THIS WEEK in Washington, if everything goes as planned, President Reagan and General-Secretary Gorbachev will sign an agreement banning short- and medium-range nuclear-armed missiles from Europe. The question naturally arises whether the world will then be a safer place, and, if so, why.

To answer this question, we would need a definition of where, in the nuclear age, safety lies. What, in other words, is the goal of arms talks? Is it the elimination of nuclear weapons from the earth? The elimination of offensive nuclear weapons together with the construction of strategic defenses? The elimination of all armaments? A continued balance of terror, but at a lower level of armaments, or with a different combination of forces, or with the addition of safety measures?

Today, however, we lack any shared answer to these questions, and without such an answer progress becomes all but impossible to measure. Arms control proceeds, but without the benefit of any consistent or broadly accepted theoretical guidance.

One argument in favor of the treaty is that reductions in nuclear arms are good in themselves. And much is made of the fact that the INF accord will be the first in history that actually imposes reductions. Certainly the number of warheads to be removed from Europe -- 1500 on the Soviet side and 350 on the American side -- is impressive; in themselves these warheads constitute an arsenal sufficient for a full-scale nuclear war. Unfortunately, far more impressive is the number of weapons that will be left after the reductions have been carried out -- more than 48,000, or enough for roughly 32 nuclear wars of the size of the one to be eliminated from Europe.

Seen against this mountain of nuclear weaponry, the agreement emerges as a very slight downward adjustment in the amount of overkill. Taken in itself, it does effectively nothing to reduce the destructiveness of a possible nuclear war.

Even if the agreement fails to appreciably reduce the destructiveness of a possible war, might it not at least reduce the likelihood of one -- might it not still, that is, add to "stability"? This question plunges us into one of the most specialized and arcane debates in the field of nuclear theory: How the United States should deploy its military forces so as to best demonstrate its resolve to defend Europe.

To simplify greatly, opponents of the treaty argue that the United States, by removing its nuclear-armed missiles from Europe, would dangerously signal a loss of resolve, thereby inviting the Soviet Union to exploit its presumed conventional superiority in the region and launch an attack. The defenders of the treaty, on the other hand, observe that thousands of American warheads will remain deployed in Europe, on planes and with ground forces, and argue that the nearly five-to-one ratio in the United States' favor in the reduction offers an opportunity not to be missed. They add that any decent agreement improves the political atmosphere, and that this, too, adds to stability.

All that the ordinary person listening to these arguments can conclude is that if the agreement adds to or detracts from stability, it doesn't seem to be by much, and that if the worth of the treaty had to be measured by this standard it probably wouldn't be very important.

There remains the possibility that the agreement still might constitute a "step" towards some distant point of safety to be reached farther down the road of arms control. The definition of a goal, however, is the task of nuclear doctrine, and on doctrinal matters the differences -- not only between the administration and outsiders but between officials of the administration -- is wider than it has been at any time since the late 1940s.

It hasn't always been so. As recently as the early 1970s, when the SALT I Treaty was negotiated, signed and ratified, a consensus prevailed in favor of the doctrine of deterrence, according to which the two superpowers supposedly prevented each other from launching nuclear war by displaying the ability and the resolve to launch an annihilating attack in return. This system sought not to dismantle the machinery of annihilation but rather to find the source of safety there, through the establishment and formalization of the well-known "balance of terror." The hope was that possession of the weapons could serve to prevent their use.

This doctrine offered comprehensive and specific guidance for arms-control talks. To begin with, it held out a hope for an answer to that slipperiest of questions in nuclear strategic theory, How much is enough? Enough was enough to annihilate the society of the other side in the retaliatory strike. (Secretary of Defense McNamara once calculated this amount to be 400 megatons of explosive power -- the equivalent of some thirty thousand Hiroshima-size weapons.)

The doctrine also offered guidance regarding stability. Stability lay in rendering weapons invulnerable (the better to carry out their retaliatory blow) and in leaving populations vulnerable (the better to assure each side that its retaliation would be successful.) On the whole, defensive weapons were judged destabilizing. Defenses of population were destabilizing because they reduced the effectiveness of the threat of retaliation; defense of nuclear arms were destabilizing because they led the other side to increase its offensive arms, which would lead the first side to do the same, and so on -- sending the arms race into an upward spiral that had no theoretical stopping place.

But if deterrence offered the promise of an upper limit on nuclear armaments, which one day might be written into an arms-control treaty, it also specified a lower limit beneath which nuclear armaments must not be allowed to sink. After all, if the safety in the system depended on the terror it produced, then the terror must not be reduced below a certain point. And that point was precisely the point at which societies might start to be safe from immediate nuclear destruction. If preventing the use of the weapons depended on possessing them, then to rid the world of them entirely -- to abolish them -- would be one of the most dangerous things possible.

The goal that the doctrine of deterrence specified for arms control, then, was a balance of nuclear forces offering the maximum of stability. In this equation, reductions had no intrinsic value. If they added to stability, as defined by the doctrine, then they were good; if not, they weren't.

The very existence of a consensus -- of almost any consensus -- was an impressive achievement of sorts. It defined a goal; it charted a path; it offered standards by which progress could be measured: It provided common ground. Yet a decade after SALT I the consensus was gone.

