The filmmaker Eugene Jarecki was a teenager in Upstate New York when Ronald Reagan was president in the 1980s. Looking back on those years, Jarecki says, it's clear that "already there was a caricature" of the 40th president, "and for a young person that caricature was largely negative."

So when Jarecki, now 41, began researching his documentary "Reagan," airing Monday on HBO, his views on the man many of his peers considered an "amiable dunce" underwent a profound transformation. In a conversation at the Sundance Film Festival, where "Reagan" had just made its premiere, Jarecki made a passionate case for remembering Reagan - who would have celebrated his 100th birthday on Sunday - not as a paragon or a pariah, but as a complex man in full.

"There are so many lies told to us today about who Ronald Reagan was and what he did, that, to a large extent, my discovery was the real Reagan," Jarecki said. "The real Reagan gives the lie to what virtually everyone says about him today."

For example, Reagan is often invoked by those advocating stringent immigration policies. "Correction," Jarecki said sharply. "Ronald Reagan was in favor of amnesty for illegal immigrants, [and] he signed a bill to give amnesty to 2.6 million illegal immigrants in the United States."

The very name Ronald Reagan has become a favorite shibboleth for anti-tax activists. "Correction," said Jarecki. "Reagan raised taxes six of the eight years he was in office. Why? Because he was a pragmatist."

From abortion-rights legislation he signed as governor of California to his support of nuclear non-proliferation and his practice of deficit spending, Reagan-the-man refuses to conform to Reagan-the-myth, a figure Jarecki maintains has been created by political opportunists to brand policies and agendas Reagan himself would likely oppose.

Sarah Palin. Grover Norquist. Barack Obama. As seen in "Reagan," each of them, among many others, has sought to co-opt Reagan's patina as a glib shorthand for . . . what, exactly, isn't always clear.

"It's like saying, 'What would Jesus do?' " the director said. "What would Reagan do? And then they sell us whatever they want to sell us, counting on the fact that we won't take the time to go research what Reagan really felt about that."

Jarecki's previous documentaries include "Why We Fight," about the Iraq War, and "The Trials of Henry Kissinger," about Richard M. Nixon's secretary of state. His approach to Reagan was almost diametrically opposed to the Kissinger film: "The public knew very little about Kissinger, and I felt the need to bring a great deal to light so that he was better understood," Jarecki said. "Here, the public feels they know a great deal about Ronald Reagan because he's as present as the air around us. . . . And yet for all that presence, there is a tremendous absence of real understanding."

The "real Reagan," as Jarecki's film portrays him, was a fascinatingly contradictory figure, a child of the Depression and Franklin Roosevelt's New Deal who came to oppose the very policies that saved his family. A one-time liberal Democrat and actors-union leader, he would find his political voice delivering a potent speech on behalf of Barry Goldwater in 1964, when he first came to the attention of conservative Republican kingmakers.

Genial and ever-smiling, he was also remote and unsettlingly disengaged, according to his son Ron, who is interviewed extensively in "Reagan." Deeply affected by individuals he met face-to-face, he could just as often turn a blind eye to human suffering, Jarecki suggests. Thus the president who quietly held the first "gay sleepover at the White House," according to Jarecki, seemed tone-deaf to the AIDS crisis that began to engulf the country during his administration.

One of enduring mysteries about Reagan has been the roots of his ideological transformation in the 1950s. The film leaves viewers with the subtle but unmistakable impression that it was Reagan's time as a spokesman for General Electric - when, like any good actor, he internalized the written script as his own - that accounts for the about-face.

"Can't we all understand that an actor who plays a role that brings the actor great success might over time grow to struggle with that gray area between who they really are and who their character is?" Jarecki mused. "Could he have become an instrument of that cutthroat corporate agenda and come to act as its figurehead and gradually lose his own moorings, to some extent?

"It would be absolutely ludicrous to pretend that this very bright, deeply driven man is anyone's tool," the filmmaker continued. "And yet he allowed himself out of his own will to be an instrument for those who needed an instrument just like him. So there's a combination of factors there."

Pronouncing himself an "Eisenhower Republican," Jarecki stressed that the contradictions that drove and defined Reagan prove far more useful to the politics of today than the hagiography or demonization that pass for commentary on his legacy. "A word is being added to the English language called 'Reagan' with a false etymology, and there's an imperative to restore the etymology to its true origins," he said. "If we have the wrong Reagan in mind, we will not apply history correctly or learn from history. We'll just apply mythology to today's challenges."

Reagan (two hours) premieres Monday at 9 p.m. on HBO.