Sandy Koufax knew because his arm told him so. Jim Brown and Barry Sanders got out while still in their prime, too, for reasons that went beyond football.
But for every Koufax, Brown or Sanders there's a dozen Favres trying to hold on even as their skills fade and the inevitable aging process takes over. The great Johnny Unitas wasn't immune when he tried to stretch his career in San Diego, and Michael Jordan couldn't even make the playoffs when he ended his second retirement to play two final seasons with the Washington Wizards.
Muhammad Ali couldn't escape punches that sadly may have cost him more than his reputation, and Joe Namath couldn't escape defenders as he tried to play on creaky knees for the Los Angeles Rams.
And who can forget a 42-year-old Willie Mays stumbling and bumbling around in the outfield in the 1973 World Series for the New York Mets.
They play because they still think they can play. They play because the money is good.
A lot of them play because they just don't know how to quit.
"The easiest thing is to become an athlete," former heavyweight champion George Foreman said. "But how do you get out? The sad thing about it is so few have been successful in doing that."
Foreman ended up being one of those, though it helped that he had a second career as a grill pitchman that made him more than he ever made in the ring. But even he retired twice and was in the gym training for yet another comeback at age 55 before his wife squashed the idea.
Favre, of course, famously threatened retirement for a few years before tearfully announcing in March that he was done after 16 seasons with the Green Bay Packers. Like many before him he almost immediately had second thoughts and said Monday in a televised interview that the Packers helped push him out the door.
"I am guilty of retiring early and there is a reason for that," Favre said.
The Packers deny that, saying they urged Favre to play and moved on only after the quarterback remained noncommittal and they had to make a decision. The team found itself in the uncomfortable position of having to finally say no to a sure Hall of Fame player, something the Mets nearly faced in 1973 before intermediaries finally persuaded Mays to retire.
Money is almost always a factor in players wanting to stay, and Favre had about $25 million remaining on his contract with the Packers. But it goes beyond that, Foreman said, beginning with the admission that every athlete has to eventually make that his or her body is not what it was in younger years.
"When you retire you're basically saying to yourself that my body is no good anymore," he said. "That's something no one will admit to."
Foreman said he cried on a few occasions after retiring because he missed the big stage so much. It took time, he said, to finally accept the fact that that part of his life was over.
"I've been successful with a lot of business things, but if I told you that took the place of boxing I'd be lying," he said. "The thrill of a crowd roaring for you after just winning a boxing match, nothing touches that.
"You can't touch that with a billion-dollar paycheck."
NOT QUITE CALLING IT QUITS
A look at a few athletes who retired and then changed their minds:
Ervin "Magic" Johnson......1979-91......1996
Source: Wire services