To negotiate what?

Ten days ago President Reagan declared that it is his purpose and intention to ''engage the Soviets in a dialogue'' aimed at reaching ''a constructive working relationship.''

United States policy toward the Soviet Union, he added, is ''a policy of credible deterrence, peaceful competition, and constructive cooperation.''

So far there has been follow-up to the speech in the form of a meeting between US Secretary of State George Shultz and Soviet Foreign Minister Andrei Gromyko. Much was made of the fact that the meeting lasted five hours instead of the scheduled two hours. But one explanation of the longer duration is that Mr. Gromyko insisted on speaking in Russian, with everything being translated - which automatically doubles the time.

At the end of that meeting Mr. Shultz had nothing exciting to report. That could mean that he ran into as blank a wall with Mr. Gromyko as Donald Rumsfeld ran into when he called on President Assad of Syria in Damascus the other day.

That can mean, and probably does, that neither the Soviets nor yet the Syrians are interested right now in a dialogue. They probably will not be interested unless there is some indication of something of value coming out of it for them.

How does one reach ''a constructive working relationship'' with the Soviet Union? We know what we want from them in what we would call a ''constructive working relationship.'' We want the Soviets to cease building military power sufficient to threaten others, cease seeking recruits to their imperial system in the third world, and cease trying to separate our allies from us.

But that would leave us in a dominant position in the world. We would have Western Europe, most of Asia, most of Latin America and most of Africa in our system. They would be left in a position of inferiority in allies, associates, and clients. And they would be under pressure to give up much of what they now hold beyond the borders of the Slavic-speaking inner core of their system.

Their idea of a ''constructive working relationship'' would be entirely different. They would be free to maintain enough military power to ensure that we and our allies would never be tempted to try to ''roll back'' their frontiers. They would be free to seek new recruits for their system throughout the third world. And we would make available to them the industrial technology to modernize their economy and allow them to enjoy both total military security and consumer goods as well.

Between these two concepts of a ''constructive working relationship'' is a lot of space. There is no easy reconciliation. But of course if both sides were equally eager to arrive at ''peaceful competition and constructive cooperation, '' they should at the very least be able to agree on mutual reduction in the enormous arsenal of nuclear weapons which threatens the survival of the human race - and of both the Soviet and the American systems.

In the days of Nixon-Kissinger detente, much was made of the word ''restraint.'' It was the key to the concept of a safe and comfortable relationship between East and West. It involved mutual restraint in an arrangement that would benefit both, by lifting from both some of the burden of the arms race. It broke down because ''restraint'' meant different things in different places. In Washington, restraint meant that the Soviets would become a satisfied power and concentrate on developing their domestic economy. To the Soviets it meant the United States would be a satisfied power and help them with our latest technology. It broke down partly because they invaded Afghanistan, they declined to give Poland its freedom, and Washington refused them most-favored trade. Trade between the two, which expanded early in detente, languished.

There can be no such thing as a ''constructive working relationship'' between the US and the USSR unless it involves both mutual restraint in seeking and using the elements of world power and also mutually beneficial trade. To work out arrangements that would meet these conditions would mean a massive amount of planning and negotiating.

There is no evidence that the policy and planning machinery of either the American or Soviet governments has yet been set to work on what would be the biggest diplomatic operation since Henry Kissinger invented detente.

Until they do, a ''constructive working relationship'' will be a pious aspiration, not a serious diplomatic project.