(FORTUNE Magazine) – Throughout his remarkable business and government career, Robert Rubin, now 65, has both worked exhaustively at reaching well-founded conclusions and rejected the idea that anything--and he means anything--can be a "provable certainty." Federal Reserve chairman Alan Greenspan once joshed Rubin about that proposition, telling him that "all is uncertain" is inherently contradictory because it asserts that uncertainty itself is uncertain.

The joshing didn't move the needle on Rubin's opinion one iota, as proved by an exchange that took place in early November between him and FORTUNE. Being interviewed in his large Citigroup office, sitting in a red-leather chair in his usual somber suit, white shirt, and dark tie, Rubin grew momentarily confused as to what day of the week it was. This writer supplied an assist: "It's Wednesday, and that's a provable certainty."

Rubin at first didn't argue: "Even in an uncertain world, I'm prepared to stipulate that it's Wednesday." But then, retreating, he said he would so stipulate "except for the cognitive issues of whether anybody knows anything--except for that caveat."

FORTUNE (a little slow on the uptake): So there actually is a provable certainty?

RUBIN (patiently): No, because then you get to the cognitive issue point, which is that in some sense you can't be sure of any reality.

FORTUNE: Can't be sure it's Wednesday?

RUBIN: Not unless you stipulate as an assumption your own cognitive capabilities. When all is said and done, you cannot prove reality. You can say what you see, but you cannot prove your cognitive capabilities. You can only assume them, and therefore everything is ultimately based on assumption. But I'm still prepared to say this is Wednesday, just for the sake of this discussion.

FORTUNE: Okay, we will stipulate that.

RUBIN (this is a man who does not let go!): Yes, as long as you are willing to agree that it's not inconsistent with my notion, which is that there's no proof that it is.

Assuming that I was really interviewing Bob Rubin and that he really has had the career I was brash enough to mention at the beginning, here is the rest of the story. Look back at a particular four words of Rubin's--"in an uncertain world"--and understand that they didn't get there by accident. That is the name of the book that he and a co-author, Slate editor Jacob Weisberg, have just brought out. Rubin, who despite his obvious taste for verbal sparring is self-effacing to an extreme, once said he would never write a book. Later, having changed his mind, he said he didn't want to write something as egocentric as a memoir. He wanted a "policy book" instead. What has finally emerged, says Weisberg, is a "policy book disguised as a memoir." The book salaams quickly to Rubin's boyhood and education and then gets to the grand chapters of his life: his work at Goldman Sachs, where he rose to be co-chairman; his years in the Clinton administration, first as head of the brand-new National Economic Council and then as Secretary of the Treasury; his return to private life and a move into Citi's office of the chairman.

His history shows that he developed those unbudgable views about "certainties" early. Steven Umin, a classmate of Rubin's at Yale Law School and now a partner at law firm Williams & Connolly, remembers the young Bob Rubin as being extremely "laid back," uninterested in finance, and so unaggressive that the mere thought he might end up at hard-charging Goldman Sachs, much less become head of it, would have seemed demented. On the other hand, says Umin, Rubin was "very intellectual" and "very much the philosophical skeptic," in the sense that he regarded everything, including opinions he might have just formed, as open to analysis. With Rubin, says Umin, there were no givens, "not even what you thought was given." (Hello, Wednesday.)

A second pillar of Rubin's life has been "probabilistic thinking," a form of decision-making based on a disciplined weighing of odds and tradeoffs. Rubin played poker in high school--"I was reasonably good," he told FORTUNE, in about as close as he gets to a boast--and then in 1966 he was hired by Goldman Sachs to do the hard-wired, Wall Street poker called arbitrage. The job, done expertly, requires the assembling of every available fact about proposed mergers, a cold-eyed assessment of risk, and an ultimate willingness to bet on the outcome with crates of money. Rubin, invincibly calm under pressure and a user of both real and mental yellow pads to size up chances, was an ace at the work. A 1977 FORTUNE article placed him among Wall Street's "Four Horsemen" of arbitrage. (One of the other riders, as it happens, was Ivan Boesky, who in 1988 went to jail for filing false documents with the government, in a matter related to insider trading.)

