After five years of living on a cloud of seemingly limitless defense increases, the administration is being brought down to earth by the budget deficit. There is a prospect of cuts that will leave Pentagon spending in 1990 back at the level of 1980. Contemplation of that vista puts pressure on the political system to make it less severe. Still, it seems safe to predict a substantial wringing-out. What matters is how it is done.

The first question must necessarily be whether cuts below the Reagan goal of regular and real 3 percent increases will hurt U.S. security. Again yesterday, Defense Secretary Caspar Weinberger insisted that they will; he sees our very "survival" threatened. Unfortunately, nothing has done more to lower the administration's credibility than his past and continuing failure to balance -- or even to accept a requirement to balance -- the demands of security against what he dismisses as "a set of perceived fiscal problems."

As a society we make budget decisions on such big-ticket items as defense and welfare on the basis of a collective instinct. A certain level of spending either feels about right or it doesn't. It's unscientific and drives many people batty, but perhaps this is the way it will work again.

Many people, I surmise, feel that the Reagan buildup has gone far enough, perhaps too far, and that the erstwhile Cap the Knife has turned out to be Cap the Spoon. Tied to this is a widespread view that the administration has misread the Soviet threat and Soviet buildup and that the world is not as dangerous a place as it seemed in 1980. The president is entitled to take some credit for this shift, though, as he contemplates the budget-cutting disposition it encourages, he may not be eager to.

Operating on instinct, to be sure, exacts certain costs. It puts the Pentagon on a roller coaster and provides no useful signals on just how defense resources should be spent. Spending fell in the 1970s, when many equated less defense with less capacity for Vietnam-type involvement, but almost all of us felt less secure at the end of the decade. Spending then went way back up from 1980 on, but -- look at Lebanon, Nicaragua, Libya and the arms-control uncertainty -- a large number of us wonder whether we have bought more security.

Then there is the inclination to treat the dollar level of defense as itself a symbol of power and will and command of the political machinery. This too blurs a focus on spending wisely and well.

So it is that the administration by its overreaching and carelessness has frittered away a great deal of the defense consensus that it commanded in earlier years. It has also left the country without intelligent guidelines by which to shrink spending now. The budget went up so fast that the Pentagon was not compelled to be smart and disciplined about it. The budget now may go down more by a process of service logrolling and political elbowing than by a reasonably balanced sorting out of commitments, threats and resources.

This is not to ignore the Reagan contributions to defense. There is, up to now, a necessary priority on making existing forces readier to fight. Recruitment, training and the retention of skilled people are pluses. Weapons programs have produced new capabilities and, especially in Europe, better conventional-war capabilities. Specialists and partisans can lengthen the list.

But these gains do not match a reasonable person's sense of value. After all, defense spending has almost doubled in just a few years. If there were not substantial gains, impeachment proceedings would be in order. But has enough of value been produced? Is there a logical patern to the buildup? Has it left us more secure? And in the second and vital range of questions, the ones where the Reagan default is now especially evident: has the buildup been purchased at too high an economic and social cost? Can or should it be sustained?

Here let brief honor be done to two prophets, David Calleo and James Chace, who early in 1981 warned us all -- you read it here -- that Reagan was tempting fate by proceeding to build up defenses without being realistic about how to pay the bills.

Someone else had issued a more resounding warning a few years earlier: President Eisenhower. A 25th-anniversary celebration of his famous farewell depiction of a "military-industrial complex" is being worked up later this month by a group called Business Executives for National Security. The purpose is to expand a center ground on which the Eisenhower balance of the desirable and the necessary, the moment and the future, might be rebuilt. A worthy task.