But to negotiate what?

Everyone is in agreement that the time has come for the two superpowers, the US and the USSR, to get back to talking and negotiating. But it is one thing to say ''we are ready to negotiate'' and another to know where and how to start.

It ought to start from a basis of having, on each side, a clear sense of what each wants its opponent to be in the future. What, for example, does the Unite States want the Soviet Union to be 50 years from now?

Is the purpose of negotiating to reduce the size, range, and military power of the Soviet Union? Is it to induce a change in the system of government? Would the US like to see a czarist restoration, or evolution from communism to Western-style democracy? Or is the US willing to recognize that such objectives are probably unattainable, hence the only real question is how to get on with the system that exists?

Obviously, there is at present no consensus in the US on the long-range purpose of negotiations. Probably many would be satisfied just with a reduction in the climate of tension that has caused so much talk of late about the danger of slipping into a war.

But if your goal is reduction in tension, then you have to try to think through the price you would be prepared to pay to get this.

That is going to be a difficult one for Americans. There is tension today partly because many Americans think the Kremlin threatens vital American interests. The reverse is also true - many Soviets, including most of those in power, have begun to think the US wants to destroy those features of today's world which the Kremlin thinks are necessary for Soviet security.

A real, long-term, peaceable arrangement between Moscow and Washington could be built only on each nation's respect for the security interests of the other.

Right there is where the biggest trouble would come. The US thinks its security is threatened every time a communist movement begins to gather a significant following in any country in the world, but particularly in Latin America, and most particularly of all in Central America and the Caribbean.

But can you imagine any official of the Soviet government agreeing that Moscow will refuse to give aid to a Marxist movement in Central America in order to reassure Washington?

The reverse of that coin is that it is impossible for any government in Washington to promise to refrain from giving public sympathy and tangible forms of help to any anticommunist movement in Eastern Europe. The US did not send arms to help the Hungarians, the Czechs, the Poles, or the East Germans to recover their lost freedom from Moscow dominance. But it has given both sympathy and aid in these cases.

Moscow, however, regards its military and political dominance over the satellites of Eastern Europe as being essential to its security. The one thing Moscow would most quickly fight against would be an attempt by East Germany to break away from the Warsaw Pact.

An understanding has existed since the 1956 Hungarian crisis and the 1962 Cuban missile crisis that the US will not give direct military support to anti-Soviet uprisings in Eastern Europe and the USSR will not give direct military support to anti-American movements or governments in Central America and the Caribbean.

But it would be politically impossible for the US to refrain from aiding independence movements in Eastern Europe. And it would be ideologically impossible for Moscow, which claims to be the Marxist motherland, to promise to withhold aid from converts to Marxism.

The Nixon-Kissinger detente years broke down because the US thought it was buying a passive Soviet foreign policy. But to Moscow, detente did not mean to refrain from seeking recruits to its empire and went right on getting Angola, Ethiopia, and Southern Yemen.

To have lasting validity, any new detente would have to mean that both superpowers would remain satisfied and passive or that both would be free to seek and acquire recruits for their world systems. But neither could agree to either of those systems. The US would not agree to be passive or to allow the Soviets to be expansive. Nor vice versa.

So what is there to negotiate about?