That's right, old age.
For the past decade, experts have said Hopkins, who turns 49 in January, was too old to keep beating fighters who were 10 to 20 years his junior.
Yet here Hopkins is, preparing to defend his IBF light-heavyweight title tomorrow night at Boardwalk Hall in Atlantic City.
"Karo Murat is hoping like others before him that he is lucky enough that I got old in the gym," Hopkins said. "There's no magic spell that hits you over the head that says, 'You're old.'
"As far as I'm concerned, old to me is not old to the average person. What is old?"
So what are you thinking if you are Murat, the 30-year-old challenger?
What doubts have crept into a corner of your mind as you prepare to face something that most laws of nature say should not exist? Probably weird thoughts, things you've never had to contemplate before but can't help but slip into the conversation.
Is Hopkins the one athlete who has held Father Time at bay? And what are the implications of his doing that?
Does Hopkins, who is by far the oldest man to hold a recognized world boxing championship, actually have alien DNA coursing through his body as he has recently claimed?
Or is Hopkins the embodiment of the mythological Achilles: a near-invincible opponent whose deeds exceed what the laws of physics say should be possible.
How do you talk about fighting a champion who by all that most of us know should be retired and sitting in a rocking chair?
Few of the great ones - not Ali, Marciano, Louis, Leonard, De La Hoya - had any large measure of success far beyond their 30th birthdays.
Hopkins has won 23 world championship fights since his 30th birthday.
Everything in the history of mankind says that ultimately the younger and stronger will vanquish the older as they become weaker.
Murat only has to Google beaten young lions like Kelly Pavlik, Jean Pascal or Tavoris Cloud to see how Hopkins has turned that theory on its side.
Murat (25-1-1) isn't considered to be of the same caliber of those fighters.
Born in Iraq, his family feared persecution and fled to Germany when he was 12.
Murat is fighting outside of Europe for the first time. He hasn't had a match since scoring a technical knockout over a journeyman named Sandro Siproshvili on Feb. 6, 2012 in Denmark. That's an inactive period of more than 19 months.
He is ranked No. 2 by the IBF but, for what it is worth, is not ranked in the Top 15 by the WBA, WBC or WBO.
In reality, he is a handpicked opponent for Hopkins - a man who is nearly two decades older.
"I'm going to surprise a lot of people," Murat said. "I've followed [Hopkins'] career since I was 12 or 13 years old. I have respect for him.
"But you see the mileage on him. He looks like a 48-year-old. He doesn't have the speed anymore. The time is now to beat Hopkins.
"I think it is a sign from God that I am here to end Bernard's career. He is an old man and needs to stop now."
Hmm, I believe Cloud, 31, said similar things before he lost his IBF title to Hopkins by a unanimous decision in March.
Cloud was 24-0 with 19 knockouts before he fought Hopkins. Now, he is 24-2 and scrambling to salvage his career.
So how much pressure is on Murat when he knows nature says he should have every physical advantage over Hopkins, yet the humans who track these things have installed him as an 8-1 underdog?
You can't lose to a man who is beyond geriatric as far as being a professional athlete, can you?
How would you explain that to your family, your friends, and the world?
On one hand, losing to a legendary champion like Hopkins, who has never been knocked out, would be no disgrace. But on the other hand, losing to a 48-year-old man would be a bitter blow to the ego and pride that could never be fully digested.
You know that Cloud is hoping Murat loses, so that he can be removed from the infamy of being the boxer who lost to the oldest champion ever.
"Make sure you are all going to be there," Murat said, "and you are going to witness a young boy from Germany who's going to come here to Bernard's home turf, and he's going to take the title from him."
Again, Murat is 30 years old. Only in a fight against Hopkins would he refer to himself as a "young boy from Germany."
That's one more psychological edge that Hopkins does not need.