Much later, as Secretary of the Treasury beginning in 1995, Rubin needed similarly to collect every salient fact about the financial disasters called Mexico and Asia and Russia, to assess which of various ameliorations might work, and then to lay U.S. dollars and the administration's credibility on the table. Overall, he succeeded so well that Clinton counselor David Gergen voted him "the administration's MVP." Clinton himself, calling at the 11th hour of this story, told FORTUNE that Rubin's Treasury performance was "matchless." Rubin in fact emerged from Washington in 1999 with his reputation not only intact but also enhanced--and how extraordinarily rare is that?

After that, his mental yellow pad could have told him he neither needed more celebrity nor more money nor a job that could in any way diminish his reputation. Figuratively and literally, he could have gone fishing, which he loves to do. Heading there would have also distanced him from the metastasizing financial crises that at Treasury he came to believe were periodically inevitable, particularly given the unchecked growth of derivatives. One friend remembers, in fact, that the post-Washington, temporarily unemployed Rubin thought the odds so plainly tilted toward trouble that he vowed he wouldn't dream of becoming a financial company CEO.

But he didn't care to entirely opt out of executive life either--he wanted to stay "current," he says--and in the end he carved out a sunny half-and-half deal at Citi. He accepted the risk of joining co-chairmen Sandy Weill and John Reed (the latter now gone, of course) at the top of a company almost certain to have its octopus arms in every crisis. Citi's reach, in fact, got Rubin himself awkwardly entangled with Enron. But his contract, though splendidly lucrative, largely insulates him from executive peril by specifically sparing him line responsibilities. Instead, his beat is "strategic managerial and operational matters." The risk assessment on all that: If the world blows up, and tries to take Citi with it, fixing the mess won't be Rubin's job.

If you want to know how Rubin got to be the introspective character he is, you might start with his rabbi. The man's name was Leon Kronish, and he got to know Rubin as a boy, after young Bob's father, a lawyer, had moved his family from New York City to Miami Beach. Kronish saw in Rubin an intellectual and questioning nature and encouraged it. Later, when Rubin was at Harvard, his mentor was philosophy professor Raphael Demos, from whom Rubin as a sophomore took a course that extolled the rigorous thinking of such icons as Plato and Aristotle and burned into Rubin the necessity of challenging assumptions and beliefs. Out of that laboratory came Rubin's conviction that there are no provable absolutes.

In the meantime, as a kid in Miami Beach, Rubin had led a normal life, unless you maybe count his first day in a Florida school. That morning his fourth-grade teacher introduced him to the class: "Robbie Rubin has gone to private school in New York and has never learned script. So be nice to him." On that very same day, when they probably wouldn't even have recognized him in the hall, his new classmates elected him class president. He held class officerships off and on after that, and as a high school senior was again class president, a fact that helped him get into Harvard. On his less regal days, he fished in the Florida Keys with a pal, collected lead soldiers, delivered newspapers, and was a long-distance expert on the Brooklyn Dodgers.

Entering Harvard, Rubin initially agonized because he knew he was scholastically behind most of the other freshmen. But he eventually graduated summa cum laude and Phi Beta Kappa, with a degree in economics. Soon after came Yale Law. There, classmate Umin helpfully lent his car to his friend one day in 1963 so that Rubin and the former Judith Oxenberg, a French and music student at Yale, could go on a one-day honeymoon. They have two sons, James and Philip, both in their 30s. Judy Rubin once worked as commissioner of protocol for New York City Mayor David Dinkins and is today active in New York theater affairs. But by determined choice the Rubins lead a very private life, avoiding the social whirl and choosing instead to stay home in either their Park Avenue co-op or a summer house in suburban Westchester County.

After Yale, Rubin tried the law, spending two years at the New York City firm Cleary Gottlieb Steen & Hamilton. But a Wall Street case he worked on, and a growing interest in making money, primed him to seize the unexpected chance that came along in 1966 to join Goldman Sachs as an arbitrageur. The firm's senior partner then was the fabled Sidney Weinberg, and his successor was the almost equally fabled Gus Levy, whose managerial style was yelling. Rubin got his share of abuse. Nonetheless, Larry Tisch, a friend of Levy's, later told Rubin that the boss had early on spotted this young, levelheaded arbitrageur as someone who might eventually run the firm.

