This week, Hillary Clinton testifies before the House Benghazi committee. If you're a Democrat, you're certain by now that the investigation exists only to embarrass her. If you're a Republican, you think journalists intentionally bury every Democratic sin and exaggerate every G.O.P. gaffe. Either way, the chances are that you ascribe a single unsavory motivation to the other side.

A useful if perhaps unintentional corrective to such dangerous simplicity is "The Last of the President's Men," Bob Woodward's brisk, provocative and often frustrating portrait of Richard Nixon as seen through the eyes of his trusted aide Alexander Butterfield. Although history barely remembers him, Butterfield is the man who told the Senate Watergate Committee about Nixon's tapes, thus ensuring the embattled president's downfall. Toward the end of the book, Woodward asks Butterfield why he did it.

The answer is of considerable importance because of our habit of viewing the past through the lens of today's knowledge. As Woodward reminds us, until the tapes came to light, some two- thirds of American adults thought Nixon innocent of involvement in the Watergate cover-up. The secret was known only to three or four top White House staffers, along with the small team of Secret Service technicians who installed the microphones and serviced the machines. Nixon, in his autobiography, insists that he never thought the public would learn that the tapes existed. Without the inculpatory evidence they contained, the chances are good that he would have clung to office.

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Politically, Butterfield had been a true believer, recruited from the Air Force by his friend H. R. Haldeman to man a desk right outside the Oval Office. He managed Nixon's paper flow and, as time went on, carried out his smallest whims -- including installing a spy within Edward Kennedy's Secret Service detail. When Butterfield decided he wanted something more, Nixon rewarded him with the job of head of the Federal Aviation Administration, adding a promise that Secretary of the Navy was likely in his future.

Why would such a man betray his boss? The committee didn't know about the tapes. One lawyer for the Republican side suspected, but nobody was sure. Had Butterfield parried the lawyer's questions, which he very easily could have done, the secret might very well have stayed safe.

Butterfield assures us that the others on the senior staff who knew -- Haldeman and Larry Higby -- would never betray the president. So what was his motive? Butterfield proves elusive. He just thought it was time the truth came out. Or maybe he held a grudge against Nixon, who was often rude and dismissive. Perhaps at heart he's an honest man. The fits and starts go on for pages.

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Part of the problem may be that Butterfield himself comes across as not terribly self-examining: "I don't feel I had a motive. I'm not sure I like the term 'motive.' I was just the guy who happened to know all this stuff and I had a bad start with Nixon." Woodward understands the futility of trying to reduce any action to a simple cause-and-effect explanation. He quotes various literary luminaries on the topic. What jumps out is a line from John le Carré's masterwork "Smiley's People": "Why did the fifth floor always think people had to have one motive only?" Woodward quotes the line correctly, but here some context is helpful.

For le Carré, the fifth floor is where the bureaucrats sit. His hero George Smiley is listening as Connie Sachs, a retired researcher, talks about a Soviet intelligence officer named Kirov who in the past had been targeted for recruitment as a British spy. Connie argues that Kirov had tagged onto a man named Otto in part because he was ordered to but also because Otto was "dishy, and anti-authoritarian, and light on his feet" -- in short, everything Kirov "could never be, not in a thousand years." The point is that the bureaucrats could never understand the mixture of motives that might give them the chance to turn Kirov against his employers. To the fifth floor, Kirov could have been following orders; he could have been half in love; he could never have been both.

People are complicated. Woodward's engrossing volume gives us an Alexander Butterfield of enormous complexity. We needn't be like the fifth floor, and imagine that he must have had one motive only for disclosing the secret that brought Nixon down.

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Perhaps the reason Butterfield is unable to explain himself to our satisfaction is precisely that there were more motives under his hat than a person can reasonably keep track of.

Which brings us back to the Benghazi committee. I suspect that there, too, nobody is acting out of one motive only. A Republican member of Congress could perfectly well believe, with utmost sincerity, that what happened when Ambassador Christopher Stevens and the others died needs to be investigated-- and at the same time be delighted that she might have the chance to wound the Democratic front-runner. Similarly, a Democrat might well be certain that it's all a partisan witch hunt aimed at tarnishing his candidate -- and at the same time believe, for reasons independent and sincere, that there's nothing about Benghazi worthy of all the hoopla.

Democracy can't work if we're constantly assuming that those on the other side are acting for bad reasons. So let's try to stay off the fifth floor. People are complicated. Motives are complicated. When we think we know what's "really" motivating someone -- especially someone we dislike -- the chances are we haven't yet looked deeply enough.

Stephen Carter is a Bloomberg View columnist and a law professor at Yale.