TO BEGIN ON A positive note, the Reagan budget reminds me of the 17-year locust. When the locusts are aswarm and nibbling the outer branches off the oak trees, many people become unduly alarmed.

Isn't it terrible, folks will say, what these locust are doing to our trees?

Can't we do something to stop them? But when spring comes round again, the oaks are full and glorious once more, actually healthier because the locusts had nibbled on them the summer before. The 17-year locusts are God's tree trimmers.

In many important dimensions, that is approximately what the Reagan administration is doing to the federal domestic budget. The immediate effect is awful to behold. But there is at least the possibility of regeneration, not in this political season and maybe not the next, but eventually when counter-forces have gotten their act together. Hacking away can be a creative act if the critics take the opportunity to rethink what is being destroyed.

That's the positive side. Reagan's budget cuts ask a generic question about scores of government programs: Who will really miss them when they're gone?

And liberal critics may respond with their own question of Reagan's nostalgic vision of the free market: What will replace the government dollars and the government protection once they are taken away? The conservative answers are most unconvincing but, in any case, those two questions ought to stimulate an era of creative turbulence, of noisy debate and serious political thinking, reexamining the purposes of government. Anything that forces politicians to think can't be all bad.

Of course, this is very easy for me to say. I always enjoy a good argument and, like most Americans, a perverse side of me loves to see politicians squirm, all politicians, regardless of party or persuasion. Besides, I am not personally inconvenienced by what Reagan is doing. Cutting food stamps and welfare and school lunches, eliminates public-service jobs and shrinking Medicaid and gutting education aid. None threatens my lifestyle.

I do have a twinge of anxiety about his plan to exclude worthy families like my own from the college loan program but, down deep, I am confident that the politicians will take care of me, member in good standing of the upper-income middle class, even as it savages other less influential groups in our society. American politics usually works that way, in a pinch.

That is the ugly side of the Reagan budget. While the broad premises for budget reform are enticing, the philosophy conceals some rather nasty political agendas -- a collection of small-minded and class-conscious aims which are hiding behind the grand rhetoric.

Sometimes the attractive and ugly sides of Reagan's creative tree-trimming get all tangled up. His team, for example, really put the blade to the Labor Department's job programs. The biggest swipe has gotten the headlines -- ending the CETA public-service jobs which provide modest wages for some 300,000 people -- but the eclectic collection of other work and job-training programs has also been cut back drastically.

It may seem cruel, but a part of me thinks: It's about time. As a reporter, off and on over nearly 20 years, I spent a lot of time examining federal job-training programs, trying to make sense of them. I concluded, finally, that they don't make sense.

I am thinking back now to the early 1960s when most of these ideas were born as "demonstration" projects, when they developed a political base as an important part of the war-on-poverty, then grew and grew. I was young and excitable and intrigued by the idealism expressed in those programs rolling forth from Washington.

The problem with poor people, it was said, was that they lacked skills for worthwhile jobs. No business would hire a street sweeper to operate a lathe or train an inexperienced black kid from the streets to do complicated work. But the government could bridge the gap: provide job training, subsidize employers to take a risk with the unskilled, set up elaborate transition programs to get the chronically hopeless into jobs with a future.

It sounded sensible, even noble. But, again and again, as one examined the original job programs up close and the modified and expanded programs which followed, it was impossible to evade the fact that the gritty reality did not match the grand rhetoric. Virtually every statistic of success I ever examined proved to be not quite what it seemed.

People were being trained, yes, but not necessarily for jobs. Many simply went from one program to another, collecting a small stipend and learning new "skills" which they would never use. Some citizens learned to "hustle" job training, both workers and employers. Millions were "helped" in a short-range and insubstantial way. But, fundamentally, nothing changed.

Over time it became obvious that training poor people, worthy as that may be, simply begs the real question: What jobs are available for those people once they are trained? As a simple matter of logic, the training approach was doomed if the fundamental failing was not with poor people, but with the size and structure of the job market. Liberals got closer and closer to confronting this contradiction but, given the taboos of the political marketplace, they opted for the least satisfactory answer: public service jobs.

This meant creating thousands of subsidized job slots at city hall and the county courthouse where payrolls were already swelling. Certainly, many of the CETA workers did useful things but, in the end, these were not real jobs. That becomes poignantly clear today as they are proposed for elimination. Few people beyond the 300,000 who will lose their paychecks are shedding any tears.

Now comes the nasty side: Throwing 300,000 people and their families into the street solves nothing. The needed jobs are still not out there. Even if Reagan's tree-trimming were to work its promised magic, the jobs would not begin to appear for some time to come. Surely nobody thinks the misery of these people will go away simply because the president preaches self-reliance.

So the budget-cutting forces liberals to ask a necessary and tough question:

What would their reforms look like in the future?

The answer, it seems to me, ought to start with work, not workers. What work does America really want done? Begin by identifying the valuable production and construction and services for which the private sector, for whatever reasons, will not allocated resources. Then build a government approach, either direct or indirect, which gets that work done. Create real jobs and ordinary people will quickly enough develop the skills to fill them.

What about rebuilding the railroads as part of our effort to refurbish America's industrial base? Does anybody think the private sector, however much it would like better railroads, is going to invest its cash in that needed enterprise again?

This is not a revolutionary proposition. Indeed, the federal government has been doing limited versions of the same thing since the earliest days when it was building canals and turnpikes. The budget calls it public works and goo-goo critics call it pork barrel, but politicians know, despite perennial criticism, that pork usually leaves something permanent, usually in concrete, to show for the tax dollars, and it also spreads a lot of money around for the folks.

If that sounds trivial, it is at least a more substantial outcome than many federal training programs have produced. When liberals start over, they might start by asking how that political reality can be married to the problems of the labor market.

Which brings us back to the unattractive political agendas in the Reagan budget. Among other things, Reagan is launching the most massive peacetime public works program in federal history -- a trough full of dollars that will slosh inflationary effects, waste and possibly corruption over one sector of the economy. Only this pork is called national defense, so people don't think of it in those terms. Still, if you want to know where those 300,000 CETA jobs are going, check the Pentagon's projections for defense employment: up 450,000 jobs in the next two years.

The abrupt reallocation from domestic social concerns to defense is clearly the most radical element in the budget. I certainly hope it makes people sleep better at night, because it is a very expensive security blanket. According to the president's projections, the Pentagon, which now spends about $162 billion a year, will be spending $342 billion by 1986.

Meanwhile, spending for environmental protection, particularly cleaning up lakes and rivers, will shrink by 25 percent. Support for education, training and related social services will shrink by 20 percent. And so on, across the landscape of social concerns.

Indeed, when one adds up the drastic cuts for academic enterprises like the National Science Foundation, the student loan program and other aspects of learning, one can reasonably conclude that the Reagan budget is anti-university, if not anti-intellectual. Reagan seems to be using today's federal budget to settle old scores from the Sixties, when campuses were aflame with anti-war protest and Legal Services lawyers were challenging the government in court. Up with the Pentagon and down with campuses? Up with weaponry and down with ideas?

That sounds too harsh. I hope so. We shall find out soon enough, as the consequences sink in. Whatever grand rationales are offered for these budget choices, they add up to a stark game of distributional politics, with much money going in questionable directions.

But this, too, is forcing liberals to think thoughts they haven't thought for years, to rediscover what had been taken for granted, to search for new answers to ancient questions. And after the locusts are gone, perhaps they will find that the oaks are stronger for it.