WASHINGTON - For the past two years, Barack Obama has made it hard for anyone to pin him down philosophically. So when he raises his hand on Tuesday, exactly what -- beyond the efforts of an eager, data-driven problem-solver -- can the American people expect?

Obama has spent his adult life tilting left while courting conservatives. That's how he won his very first campaign, for president of the Harvard Law Review.

He has been known to call himself a "progressive," and when he occasionally uses the word "ideological" in reference to his own leanings, he clearly casts himself somewhere left of center.

Yet on most occasions, his references to ideology are disdainful and dismissive. In discussing his economic stimulus package, he speaks of judging his proposals by how many jobs they produce and how quickly they will move the economy. Other criteria are inadmissible.

There are at least three keys to understanding Obama's approach to (and avoidance of) ideology. There is, first, his simple joy in testing himself against those who disagree with him. Someone who knows the president-elect well says he likes talking with philosophical adversaries more than with allies.

This part of him was once the detached writer and professor who could view even his own life from a distance and with a degree of abstraction. Seen with perspective, after all, the ideological differences in the United States are rather small. We have no major socialist party, and when it comes down to it, even conservatives are reluctant to dismantle our limited social insurance and welfare programs.

But Obama's anti-ideological turn is also functional for a progressive, at least for now. Since Ronald Reagan, ideology has been the terrain of the right. Many of the programs conservatives pushed were based more on faith in their worldview than on empirical tests. How else to claim that cutting taxes would actually increase government revenue, or that trickle-down economic approaches were working when the evidence of middle-class incomes said otherwise?

Thus the second key: Being empirical right now is in the progressive interest. Note that data show that the parts of the stimulus package most congenial to liberals (increases in unemployment insurance and food stamps, fiscal aid to the states, government spending on public projects) are also the parts that have the most economic bang. In other words, progressives don't need ideology to make their case.

In this respect, at least, Obama is rather like Franklin D. Roosevelt, who dismissed the conservative economic doctrines of the 1920s. "We must lay hold of the fact that economic laws are not made by nature," Roosevelt said, directly countering the central premises of orthodox economics. "They are made by human beings." Thus did Roosevelt make pragmatism and experimentation the enemies of conservative ideology. Obama is doing the same.

But there is a third respect in which Obama's anti-ideological talk is not just a vehicle for progressive inclinations but the real deal. Obama regularly offers three telltale notions that will define his presidency -- if events allow him to define it himself: "sacrifice," "grand bargain" and "sustainability."

To listen to Obama and his budget director Peter Orszag is to hear a tale of long-term fiscal woe. The government may have to spend and cut taxes in a big way now, but in the long run, the federal budget is unsustainable.

That's where sacrifice kicks in. There will be signs of it in Obama's first budget, in his efforts to contain health-care costs and, down the road, in his call for entitlement reform and limits on carbon emissions. His camp is selling the idea that if he wants authority for new spending, Obama will have to prove his willingness to cut some programs and reform others.

The "grand bargain" they are talking about is a mix and match of boldness and prudence. It involves expansive government where necessary, balanced by tough management, unpopular cuts -- and, yes, eventually some tax increases. Everyone, they say, will have to give up something.

Only such a balance, they argue, will win broad support for what Obama wants to do, and thus make his reforms "sustainable" -- meaning that even Republicans, when they eventually get back to power, will choose not to reverse them.

It is riotously ambitious. But last November, Americans elected a man who counts "audacity" as one of his favorite words.

E.J. Dionne's column is distributed by the Washington Post Writers Group.