The draft debate that counts most is in President Reagan's head. To judge by a fresh report from an authoritative source. Defense Secretary Caspar Weinberger, it has taken an important new turn. The president, he indicated to the Los Angeles Times, is going beyond his old (and persisting) philosophical objection to the compulsory element of conscription, and is focusing on the question of how, if there were a draft, it could be made fair.

This is good. The philosophical or idelogical argument over the draft has no end. I happen to differ with Reagan: compelling service seems to me less of a minus than sharing the burden is a plus. But both positions represent values deeply prized in our society. When; values held with intensity clash, it is best to shift the discussion to other terrain.

There are two choices. The first is to make the volunteer Army work better so as to moot the question of a new draft. This was the Carter policy, and it is Reagan's. There is a familiar list of incentives than can be jiggled (pay, GI bill, etc.) and programs that can be managed (recruiting, professional rewardds, retention, etc.) to ensure that the military gets and holds the numbers and kinds of volunteers it needs.

Pentagon manpower chief Lawrence Korb says that monthly quotas are being reached and the percentage of high school graduates among recruits is up in a year from 37 to 66. Retentions are up from 55 to 63 percent. He attributes all this to higher pay, the ills of the economy and a new patriotism in the air. Quite soon, however, as almost everyone agrees, requirements for new recruits will be pushing hard against the declining youth population and a planned expansion in the size of the armed forces.

"Manpower, not procurement, will be the issue of 1982 or 1983", says Assistant Secretary of Defense F. J. West Jr. "How to draft -- or not to draft -- will be the question that will challenge the secretary's ability to retain credibility and confidence on the Hill, with the president and with the military. The issue could well be dramatized by a serious crisis or conflict."

Reagan, says Weinberger, "remembers all the problems" of the Vietnam War draft of the 1960s. "Now, the draft is a very hard thing to administer fairly, and if it isn't administered fairly, isn't perceived to be, you have all kinds of trouble, even if you don't have a Vietnam War," College exemptions and a rich poor split, Weinberger indicated, are what the president has in mind.

The fairness question is the right and necessary terrain. Those who favor a draft should be pleased to see the question raised, since if it is not answered well, a draft loses political feasibility and moral merit. Those who oppose a draft can either hope the draft will fall on the fairness question or accept equity as consolation.

We are talking about a peacetime draft that might scoop up only a few thousand young men now, or perhaps a hundred thousand in a few years, out of the 2 million or so coming of age annually; adding women would sharpen the fraction. True, a lottery would be "fair," though it might not seem so to those selected, but it would not ensure the range of skills the services need. Pushing up entrance requirements to get those skills would not be fair. What other manner of selection could pass the fairness test -- the "Why me?" test -- that the country's young and the political community would surely demand? Any? That's the test.

James Fallows argues for a draft to raise military self-esteem and effectiveness and show the world we're serious. Good point. Maxwell Taylor believes a draft essential to replenish the reserves and keep the ranks filled once shooting begins. Good point. Both worry that the volunteer Army is undemocratically poor and black. Good point. But the politically and morally crucial "Why me?" test remains.

The draft's "problem of capriciousness," as Fallows calls it, has pushed serious people like him and Sen. Sam Nunn (D-Ga.) toward an alternative system. Comprehensive rather than selective, it would place on all young people an obligation of some kind of national service, military or civilian. Nunn, to avoid a bureaucratic monstrosity, would hope to enforce this new system with an honor code.

Can you see a president with the libertarian instincts of Ronald Reagan trying to ease "the problem of capriciousness" by extending the claim of the government to compel service from its citizens? Can you see Congress, which more than makes up in political sensitivity for what it lacks in ideological consistency, going that route?

The draft is an unusual kind of problem, on more of attitudes than resources. It changes shape as we think about it. Let's all of us, the president included, keep thinking about it.