That's not to say that Rubin's early reputation at the firm was pristine. Goldman has always placed a high value on teamwork, and the young Rubin had a trader's fault: a tendency to be brusque and short with other people. Ultimately an older partner--an ancestor of today's executive coaches, no doubt--told Rubin that if he wanted to move ahead in the firm, he might want to soften his style and start thinking about the opinions and views of colleagues. This deficiency notice surprised Rubin, who hadn't realized he needed it. But he promptly changed, as if remaking himself was the easiest thing in the world, and became very attuned to working with others. That still left him a long way from touchy-feely. Years later a Treasury staff member, Michael Froman (who is now at Citi, having been brought in by Rubin), teasingly captured the boss's character by dismissing the very idea of what some viewed as a presidential dream ticket, Gore and Rubin. Scoffed Froman: "They'd be fighting over who had to kiss the babies."

After Gus Levy died in 1976, the Goldman pair known as the "two Johns"--Weinberg and Whitehead--took over management of the firm. Then, in 1990, Rubin and Stephen Friedman rose to co-chairmen, assuming the reins of a firm that had burgeoned in size and geographical reach and would continue to do so.

Brainpower surely drove Rubin's ascent. But there is a line in his book that is also revealing, in which Rubin describes himself as "not manifestly aggressive." Quizzed about that stuck-in modifier, "manifestly," Rubin laughs and concedes euphemistically that he is "proactive" when something needs to get done.

Two points about Rubin's 26 years at Goldman stand out for how they relate to his later years at Citi. The first is that Rubin and Steve Friedman worked beautifully together as Goldman's co--chief executives, managing to iron out their rare disagreements amiably. The experience, no doubt helped by Rubin's great willingness to listen and entertain views contrary to his own, convinced Rubin that co-CEOs were a perfectly sound arrangement, and years later he joined Citi believing that.

But then things ruptured between Sandy Weill and John Reed. In a tense, fractious Sunday meeting of Citi's board in February 2000, director Rubin was at one point asked whether he'd consider taking the CEO's job himself--he said no, adamantly--and then was asked whom he would support as a single CEO, Weill or Reed. On the grounds that the company's management was by that time almost entirely made up of executives whom Weill had brought with him from Travelers, and that the management team was very good, Rubin picked Weill--in effect, throwing him the victory. The total experience, says Rubin understatedly, "left me with the feeling that the odds of co-CEOs working together effectively in a corporate setting are actually very low."

The second point is that in 1987, Goldman suffered a scandal in which one of its partners, arbitrageur Robert Freeman, was jailed for four months for insider trading. Subsequently Goldman's management--very much including Rubin, who was then a co--chief operating officer--put in a "zero defects" policy aimed at ridding the firm of all possible sins. The policy obviously didn't have permanent effects: A former Goldman economist recently pleaded guilty to insider trading, and the firm itself also is scheduled to pay fines in the "global settlement" concerning stock research and IPO allocations. As a slight mitigation, though, Goldman's involvement in the big corporate blowups--Enron and the like--has been less than notorious.

But Citi, of course, has been mired in trouble, and Rubin himself has not entirely escaped. His problem was a call about Enron he made in November 2001 to undersecretary of the Treasury Peter Fisher. In the call Rubin asked whether Fisher might not want to ask the credit-rating agencies to hold off on any downgrading of Enron's debt--Rubin's point being that delay might allow the company's bank creditors to scramble a rescue.

Rubin, who was phoning at the instigation of Citi's banking head, later told a bipartisan congressional staff investigatory team that he prefaced his conversation with Fisher with a qualifier about this being "probably a bad idea." He'd chosen to put it forth nonetheless, and he wasn't just any caller; he was the former Secretary of the Treasury, weighing in from a company that had huge exposures to Enron. Even so, Fisher thought Rubin's idea was bad, and the matter ended. Later the staff investigation concluded that nothing illegal had happened, leaving the question of impropriety hanging in the air. Rubin says today he can see why the call might be questioned, but that given the circumstances of the time and what he knew, he would make the call again. You have to wonder, though, whether he really would.

