The Reagan administration is about to tell us who it is -- or at least to begin that march toward self-definition that had to get started just about now. This is right on schedule. The first three months or "hundred days" of any new presidential term will be spent measuring statements and decisions and apppointments against the alternative that was vanquished at the polls. ("We are not Democrats . . . We are not liberals . . . We are not more of the same.") The press is very attentive during this phase, ever on the lookout for evidence that, frankly, nothing will change, that the promised new departures so incandescently urged during the campaign will be dropped because -- well -- because they always are.

Reagan has succeeded brilliantly in confounding the traditional skepticism on this score: No one can believe that the old order hasn't changed, and changed dramatically, so far as both foreign and domestic policy are concerned. Understandabaly his political and journalistic cheerleaders love this, and never tire of declaring how dumb and misguided the now out-of-office and out-of-fashion ideological opposition was -- and is. But Reagan's chore, alas for them, is not simply to reject advice that might have been proffered by Eleanor Roosevelt or Jane Fonda. It is to come down on one side or the other in what are a number of wretched choices and, in doing to, to choose as well among the various schools of thought that populate his administration.

AWACS, the grain embargo, to compromise or not on the economic package -- by last week the self-definition was starting to be made. The first thing that needs to be said about it is that the reality of this administration defies its conventional critics. This is no simple-minded, monolithic wreck-the-poor and bomb-the-commies crowd that some imagine. It is a coalition around Reagan of men and women who are passionately devoted to different aspects of his presidency, what it stands against as well as what it stands for.

But these people come out of factions and groups that are in some ways political, social and emotional strangers to one another. Lyn Nofziger, David Stockman, Jesse Helms, Jeane Kirkpatrick, James Watt . . . Coors and Dart, Kemp and Kristol . . . Malcolm Baldrige, Phyllis Schlafly -- what do these people have in common, except that Ronald Reagan represents for them a profoundly preferable alternative to what went before?

I am not saying that this situation is even especially distinctive, let alone unique. Plenty of administrations are complicated puddings that never set. What I am saying is that right now we are seeing Reagan having to begin to adjudicate the predictable conflicts of purpose within a government composed of people who have radically different ideas of what conservative priorities are.

Here is an example. I sometimes think that Amy Carter and I are the only two people in this country who care about nuclear proliferation anymore. And this is assuredly not a "front burner" issue in the Reagn administration just now. But there is beginning to be more public attention called to the chilling prospect of a nuclear-armed Middle East, and there are a number of decisions the administration is soon going to have to make that will influence the pace and likeliness of more countries' acquiring nuclear weapons.

Interestingly, within the Reagan national-security apparatus -- among Reagan's own appointees -- you will find a number of men who have been among the fiercest opponents of the uncontrolled export of nuclear materials -- materials that can be diverted from peaceful to bomb-making purposes. (These tend, incidentally, to be "hawks" on other nuclear-armament issues.) You will also find men who believe strongly that next to no curbs should be put on nuclear exports and next to no pressure on either our allies who wish to sell these materials to potential bomb-making Third World nations or on the nations that wish to purchase them.

You will find, however, no one who believes it would be wonderful if Libya, Iraq, Taiwan, South Korea, Brazil, Argentina, Israel, Pakistan, and South Africa (to take a few) acquired nuclear weapons. And you will find among all the disparate factions agreement that U.S. policy should be to discourage this. But how? Pretty soon the president is going to have to decide which of the divergent schools of thought in his government has the best idea of how to get there from here.

In the decision to supply the Saudis with AWACS, Reagan brought to a rolling boil the anxieties of a whole range of people whose support for him had something to do with a theory that Carter was being blackmailed by the oil-bearing Arab states. Where do hard-liners take their comfort in the lifting of the grain embargo? How far can Reagan go to appease the anxieties of those in Congress, especially those on his side, who believe his economic reading is wrong? A tourist to the Reagan administration, one who merely notes the occupants of the various desks and listens to the lyrics sung that don't seem to belong in the same production, will conclude that this president is only now embarking on the really hard part of his presidency.

From time to time I think that Washington doesn't actually need all its political commentary so much as it needs cultural anthropolgists and students of social comedy to tell its story. Margaret Mead, Jane Austen should be living at this hour. Where does the remorelessly serious eastern urban intellectual who has worked his way to a rejection of a lifetime of loyalty to a different political culture -- your basic "neoconservative" -- fit with the smalltown, anti-intellectual representative of let us say, the Moral Majorty?

All right -- this is unfair, gossipy, play. But the larger point isn't. Jimmy Carter put one of everything (only different everythings) in those offices too, people bound at most by a fairly tenuous and vague compatibility. To the end he was seen as never really choosing. Choosing -- defining -- is risky. The onlyt thing riskier is failing to choose. We are right at the edge of finding out whether Reagan really can.