adviser plays a major role in designing foreign policy and manipulating the levers of power, the Congress can neither pass on his nomination nor call him to testify. Still, it would be wrong to try, as some in Congress have proposed, to correct this anomaly by making the post subject to confirmation. That would merely legitimize a structural aberration. The proper solution was provided by Henry Kissinger in 1973. Having used his own position of national security adviser to dominate foreign policy and eventually replace Secretary William Rogers, he quite understandably insisted on retaining his national security title. By holding both jobs, he could speak and act incisively without fear of being undercut.

It was an easy and sensible solution, since the problem was not ordained by the Founding Fathers but created almost by accident. When the National Security Council was established in 1947, an anonymous bureaucrat collated the submissions of relevant departments for the council's consideration; not until the Kennedy administration did a president treat his national security adviser as a substantive officer. President Kennedy's motive was to utilize the exceptional talents of a trusted friend, McGeorge Bundy; he certainly did not contemplate that, thereafter, all occupants of the post would play a policy role rivaling that of the secertary of state or that they would uniformly be professors with little or no practical foreign-policy experience.

But, though acquired characteristics are not inheritable, bureaucratic biology often endows improvisations with genetic vitality. Thus, today the job of national security adviser is presumed to belong to academia in the same way that the treasurer of the United States is automatically a woman or, in Britain, the post of Earl Marshal belongs to the Duke of Norfolk. Uncritically accepting that presumption, the press regularly accords to the national security incumbent - whether Kissinger or Brzezinski - the accolade of "doctor" - though it withheld that title from five members of the pre-Thermidor Carter Cabinet equally endowed with Ph.Ds.

Moreover, because Kissinger used his national security post to displace Rogers, such a progression is now regarded as inherent in the job. Without question, the national security adviser is in a strong tactical position. Unlike the secretary of state, he has no protocol responsibilities, no obligation to spend his days before congressional committees, no need to attend international conferences or defend and administer a complex department. Physically based in the White House, he is constantly at the president's side.

How much advantage that gives him depends to a large extent on the secretary of state. If, like Acheson, Marshall or Rusk, the secretary fully uses the diplomatic establishment and is not addicted to "personal diplomacy," he should be able to guide policy effectively. But if he follows the Dulles and Kissinger pattern, flying frantically about the world, he will leave the field wide open for an ambitious national security adviser. That is the situation today.

Who enunciates our foreign policy? Well, there's Brzezinski, and Strauss - and, oh yes, the secretary of state.