In another strike against him, Rubin did not urge Citi to put in something like a "zero defects" program when he arrived there in 1999. In fact, Rubin himself asks today the very large question of why he--along with many others, including the media--did not recognize earlier the investment-banking and research conflicts that have led to so many scandals. His answer, not too satisfying, is that perhaps the great bull market masked many sins or created "powerful incentives" not to dwell on problems when all seemed to be going so well. He calls that a "natural human inclination."

As for Rubin, many of his human inclinations stem from his 26 years at Goldman, which he refers to as "the base for everything." Indeed, many years later, when he was trying, post-Washington, to pick from a fistful of offers to join corporate boards, his single choice was Goldman's longtime client Ford Motor. Today Ford's chairman, William Ford Jr., has a strong sense of how lucky he was to get Rubin as a director and counselor--and indeed a mover of men, since Rubin was highly influential in 2001 when the Ford board ousted Jacques Nasser as CEO and installed Bill Ford in his place. Bill's reading on Bob's board style: "He's amazingly humble, but he always nails the heart of a situation." When Rubin winds up to talk, says Ford, "I go into red alert immediately."

While he was climbing up Goldman Sachs, Rubin was also following the example of his maternal grandfather and involving himself in Democratic politics. Rubin considers himself a "centrist," but the name fits only if you average out the polar opposites in his thinking: He is passionate about fiscal discipline, but he is also a social liberal in his beliefs that government should help the underadvantaged--the middle class and particularly the poor--whom the free markets that he admires do not, by his thinking, serve well. Out of those dissonant views, Rubin found his grandfather's party a zone of comfort, and he began fundraising for the Democrats way back in 1972.

That led to lightning when he met Bill Clinton in 1991 and saw in him a deep grasp of issues, intellectual curiosity, and an ability to listen, all qualities that play back what Rubin is like himself. It was a short leap from there to Rubin's accepting Clinton's invitation to become head of the new National Economic Council and another short leap, upon Lloyd Bentsen's leaving the job in early 1995, to Treasury head.

From the start, Rubin was intent on persuading Clinton to forgo some of the campaign promises he'd made about lowering taxes and to focus instead on reducing the huge budget deficits of the early 1990s. Politically difficult as it was to do so, Clinton accepted Rubin's arguments, and during his administration the budget moved from deficit to the surplus that was handed over to George W. Bush. Waging the deficit battle, Rubin often encountered stiff opposition from tax-cutting proponents in Congress and spending enthusiasts in his own administration, among them Secretary of Labor Robert Reich. At one tense moment in 1995, Rubin appears to have done a little probabilistic thinking, found the odds not great for his side, and concluded he should be proactive. Making an SOS call to New York businessman and former cabinet officer Peter "Pete" Peterson, Rubin asked Peterson to help arrange a bipartisan display of support for fiscal discipline. Peterson engineered a double-truck newspaper ad that pleaded for balanced budgets and that was signed by 96 high-level businessmen (yes, they were all men). One of the six lead signers, along with Peterson, was CSX chairman John Snow, now Secretary of the Treasury and custodian of a 2004 budget deficit forecast to be $480 billion.

Working with Rubin in all his economic battles, including those that focused on the financial morasses of Mexico and Asia, was a team of people he drew deeply into his decision-making process and who came to be highly respected. Mark Malloch Brown, who worked with Treasury when he was at the World Bank in the 1990s and is now at the United Nations, calls this team "probably the most formidably excellent since Alexander Hamilton's." The names of Hamilton's folk may not pop readily to all minds, but Rubin's included Lawrence Summers, Rubin's successor as Secretary of the Treasury and now president of Harvard, and Timothy Geithner, an undersecretary to Rubin who was just named to head the New York Fed. Both admire Rubin greatly, and that is probably not just because he helped each get his job. Summers, now 49, elegantly focuses on Rubin's decision-making powers: "For some people, the defining act is a speech, for some an argument, for some an essay. For Bob, the balanced and nuanced decision is the defining act." Geithner, who is 42, says that "while Rubin enjoys an amazing amount of public respect and credibility, people who have worked with him know that he's substantially better than even that exalted perception."

Part of Rubin's approach to decisions at the Treasury was to put them off as long as possible. Some people might call that procrastination; Rubin called it getting that one last fact or well-judged opinion, from whoever at the table might offer it, that might make a decision the right one. Geithner says the young members of the Treasury staff would on occasion rush into Rubin's office, imploring him for a decision about something consequential. Rubin's first question would often be, "How much time do we have before we have to decide?" Summers tabs this Rubin's habit of "preserving his optionality."

