Start START

Anyone knows that serious negotiation can only take place behind closed doors. So the flurry of pronouncements by Moscow and Washington on the subject of reducing medium-range nuclear weapons in Europe must be seen as part of the public posturing that goes on outside the negotiating room. Last November President Reagan deftly took the diplomatic initiative by proposing the so-called ''zero option.'' Leonid Brezhnev, playing to audiences in Western Europe, has made public his offer of a ''two-thirds option.'' Rejecting this proposal, Mr. Reagan, in turn, has announced the formal presentation of a treaty based on the zero option.

If the two sides are engaging in shadow boxing, however, we would not underestimate the significance of it. Mr. Brezhnev's proposal clearly is unacceptable to the United States. If each side were to cut its medium-range weapons by two thirds before NATO's planned deployment of cruise missiles and Pershing IIs, this would leave the Russians ahead, perpetuating their superiority. But the Soviet leader is at least talking about reductions of weapons instead of calling for a freeze as was once the case. Some Western arms control experts believe the Russians would in fact accept close to a zero option since they do not really need their SS-4s, SS-5s, and SS-20s and are primarily concerned about forestalling the deployment of ground-based cruise missiles in West Germany.

That, however, raises the subject of broadening the Geneva talks to include intercontinental as well intermediate-range weapons. In short, SALT. We were heartened by Secretary of State Haig's assertions last summer that strategic arms control was too important a subject to be made a hostage to other aspects of East-West relations. It is therefore disappointing that, because of the adverse developments in Poland, he has backed off that position and linked arms negotiations with Moscow's responsibility for the Polish military takeover. Why is strategic arms control any less crucial now?

The fact is, it is difficult if not impossible to separate out strategic weapons in the Geneva talks on theater arms. A treaty on the latter would not eliminate the threat which Soviet ICBMs would still pose to Western Europe. Without a SALT agreement - or START (Strategic Arms Reduction Talks), to use Mr. Reagan's designation - the Soviet Union could go on building nuclear warheads far beyond the numbers permitted under SALT II and a theater arms agreement would have limited value.

Postponing these all-important discussions does not make sense. In economic terms alone, the US has much to gain by a strategic arms agreement. It is an irony that at a time when the US wrestles with gigantic budget deficits which are fueling high interest rates and recession the administration is embarked on a costly military buildup. The Soviet Union, too, goes on diverting resources to defense even while it is straining under a slow growth rate, a $20 billion debt to the West, and mounting economic burdens in Poland and elsewhere. Both sides would benefit by putting a cap on their nuclear arms plans.

It is the paramount interest of global security and peace which should be propelling the two superpowers toward agreement, however. Unless some restraints are put on their relentless race in nuclear arms, the danger of nuclear war grows. Each side becomes more and more fearful that, unless it strikes first, it may be subject to a strike by the other side. There is no time to lose. It is not only that they already have some 16,000 nuclear warheads between them, enough to pulverize the earth. We are now in a period in which weapons are becoming increasingly difficult to bring under control. The air-launched and submarine-launched cruise missiles, for instance, which the US is planning to build in huge numbers, are considered a major threat by the Russians because there is no way to distinguish through satellite photographs between nuclear and conventional cruise missiles. Without a reasonable ability to verify, no power will sign an arms control agreement.

The people of Western Europe and the United States are speaking out more and more on behalf of a vigorous effort to reduce this nuclear peril. It is to be hoped their voices are being heard - and that beyond the public fencing Presidents Reagan and Brezhnev are prepared for serious negotiations.