THE DEFENSE budget is real. Much of the discussion concerning it is not. That is why it is worth considering some of the home truths expressed by Harold Brown, the Defense Secretary-designate, at a Senate confirmation hearing Tuesday.Jimmy Carter had already heaved a load of cold water on Democratic hopes of financing new or expanded domestic social programs from money saved on defense. Mr. Bown added another pitcherful. He observed that whatever savings the Carter administration might be able to make by cutting down Pentagon "waste" could well be offset by a requirement for "real growth" in certain military programs.

You do not have to be Dr. Strangelove to see the point. It is that the defense budget can no longer be regarded as a kind of untapped fiscal reservoir. Yes, there are plenty of marginal extravagant expenditures undertaken in the name of national security that should be curtailed. And yes, there are savings to made. But at some point those who have cherished it are going to have to abandon the illusion that the resolution of our domestic ills a awaits only that fabled "reordering of priorities" which will buy peace and progress at home with funds that would otherwise have been spent on an arsenal of exotic weapons which we don't, in any case, need.

The argument has never been as persuasive as those making it believed - nor even as morally attractive. At some point when you diminish military expenditures you may also diminish options to respond to the threatening acts of others in relatively bloodless ways. It is also true that out big defense expenditures, the ones that would have to be curbed to create any kind of social "dividend," have far more to do with military pensions and payrolls than with exotic weapons; so making really significant cuts in the Pentagon budget involves making decisions about volunteer forces and relative pay scales and the rest that are not reducible to hawk-dove pieties.

Speaking of those pieties, arms-control agreements are not necessarily big money-savers, as the pious hold; in fact, in their initial stages some are likely to involve increased expenditures. Finally, there is the matter of that much-discussed Soviet arms build-up, against which Mr. Ford warned in his valedictory address the other night. You will be relieved to learn that we do not intend to offer our won Washington Post Intelligence Estimate of what the Soviets are really up to. But we will go so far as to say that only loons and terns would deny that the Russians have been making an extraordinary effort, and it is not necessary to know its every detail to see that it is precisely the possibility of this kind of development that makes the defense budget such an unreliable prospective source of money to meet urgent domestic needs.

Richard Nixon used to like to say that in his time in office the lines of social and military spending had crossed, that the military had become a smaller percentage than the social of total federal expenditures - and also had declined as a percentage of GNP. About that, at least, he was right. In the face of it, however, a number of strong-willed liberals (of both parties, but mainly Democrats) have continued to invoke the defense budget as the easy answer to the question of how they mean to pay for the grand projects for which they regularly call.

Mr. Carter, as it seems, knows better. So does Harold Brown. ANd all that is good news. It is good news not because the defense budget is sacrosanct or because what the nation needs is a chemical agent in very pot and a death ray in every garage. It is good news because it may compel the Democrats to look at social programs in a different way. To the extent that many of those programs have failed to alter unhappy reality, it cannot be said that a want of funds was the principal source of failure - and it cannot be hoped, except idly, that the Pentagon will somehow be made to provide funds that will make a difference. To accept those facts may be the beginning of wisdom.