He has less serious habits, of course, and the Treasury team got to know them all. Rubin quit wearing a watch about 20 years ago--he thought it made him too conscious of time--and he doesn't carry a briefcase, instead toting what he needs in brown, expandable legal folders jammed under his arms. At the office he likes to work in stocking feet. For the 6 1/2 years he was in Washington--he tends to be very precise about that elapsed time--he lived like a bachelor at the Jefferson Hotel, flying to New York on weekends to rejoin his wife. He reads voraciously, keeping several pieces of fiction and nonfiction going at once. But he is largely oblivious to popular culture: His staff found that he hadn't heard of Aretha Franklin or Eartha Kitt, and until corrected he thought that Jimmy Buffett was Warren's son. He also once suggested to the board of the American Ballet Theatre, on which he sat, that it help along a cost-reduction drive by cutting 10% of the swans in Swan Lake. His advice, he says, was not well received.

When Rubin left Treasury, in 1999, he naturally did not leave his interest in politics behind. Today's budget deficits appall him, because he is sure they will produce high interest rates and policy chaos down the road. Even so, he has up to now refrained from backing any one 2004 Democratic presidential candidate (though, interestingly, he did prep Wesley Clark for a debate). It's a problem, he said recently, to choose among nine "squabbling" contenders--and then he promptly retracted that blunt description, no doubt thinking it less than Rubinesque. The one thing clear, he says, is that he himself will not be running.

Upon his return to New York after Treasury, Rubin had a range of job offers, including the opportunity that soon began to be relentlessly pressed on him by Sandy Weill. For Rubin, one attraction of Citi's offer to join its office of the chairman was that it promised him--and later contractually assured him--that he could hold the post and still pursue his interest in public policy and charitable causes. He has in fact shoehorned those into his Citi life, hectic though that is. In one key post, he is chairman of Local Initiatives Support Corp. (LISC), which works with on-the-ground organizations that are sponsoring projects for the poor, such as playgrounds.

A second strong attraction of Citi clearly was money. Although Rubin's Goldman days left him well-heeled, he remains, in his own words, a "reasonably commercial person," and among his job requirements in 1999 was something that would be "financially rewarding."

What he got at Citi was a rich three-year contract (since renewed twice for one year) that gives him at least $15 million a year in salary and incentive compensation; stock options; and Citi planes for his personal use. Through 2002 he actually beat the $15 million by upwards of $1 million. As for stock options, he was at that point up to a total of 4.6 million, some awarded when Citi stock was in the 30s, rather than the mid-40s of today. Rubin has never exercised any options.

There is an irony to Rubin's pay. It is related to the fact that $14 million of his take is guaranteed incentive compensation--guaranteed, that is, unless there are, so his near-impenetrable, punctuation-challenged contract says, "extraordinary circumstances drastically negatively affecting Citigroup reported operating results." Even in that disaster scenario, Rubin's pay will not be taken down unless the other member of the office of the chairman--that would be Sandy Weill--has his pay docked too. So for all practical purposes, we are talking about circumstances that are just never going to occur, which means that Rubin's incentive compensation really is guaranteed, just as we started off saying.

So what's going on here? The plain answer is that these contract contortions are intended to let Citi get around the tax law clause called 162m, which prohibits the deduction of executive pay exceeding $1 million unless the excess is tied to performance. Citi is not alone in its acrobatics; all over America corporations are plotting to skirt 162m. But the special point here is that a National Economic Council chap named Bob Rubin helped shepherd 162m through Congress in 1993. "Life," he agrees, not seeming wild about the whole subject, "is full of little ironies."

Beyond that wrinkle, there is the overriding question of whether Rubin is worth his $15 million and more. That's a matter of judgment, of course. But here's a judgment that counts: Charles "Chuck" Prince, Citi's new CEO, says unequivocally that Rubin has been worth "multiples" of what he gets.

One of Rubin's jobs at Citi is occasionally to take his high-powered self on visits to customers. But the major part of his work goes on inside, in a role that sometimes gets called consigliere or minister without portfolio. Under any name, he makes himself available to Citi's executives, CEO on down, as a sounding board, and he also attends many group meetings.

