We like the crazy in sport. We like the chasers of records, the hunters of the unthinkable, the pursuers of legend. Like Roger Bannister breaking the once-mythical four-minute mile in 1954 and pole vaulter Sergey Bubka flying over the six-metre barrier in 1985.
Like Mark Spitz predicting six Olympic swimming golds in 1968, winning seven in 1972 and being overtaken by Michael Phelps with eight in 2008.
It's not always about the succeeding, anyway, it's about the quest.
The crazy ones, noted an Apple advertisement, "change things. They push the human race forward". They were really talking about rebels - Muhammad Ali, John Lennon, Amelia Earhart, Pablo Picasso - and so Novak Djokovic, a sublime technician, doesn't seem to naturally fit that company.
He is more tennis' Martian, a gluten-free god who's trying Matt Damon-style to "science the s*** out of this" game.
But Djokovic is crazy. Because in China this Serb admitted he is eyeing the Swiss' record of Grand Slam titles. Who says stuff like this? Only the psychologically mighty.
With 10 Grand Slam titles so far, Djokovic is an entire Jimmy Connors career away from overtaking Federer's 17. Yet there he is, articulating his appetite: "Nothing is impossible. I have that kind of mindset. I know that it's still a long way to break his record... but it's one of the things that motivates me to keep going."
He's communicating his fearlessness to us, yet also to himself.
Here is what is truly crazy. Djokovic is admitting Federer's record is on his mind and we do not think it is an entirely preposterous idea. Because right now, on most days, it feels like you could give the Serb a wooden racquet, bathroom slippers and an eye patch and he'd still win a Slam.
He probably thinks so, too. And that's not crazy because as Joe Namath, the American quarterback, once said: "When you have confidence, you can have a lot of fun. And when you have fun, you can do amazing things."
Djokovic did not mention the number 18 nor state it was his life's mission. Maybe he's thinking 20 and is too polite to say so. Yet it was intriguing that he didn't parry the question because athletes can often be cagey about their ambitions.
To state a goal, after all, is sometimes to be imprisoned by it.
Tiger Woods' pursuit of Jack Nicklaus' 18 Majors became golf's predominant storyline to the point where even Woods' website has a section titled "Tiger v Jack". It was a chase that was tense, compelling, dividing and yet it did Woods a disservice.
Seen in isolation, he is an outrageous talent; seen in the context of 18 he seems a talent who has fallen short. Impossibly, it has made his 14 Majors appear less than majestic.
But crazy people don't scare easily, not by what might happen, not by the idea of possible failure. Djokovic, only 28, is just revelling in his temporary invincibility and in how nervous he makes everyone else.
Eighteen is eight more Slams for him. If you consider he won three Slams this year, he could achieve that feat by 2018. Except that the last time Djokovic had a three-Slam year was in 2011 and the next three years he won one Slam annually.
No one knows what is going happen next in sport. After all, baseballer Glenallen Hill once injured himself years ago while trying to escape the spiders he was dreaming of.
Sport has a terrific capacity to build fairytales but also to interrupt them. Form can just evaporate, just ask any golfer with the yips. Injury can bruise the strongest minds - just ask Rafael Nadal, for whom 18 Slams once seemed almost inevitable.
Greatness can abruptly erode, just ask Tseng Yani, who won four of eight Majors in 2010-11 and since has had only one Top 10 finish. Rivals emerge from the mist, just ask Rory McIlroy, golf's Chosen One a few years ago who is now being bullied by Jordan Spieth and Jason Day.
But, currently, Djokovic is undisturbed. He's feline fast, boxer fit, rival-proof and on a negativity-free diet.
He's won eight titles this year, lost five matches, is 3-0 against Nadal in 2015, 3-0 against Federer in their last three Grand Slam finals and 3-0 against Andy Murray in their last three Grand Slam meetings. No, Kei Nishikori and Milos Raonic are not making him quiver.
Djokovic admits he is at his peak and for all the science in sport we can't precisely tell how long that lasts. Long enough, we hope, to captivate us with his chase. Long enough also for us to reconsider this idea of judging greatness by one number.
As if years at No.1, head-to-head, effect on the game, surfaces won on and consistency over time is irrelevant.
Sometimes we just seem like hysterical mathematicians trying vainly to reduce great art to cold digits. After all, Djokovic, the great impersonator, might mimic Federer's numbers one day but emulating him is quite another business.
Yet even as we debate and dissect, calculate and cogitate, the truth is that there is no such undisputed position in sport as The Greatest.
Well, there was, but now Ali owns it forever.
This article was first published on October 16, 2015.
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