An assault on it was mounted from two sides; from the Reagan administration and from the peace movement of the early 1980s. The Reagan administration -- in rejecting the consensus and in failing to reach any arms-control agreements in its first term -- deprived the public of the reassuring feeling that the nuclear arms race was at least being "managed," and this set the stage for the peace movement. The rise of the movement, in turn, seemed to provide some of the impetus for President Reagan's embrace of the Strategic Defense Initiative, or Star Wars, with its promise of an escape from the balance of terror and the abolition of all nuclear weapons.

Of the two challenges, the president's was incomparably the more radical and consequential -- not only because it was backed by the authority of the president but because it constituted a full-scale rejection of the doctrine of deterrence and its replacement with another doctrine. The peace movement, by contrast, had no rival doctrine to offer. Its main practical idea was the nuclear freeze -- a measure that could be (and was) justified within the framework of the deterrence doctrine. Asked what would come after the freeze, its advocates tended to answer that cuts, or deep cuts, would be next.

The president was bolder. He put forward his ambitious "peace shield" of defensive weaponry, aimed at protecting the population of the United States against Soviet nuclear attack. If possessed by both sides, the shield would, in the president's view, render all nuclear weapons "impotent and obsolete," and they could all be dismantled forthwith.

The new doctrine offered a clear ultimate goal (the abolition of nuclear weapons), a path to follow to the goal (the creation of defenses) and clear standards for measuring progress (success in developing the defensive weapons, sucess in dismantling the offensive ones). The problem, of course, was that few people other than the president believed that the peace shield was technically possible. Even the head of the SDI organization, Lt. Gen. James Abrahamson, has stated, "A perfect astrodome is not a realistic thing."

Meanwhile, the Soviets were rejecting SDI in all of its forms as a "trick" designed to give the United States a first-strike capacity. The United States, they said, wanted to be able to launch a first strike and then use the defensive shield to repel the Soviet Union's retaliation. SDI, they were saying, undermined deterrence.

The destruction of the old strategic consensus has left the arms-control process without a guiding doctrine. It is thus impossible to measure the value of the INF proposal, or of the proposal to cut the arsenals of the two sides in half, or of any other agreement, for that matter, as a "step" toward some ultimate goal or other because there is no measuring rod with which to do so. Before one can decide whether a "step" is heading "in the right direction" one must know which mountain one is climbing. But none has been decided upon; or rather, many have been decided upon, and the participants, it seems, are all heading up different slopes. The president is in disagreement with the officials he has charged to carry out his own program, and both are in disagreement with the Russians and with officials of past administrations, many of whom seek to restore support for the old, broken consensus. The arms-control talks proceed unguided by any doctrine because, in the late 1980s, more than 40 years into the nuclear age, doctrine has fallen apart.

Notwithstanding the modesty of the INF agreement, its ambiguous contribution to stability, and the doctrinal confusion surrounding it, the agreement stirs hope.

One reason lies in the acceptance by the Soviet side, for the first time in the nuclear age, of the principle of on-site inspection. United States has always accepted.) This one concrete accomplishment of the agreement towers over every other. It is so important that the agreement might well be re-named the On-Site Inspection Agreement. The Soviet refusal to allow such inspection over the decades had placed a tight limit on the extent of arms reductions that were likely to be considered. Its removal -- perhaps the most striking fruit of glasnost so far -- makes sweeping nuclear disarmament (guided by whatever doctrine, toward whatever final goal) thinkable.

A second reason for optimism has to do with the apparent recognition on both sides that we have far more nuclear weapons than is required by deterrence. It is true, as some of those who discount the importance of the treaty point out, that the number of warheads to be withdrawn from Europe fails to change the overall destructive power of the two sides' arsenals. Yet if the removal of the warheads has few strategic costs, then it follows that their presence supplied few strategic benefits in the first place, and from this it follows that until the agreement was made the two powers were clinging tenaciously to useless deployment of nuclear weapons. Seen in this light, what is reduced in the INF agreement is not so much the danger of nuclear war as the craziness of possessing many dozens of times more weapons than we need for the project (crazy enough in itself) of destroying ourselves. The "step" here taken is one toward realism and sanity in the nuclear age.

What may be most important, however, are the political conditions that have led this particular administration to conclude an agreement with the Russians. The very lack of a guiding doctrine makes one wonder just what propels the arms talks at present. Certainly glasnost is one thing. The peace movement in the West is no doubt another. Probably Soviet anxiety about SDI is a third.

But also frequently mentioned is a force that many observers are pleased to call "history." No administration has been more resistent to arms control agreements than this one. In its first term, a dominant faction seemed to oppose any agreement whatsoever, on the ground that it would weaken the United States. Yet now that President Reagan is approaching the end of his time in office, many observers say, he wishes to secure for himself an important place in history, and finds that the way to do this is to negotiate an arms-control agreement after all.The president, who for so long seemed not to understand that the overriding imperative of our time is relief from the nuclear peril, now appears to have grasped the point. What the present could not teach him, the future has. The president has opted for history.

Jonathan Schell is the author of "The Fate of the Earth." His most recent book is "History in Sherman Park," which looks at the 1984 campaign through the eyes of two voters in Milwaukee.