IT'S MORE THAN a routine statisticians' quarrel over President Reagan's budget numbers. There's a genuinely radical element in his fiscal policy -- a deliberate rejection of past rules and conventions. Orthodox economics carries on a careful sifting of recent experience, to identify the controlling patterns. Orthodox economics concedes that the patterns sometimes change, but holds that experience is the only real guide -- and, most of the time, a pretty accurate one. The Reagan administration disagrees. Recent experience isn't relevant to its plans, it argues, because it intends to lift the economy out of the accustomed track altogether.

That's the heart of the brief run-in the other day between Mr. Reagan and the Congressional Budget Office. The CBO is a technical staff that does fiscal analysis for both houses of Congress, and it has become a target of some of Mr. Reagan's more enthusiastic supporters. Its methods tend to be orthodox, and don't give much room to the radical vision. That's where the trouble arises.

A federal budget requires a lot of informed guesses about the state of the economy a year or two ahead. The inflation rate sets the cost-of-living adjustments for the Social Security checks, and the interest rates set the cost of the federal debt. The unemployment rate drives the cost of unemployment insurance and welfare. Any budget begins with a set of economic assumptions.

On Monday, the Senate Budget Committee made public some preliminary figures from the CBO's annual budget analysis. It showed spending at least $20 billion higher than Mr. Reagan had estimated. On Tuesday, responding to a question, Mr. Reagan called the CBO numbers "phony." Then, cooling, he amended that remark to say that their assumptions differed from his. And that's exactly right.

The Reagan administration is betting that its economic program can transform the way Americans think about money, inflation and investment. The administration shrugs off the criticism that its figures don't fit together. The White House and the Treasury are saying that they don't intend to manage the economy in the usual fashion. They intend to change Americans' economic behavior in fundamental ways.

The key difference involves anticipation. By changing the national atmosphere, the administration insists, it can induce people to behave in ways that anticipate changes still years ahead. Orthodox economics agrees that a tax cut can increase investment, but says the cut has to take effect before investment responds. The Reagan plan holds that the effect can precede the cause.

Under the Reagan tax plan, tax rates for most taxpayers would actually be a little higher this year than in 1980, because inflation will push incomes up about 10 percent while tax rates are cut only 5 percent. Under the Reagan plan, tax rates in 1982 return only to approximately the 1980 level. But, the plan says, the prospect of larger cuts in the future will begin to stimulate better economic performance immediately. Investment today will begin to raise productivity only slowly, over the years. But the plan holds that the prospect of it will begin to bring inflation and interest rates down at once. Mr. Reagan is seeking to establish an idea of the future that will change the present.

It's not impossible. But it's not plausible, either, and the pragmatists in Congress are likely to change it substantially. It's an unusually interesting case of one of the great themes of politics, the collision between an expansive, romantic hope and the doubts of experienced skeptics.