A new boy's description of how Rubin operates is proffered by Stanley Fischer, who joined Citi in 2002 as president of Citigroup International, formerly having been at the IMF. Talking about Rubin's "immensely influential" ways, Fischer refers to meetings held under Weill. "There was never any question at any point that Sandy was the CEO and that Sandy made the final decision. On the other hand, in every meeting Bob was listened to in a way that almost no one else would be. But because of the way he says things--'Aw, shucks, I'm just another humble working peon around here who doesn't know very much stuff, and you guys know more than I do, but don't you think ...'--it's very disarming and effective. Bob did it all in a way that produced very little friction with anybody, and that's a tough act to carry off."

Fischer refers also to Rubin's other role as a "trusted advisor" with whom people feel they can discuss issues--say, "Should we introduce this new product? Is there a better way to do this?" That description fit the Wednesday on which FORTUNE interviewed Rubin. He said in passing that he was lunching with Fischer "because Stan has an issue he wants to discuss."

For both Sandy Weill and Chuck Prince, Rubin has been a true consigliere. Weill and Rubin are almost comical opposites, in that Weill is instinctive and quick in coming to decisions, even as Rubin is asking him for just a little more time--the weekend, say--to mull over a problem. But they seem to have a mutual regard. And Weill has picked up important management ideas from Rubin, including the institution of a weekly business-heads meeting that is now viewed around Citi as virtually indispensable. Or to use Rubin's term, it has become an "organic part of the whole." Another portion of the whole that Rubin can take credit for is Banamex, the big Mexican banking company that Citi acquired in 2001 and that today contributes significantly to Citi's profits. Rubin got the call about that opportunity from Goldman, and then helped Weill make the deal. Weill says he doubts that Citi would have Banamex had it not been for Rubin.

Weill often drills into Rubin's fund of knowledge personally: "I just love talking to him about the economy, interest rates, markets, countries, currencies." He pauses, then adds "risk" and "the understanding of all those models and how much they should be relied on." Weill could even have added the Internet to his list: After John Reed left in 2000, taking his Internet strategies and philosophies with him, Rubin stepped into the vacuum and put his weight behind the formation of a new Internet Operating Group. Rubin didn't head the group; vice chairman Deryck Maughan did. But for Rubin it was not a casual matter. One technology expert who has left Citi remembers Rubin asking for a private tutorial: "I know absolutely nothing about technology," Rubin said. "Take me through it, including how I turn on the computer."

Chuck Prince has been CEO only since Oct. 1, so his requirements in that job for a consigliere have yet to fully unfold. But the value of Rubin's wisdom had become eminently clear to him about a year earlier, when he moved from general counsel of the company to head of the scandal-torn Global Corporate & Investment Bank (GCIB). Devoid then of operating experience, Prince pulled Rubin into two months of meetings on restructuring. Prince says that Rubin's enormous contribution was his steady insistence that the people meeting get beyond the nitty-gritty of the moment and instead be forward-looking in their thinking.

Those meetings and others have made Prince think that Rubin is a level up from the cerebral descriptions he usually draws, such as "thoughtful" and "reflective." If you work with Rubin, says Prince, you get easily to the word "forceful." Vice chairman Maughan ratchets the description still higher: "Behind that laconic exterior lurks a highly competitive man."

The one-year contracts that Rubin is signing these days raise the question of whether this highly competitive, forceful, reflective, and thoughtful man is contemplating moving on from Citi. He says not. That certainly suits Prince, who says his favorite term of office for Rubin would be "forever."

It is intriguing to consider whether there is any other career that parallels Rubin's: business leader of note phasing to high-level and respected government official and then phasing again to business leader of note. FORTUNE asked Rubin whether he could think of others fitting this bill of particulars--and then realized this prompting would utterly fail. Answered Rubin: "One of the problems with trying to respond is that I don't really think of myself that way." Okay, so how does he think of himself? That's simple: "I did what I did," he says, "and then I went to government, and now I've done this."

And that's pretty much what you need to know about Robert Rubin, assuming there is a Robert Rubin.

RESEARCH ASSOCIATE Patricia Neering FEEDBACK cloomis@fortunemail.com