Survivor sang about it in Rocky III, putting it succinctly for those who didn't know the difference between a fighter that still hungered for the win and one that couldn't take it anymore.
"Hangin' tough, stayin' hungry.. it's the eye of the tiger, it's the thrill of the fight."
It's become a great anthem about wanting victory more than your opponent and about needing to still have the desire to push through the pain barrier.
For many fighters it is a very public event when they are exposed as no longer having 'the eye of the tiger'.
For Ricky Hatton that moment came on Saturday night.
The former welterweight and light-welterweight champion capitulated to a body punch from the capable, but uninspiring Ukrainian Vyacheslav Senchenko, and to the dismay of his fans didn't even try to beat the count. He got off the mat in tears complaining about not wanting to 'keep picking his arse up off the floor'.
Hatton's look of utter resignation almost mirrored the moment seven years before when his career really went stellar. That was back on June 4, 2005, when his opponent, the world welterweight champion, Kostya Tszyu also hit that infamous wall.
Tszyu, a top five pound-for-pound fighter with an enormous reputation, sat dejected on his stool at the end of the 11th round, exhausted, unable to counter Hatton's stamina and punch output and unwilling to continue the fight.
Like many boxers who reach the same point and still find it hard to admit to, the Australian-based Russian didn't announce his retirement, but took time off and never fought again.
Just as this weekend will have been tough for Hatton we have seen over the years the same reaction in many a great fighter.
Around the same time Hatton defeated Tszyu, Mike Tyson famously quit on his stool at the end of the sixth against his overmatched opponent Kevin McBride, saying afterwards: "I'm not going to fight again. I haven't got the fighting guts or the heart any more."
Tyson's then-trainer Jeff Fenech, himself a former three-time world champion, had suffered his own humiliating loss to Azumah Nelson 13 years before, and was to capitulate first to Calvin Grove and then Phillip Holiday, in two rounds, attempting to get his career back on track.
Once that air of invincibility had gone so did Fenech's belief - he was a changed man.
Virtually all the young warriors who took their division by storm, flashy and fierce and unable to even imagine defeat, must some day face it.
Few great fighters have retired undefeated: Rocky Marciano, Joe Calzaghe... possibly someday Floyd Mayweather.
For the rest there is a sad awakening that usually comes in one violent moment when they are relieved of their senses, and come round to a new reality in which they are no longer indestructible.
Occasionally fighters respond to this with grit and character and are able to successfully rebuild their careers after a knockout.
Lennox Lewis came back from lights-out defeats to Oliver McCall and Hasim Rahman to avenge both losses and continue his rise as one of the great heavyweight champions.
Amir Khan responded to a devastating knockout by Breidis Prescott to come back and defeat the respected Marco Antonio Barrera and then added the very good scalps of Paulie Malignaggi, Marcos Maidana and Zab Judah before being derailed by Lamont Peterson and TKO'd by Danny Garcia. Where he stands now, only time will tell.
For the hugely entertaining and brash Prince Naseem Hamed the moment where he knew he was no longer up for the fight came not with a knockout but with his humiliating loss to Barrera in 2001.
The undefeated featherweight champ went into the bout favourite, but was comprehensively outboxed by the Mexican, who in the 12th round spun Hamed around and forced his head into one of the turnbuckles in a macho display of his overall superiority. All Hamed could do was smile weakly in response. At that point his career was effectively over.
Although Hamed fought one more time he walked away from the ring at the age of 28 without ever officially announcing his retirement.
For Hatton, it has taken the past 42 months of thinking about leaving on a high note to get him back in the ring, but once there, he knew he no longer had the heart for it.
The seed was planted five years ago when Floyd Mayweather knocked him out in the 10th round of a WBC welterweight title fight. His two-round annihilation by Manny Pacquiao 18 months later really just emphasised the fact that he no longer had the same belief in himself.
Hatton struggled with those losses, and his life spun out of control with booze and drugs and thoughts of suicide, before the hope of redemption straightened him out again.
Most fighters don't know when to quit, and sadly it's often the place where they enjoyed their greatest successes that reality is finally brought home to them.
With any luck Ricky 'the Hitman' Hatton will find some peace in understanding the same fate awaits all great boxers.