In a letter mailed to about 12,000 former contributors, Gary Hart fessed up: "Seven months ago, I let you down and caused my family great pain. Words alone cannot express how deeply I regret the disappointment and anguish I caused." And then Hart used words alone to characterize his dalliance with Donna Rice. A "damned fool mistake," he called it.

"Mistake?" A weekend spent in Washington with a woman not his wife is a mistake? Another weekend on a boat off Bimini with the same woman is a mistake? Weekends spent sexually, non-sexually or devoted to fierce games of Parchesi while others toil in Iowa or New Hampshire are a mistake? If these were mistakes, then so was Richard Nixon's attempt to cover up the Watergate burglary or, for that matter, every one of Elizabeth Taylor's many marriages.

Hart's choice of words is interesting -- and not, apparently, casual since he also described the Rice incident in a similar manner to former NBC newsman Marvin Kalb. In his formulation, Hart makes it sound as if the most gross errors of judgment, not to mention troubling questions of character, are the equivalent of hitting the wrong key on a typewriter or forgetting to carry the "2" while doing your sums.

As is sometimes the case, Hart speaks for many people. The young, in particular, sometimes refer to purposeful acts as mistakes. Maybe they're entitled. Youth is a succession of misjudgements, of education by failure.

But maturity entails something more than gaining wealth, power or status. The ultimate achievement is self-knowledge -- finding out, sometimes the hard way, who you are. This can be a painful epiphany -- for instance, the discovery by a man that he is not, fundamentally, his father's son, but his own man. Some people learn that money is not everything; others that it is. On this journey, we are always alone.

In an end-of-the-year interview with David Broder of The Post, Sen. Joseph Biden indicated that the pain of the last year had not been for naught. He had been forced out of the presidential race, accused of being a plagiarist and a blowhard. But in his interview, Biden did not blame the press for his fate or characterize his behavior as a mistake. Instead, he took himself to task. The self-descriptions were harsh: "cocky," "immature" and "naive." He had a lot to learn, he said. It's okay, Joe. So do we all.

But Hart seems to have learned little. What he calls a mistake is a snapshot of who he is. He's a man who, already suspected of womanizing, proved the accusation in the most public manner. It just so happens the incident in this case apparently involved sex, but, really, morality is almost beside the point. What's more to the point is that a pattern, a life style, an urge to take needless risks and to dismiss damaging personal behavior as unimportant, emerged. One of the essential requirements of the presidency is self-discipline, and Hart showed he lacked it. These behavioral traits put a shoe on Hart he clearly does not like. Nevertheless, it fits.

The question of character dogs Hart, and it's made worse, or at least more sensational, because its connected to sex. Both words -- character, sex -- are terribly loaded and tend to confuse us. The more we know the less we can be sure of. For instance, is the lie of adultery worse than that of Watergate? Will a man who cheats on his wife cheat on his country? Does the sincere and dogged commitment of a Sen. Edward M. Kennedy to racial and economic justice tell you more about his character than, say, Chappaquiddick?

The answers to these questions are elusive. But sometimes the questions need not even be asked. Just as a person will sometimes discover that he is not cut out to be what he always wanted to be, so Hart ought to realize that the presidency is not for him. That has nothing to do with his essential self-worth, his considerable intellect or his courage. It's just a statement of fact, a realization that by now he ought to have come to himself. Not everyone need be president. There are other ways to serve.

But by characterizing the Donna Rice episode as a "mistake," Hart shows that we have learned more about him than he has about himself. He persists in his presidential race as if behavioral patterns were slips of the tongue or blunders made at the end of a long and tiring day. They are nothing of the sort. What he calls "a mistake" is representative of who Gary Hart is. His real mistake is not realizing that.