American strategic policy is changing at a dizzying pace. Most of us outsiders are flailing to keep up, and there is reason to doubt that many policy makers have yet caught their breath.

Consider the speed at which President Reagan's embrace of defense against strategic nuclear arms has moved from the status of an eccentric presidential idea for the distant maybe to a program that, in the minds of leading officials, is an all-but-sure thing, just around the corner, the centerpiece of current strategic planning.

It was less than two years ago that Reagan "deregulated" strategic theory: He broke the monopoly previously accorded to deterrence -- threatening retaliation -- and forced it to start competing with defense -- knocking Soviet missiles out of space. The president unveiled his Strategic Defense Initiative in March 1983. A month later the Scowcroft commission filed its consensus-hugging save-the-MX report, which ignored the SDI. But the idea of defense has since pushed far into the old constituency of deterrence.

What happened? The partisan explanation is that Reagan's intuition drew him unerringly to an idea whose time had come but which had not yet struck home with the heavy thinkers because of their intellectual and political commitment to old ways of thinking about arms and arms control.

Another explanation arises from the currents stirred by the nuclear freeze movement. It has been duly credited with adding to the political pressures that drew Reagan back to the bargaining table with Moscow. Its more enduring contribution -- one even more bittersweet for the freezers -- may turn out to have been its assault on deterrence.

For what else were the freezers saying but that deterrence was unstable and dangerous and that something bold had to be done to reduce the threat of nuclear devastation? They built a public for that argument. Reagan bought it. But he put on a different bottom line, calling not for a freeze but for strategic defense, a non-nuclear defense, no less, his stated purpose being not simply to reduce offensive arsenals but to eliminate nuclear weapons altogether.

No wonder the freezers and traditional arms controllers are pinching themselves, trying to figure out how to reclaim the issues Reagan stole from them: their issues.

In the excitement, however, the administration has surged ahead of any position that has yet been validated by the sort of rigorous extended debate we should demand of a major policy departure. Already the administration has something it describes as its "strategic concept," which, it says, "can be summarized in (a) single paragraph." The paragrph says that for the next 10 years we should seek radical reductions in the number and power of nuclear arms, offensive and defensive. Then we should begin a transition to non-nuclear defensive forces. Then we should eliminate all remaining nuclear arms, leaving a nuclear-free world. This "concept" takes weapons that do not exist and that we may never have, builds on them a structure that defies the historical record, the arms control record and the Soviet-American record, and arrives at a place (a nuclear- free world) previously considered the exclusive property of the left fringe. A sweet dream, but a dream.

Then there is Max Kampelman's New York Times article of Jan. 27, written with Zbigniew Brzezinski and Rob SDI, it's important as the first glimpse most of us have had of what may be in the head of the new chief of our Geneva negotiating team, head of its space-and-defense sub-team to boot.

The article's premise falls at the alarmist end of the spectrum: a Soviet first strike "can become a practical option." It minimizes, as SDI proponents tend to, the existing American deterrent, passing by, for instance, our bomber force. Reagan has said the United States would "discuss" an eventual deployment; the article says SDI should not be traded "for promises that can be broken at any time." It asserts, against much contrary expert testimony, that by the early 1990s we can have a two-layer, $60 billion defense catching nine of every 10 Soviet missiles -- enough to discourage Soviet attack. Like other briefs for SDI, it avoids the simple crucial question: Why, when we are working on a missile defense, would the Soviets ever want to negotiate down their offense?

It's not that my points are unanswerable. It's that they lie in the realm of a discussion that has scarcely begun. If the administration has a winner, it will have to show so by its judgment, not by its enthusiasm alone.