He is the oldest fighter to win a world title and has broken his own record twice; most recently when he became IBF and WBA (Super) light-heavyweight king in April 2014 at the age of 49 by defeating Beibut Shumenov.
Hopkins originates from the housing projects in Philadelphia and following a tough upbringing, it was in prison that he discovered his passion for boxing - turning professional after being released in 1988.
Mental strength can be seen through a fighter's cardio- how hard they worked before the fight, and if they keep going when things get tough- as well as their desire and ability to impose their will on an opponent. Will he be able to take Jose Aldo's best at UFC 156? But for now, we only know that trucks can hit him and he will keep on coming.
He didn't leave much to mystery with his loud mouth, but it was his famous graveyard ring walk on Halloween 1998 that cemented 'Prince' Naseem Hamed's place on the list.
Being hard to knock out, plain and simple, is important. In fact, this may be the single greatest predictor of who we view as tough. Big fights where toughness was needed and delivered on- check. An ability to impose one's will on another through great cardio and mental toughness- check. The bottom line is that Nick Diaz is the kind of tough guy are forced to respect. No one has proven capable of taking a shot like Frank Edgar to date.
Later, he was re-incarcerated for assaulting a police officer.
For much of his career, his contract was majority owned by Frankie Carbo, a one-time mob hit man and senior member of the Lucchese crime family, who ran boxing interests for the Mafia.
So, the criteria are (drumroll please): Being tough isn't about potential for being tough; it's about having shown your mettle in great fights.
One of the criteria for being a tough guy in MMA is to have fought in great fights that showcased this.