PRESIDENT REAGAN'S proposal to bar all land-based intermediate-range missiles from Europe won the spotlight in his debut as an arms controller last week, and deservedly so. Yet his address contained a general approach to negotiations with the Soviet Union, and a specific approach to the mutual reduction of strategic arms, with even more far-reaching implications for relations between the two great powers.

Very quietly, Mr. Reagan did something important. He distanced himself from a whole set of ideas -- ideas about the nature of the enemy -- that he had repeatedly cited and that undermine the very idea of negotiations. He did not say the Soviets cheat, lie and cannot be trusted. He did not say they are bent on expansion and conquest and can be dealt with only by power. He did not say they have a demonic idealogy and are evil at heart.

No, he simply accepted the familiar and frayed but right and unavoidable premise that all other presidents have accepted, that the Soviets must be dealt with somehow. Without further ado, he stepped up to the plate and took a cut. Leonid Brezhnev, the Soviet president, had taken the first cut in his Der Spiegel interview of Nov. 2. Mr. Reagan replied specifically to him. It is not that negotiations, in this instance on theater nuclear forces, will begin on Nov. 30. In a real sense -- both leaders are coming out of their corners, preparing concrete proposals on a wide range of subjects and starting to seek public support -- negotiations have already begun.

But this is not all. On arms control, again quietly, Mr. Reagan made a major shift. He took his old theory, that arms control is out of the question without a prior or parallel agreement with the Soviet Union on the rules of international conduct (Afghanistan, Cuba, etc.), and abandoned it. That old wearying word "linkage" was reduced at one swipe from a concept effectively barring the way to an arms control dialogue, to a simple fact of life that nations must accept as a political reality but that they must not allow to dominate their policy, or to substitute for policy. From now on, Mr. Reagan made clear, there is only one precondition to American participation in arms control talks with Moscow: adequate preparation.

That is not to say Mr. Reagan is operating without a guiding intellectual rationale for arms control. He has a new one, which is gradually becoming known. It is that arms control can help convert the Soviet Union from a challenger of order to a co-guardian of order. As administration strategists see it, the chief danger to world peace now comes not so much from a Soviet threat of war as from a Soviet threat of nuclear blackmail, and arms control, backed by a readiness to build new arms and to use power when necessary, is the way to turn this around. If this sounds as though Mr. Reagan has reinvented the wheel, or invented a new wheel quite like the old one, there should be no complaint as long as he appears to be rolling.

He does appear to be rolling. It turns out that the administration has begun transmitting some of its ideas on strategic arms to the Kremlin. What is more, in some measure the Kremlin is responding. The administration takes the view that the Soviets cannot be expected to agree with the new American ideas unless they understand them. So the Soviets have been told that the principal old measure of strategic equality, the number of deployed launchers, has been undermined by the march of technology and time, and that new measures, centering on equivalence of capacity for deterrence, must be found. The administration awaits a reply.

The Soviets have also been told that, for verification, the old electronic spies in the sky ("national technical means") must be supplemented by what President Reagan called "openness and creativity," meaning more intrusive on-site procedures. The implication of the "yellow rain" and of the Smolensk events -- that Moscow broke its treaty word -- reinforces this demand. It was put to Moscow before Mr. Reagan spoke and Moscow privately responded. Mr. Brezhnev then went public with the response, telling Der Spiegel that "national means must have priority" but that "some other forms of control might be worked out, given confidence." Interesting.

In brief, one cannot say where they will go, but things are moving. This is far and away the most important international development since the Soviet-American dialogue was broken as a result, primarily, of events in Iran and Afghanistan some two years ago.