STEP BACK for a minute from the epic argument raging over President Reagan's "Star Wars" idea. How is it that Mr. Reagan and his critics, and the United States and the Soviet Union, find themselves at this pass?

1)Arms control had reached a stalemate, if not a general crisis. This was signified not merely by the Soviet Union's boycott of the START and INF talks. A substantive deadlock had been reached in those talks. And, in the view of many experts, difficulties in agreeing on a strategic balance, in arranging verification and, in the West, in securing political support for the arms control process had made it increasingly difficult to move ahead. So there was a readiness to look for some new way to transform the situation or at least to create a new chemistry or a new combination.

2)Deterrence had engendered ever wider skepticism and doubt bordering, on the left as well as on the right, on fear and contempt. For years the right had feared a "window of strategic vulnerability." More recently, the left had come to fear a general breakdown of the nuclear peace. In broaching his idea of a Strategic Defense Initiative, President Reagan was responding to a pervasive discontent with the viability of the theory of mutual deterrence. There was a market for another theory.

3)Technology as always was marching on, making possible inquiries and inventions that had not been thought of in earlier years. All of the separate pieces of the SDI that are now in the administration's budget were already being worked on, separately, at the time he made his maiden speech on the idea two years ago. Mr. Reagan did not invent the idea of defense. He assembled its potential in a form that caught the public's attention. Sooner or later the country was going to have to deal with the idea of strategic defense in some form.

A crisis of arms control, a perceived erosion of deterrence, the march of technology: this is how we got where we are today with the Reagan administration's carrying to Geneva for talks opening Tuesday a set of proposals that are fundamentally new and, to many, upsetting in their current combination but are not new in their separate conception. "Star Wars," that is, is not an idea born out of nowhere. It is a particular solution to problems that were recognized as problems even, especially, by many of those who are now sharp critics of the president's proposed solution to them.

Whether SDI is the right solution is, as far as we are concerned, a long way from proven. Certainly on its face it presents extreme new difficulties in technology and no less in politics. A strong case can be made that President Reagan, in investing it with the certainty and fervor at his command, has raised anxiety and opposition unnecessarily and, more important, has made a commitment far in advance of and in excess of what further inquiry will show to be sensible.

Meanwhile, however, the president -- by making SDI the centerpiece of his Geneva offer and of his whole global strategy -- has made it unavoidable for all of us, no matter how we feel now about SDI, to keep studying it. We note that no one has a greater responsibility to have an open mind than President Reagan.