THE PRESIDENT'S new defense idea is pure Reagan: simple at first glance, complex at the second, running against the grain, sure to arouse a storm. It is the product of Ronald Reagan's peculiar knack for asking an obvious question, one that has moral as well as political dimensions and one that the experts assumed had been answered, or found unanswerable, or found not worth asking, long ago. In this instance, the question is: why are we and the Soviets basing our defense and survival on the terrible and incredible threat of mutual annihilation? Is there not a better way?

To that question, a whole generation of strategists has said no. Defending against nuclear threat has been accepted as tantamount to announcing an intent to bring an offensive threat against the other side. Deterrence--carrying with it the threat of inflicting and incurring mind-numbing damage--has come to be enshrined as the guiding strategic principle. The effort of both Americans and Soviets has been, as variously interpreted, either to gain a margin of superiority or to attain parity or stability.

Deterrence has worked in the sense that nuclear war has been stayed. But the requirement to maintain a usable and invulnerable deterrent, against the rush of technology and the fear of the other side's moves, is precisely what "arms race" means. It has led, in hardware terms, to such tortured constructs as putting huge missiles on a racetrack in the western desert, running them around from one garage to the next, and occasionally opening the ceiling doors to let the other fellow's cameras peek in. That particular scheme was shelved, but no matter what other scheme to maintain a deterrent is finally accepted, it will keep alive the specter of mass death and destruction in a nuclear "exchange."

Against this specter Mr. Reagan now suggests that we slowly start investigating whether in the next century technology may offer a solution to our security that does not rest on the prospect of mass and mutual death.

Is it a good idea? Scarcely was it out of the bottle than it was denounced as an escape from reality to the nirvana of high tech ("Star Wars"), a step toward the militarization of space, a gimmick with which to distract the freeze movement, a calculated assault on the jewel in the arms control crown, the antiballistic missile defense treaty, and, last but not least, a reckless provocation to the Soviets, who could only be expected to take the proposal as a prelude to a nuclear showdown.

Perhaps it is all these things. Perhaps, too, it is none of them. At this point it seems enough to say that President Reagan has given impetus to what is already a major gathering review of the strategic principles this nation and the Soviets have adopted in the last generation. These principles, keep in mind, were not written in stone. They represent merely the best guesses made by harried men groping with the historically unprecedented circumstance--the capacity to end the world as we know it --that technology had put in their hands. Their answers created the uncertainty and peril with which Mr. Reagan, not alone, is attempting to cope now.