What if this is simply about the numbers? What if the debate about who deserves to be called the G.O.A.T., or the Greatest of All Time, isn't really a debate at all? Serena Williams is among those who contend that this isn't subjective; this is arithmetic.
For Williams - whose victory at the Australian Open brought her a 19th Grand Slam title, 'just' three short of Steffi Graf's modern-era record of 22 - believes that the only way to achieve true greatness is to win more majors than anyone else. "I go by the numbers," the Californian has said. "I go by what is written down." And her coach, Patrick Mouratoglou, is also of the opinion that this is entirely driven by the data, with the Frenchman once saying that Williams has to keep on winning if she wants to become the alpha female across all generations: "Serena is already one of the greatest, but it is up to her to become the greatest by beating the records."
Sounds simple, if winning 20-odd Slams could ever be described as such. But the reality is that it isn't as straightforward as this (tennis's chattering classes won't be denied the pleasure of debating this one). Which record, you might well ask, is Williams supposed to be aiming for?
Perhaps you think it should be Margaret Court's, whose career was across both the amateur and professional eras, and who has the largest collection of Grand Slam titles, with 24. If it's all about the numbers, don't you have to go with the biggest figure of all? But there's the counter-argument that the Slams that Court won during the amateur era are somehow worth less, and that her tally was inflated by scoring so many Australian Open titles - 11 in the amateur and professional eras - at a time when many leading international players didn't always make the trip Down Under.
There's a reluctance to step too far back into sepia. You don't always hear Court's name during the G.O.A.T. debate, with many observers arguing that the true record, and the number that Williams is trying to equal and then surpass, is Graf's modern-era portfolio. But there isn't total consensus there, either. Maybe, some would say, there should be an asterisk against Graf's total as would she have won so many majors if Monica Seles hasn't been absent after that horrific day in Hamburg? "I look at Graf's numbers and I look at what happened to Seles, and I think, 'Wow, Steffi is great'. But that [Seles' absence] is what padded her 22," Pam Shriver once told ESPN.
So you will appreciate that people in tennis can't even agree on the significance of the numbers in front of them. Looking at the list of the serial Grand Slam winners is clearly the starting point for any sensible debate on this subject - who is going to champion a player in low singles figures for majors? - but surely greatness is about more than just the numbers. It's also about the way you play the game, how you go about your business. A personal view is that Williams doesn't necessarily have to surpass Graf and then Court to be widely regarded as the G.O.A.T., though doing so clearly wouldn't harm her credentials.
Four women tend to be at the centre of these discussions: Williams, Graf, and the two players tied on 18 majors each - Chris Evert and Martina Navratilova. It's often said that it's not easy to compare generations, because of the advances that have been made in technology, physical preparation, and because the quality and the depth of the opposition varies from one era to another. But that doesn't mean that you shouldn't try. For a while now, John McEnroe has been saying that Williams should be regarded as the greatest: "I've seen them all - Martina Navratilova, Billie Jean King, Chris Evert was a machine, Monica Seles and Steffi Graf, but I think we're watching the greatest female player that's ever played this game."
Longevity matters, too. It's a remarkable statistic that Evert won at least one Grand Slam title every year for 13 years. Consider also how Navratilova won her slams over 12 years, the last of which came when she was 33 years old. Williams is 33 now, and a young 33 at that, because of the long absences she has had from the sport during her career, and the indications are that she still has several more Slams in her. Already Williams has spread her Slam victories over the past 16 years, having started her collection at the 1999 US Open. To win her major titles, Williams has fended off several different generations of players.
Some weight should also be given to players who have transformed the sport - it was Navratilova who introduced sports science and physical conditioning to the women's games, and Williams has taken athleticism, power and speed in tennis to new heights. In tennis, as in all sports, the latest is always the greatest - it's difficult to resist the idea that the best current player would trump the champions of yesteryear. "I'm a big believer that every generation gets better so Serena should be, could be, the best that has ever lived," Billie Jean King once said.
Evert has suggested that you can't just be a slave to the numbers: "Serena doesn't have the greatest record. Margaret Court does. And Steffi Graf has a better record, too, and Serena is the first to admit that. But nobody has had a game like Serena's," Evert once said. "Nobody has the power and the shots and the serve and the complete package that she has so she's the best tennis player."
It is hard to look beyond Williams. The argument is frequently made that Williams has the greatest serve in the history of the women's game - you could go further and suggest that it is the greatest shot of all time. But this is about much more than her physical and technical capabilities; what makes the American's candidacy so compelling is her strength of mind, her bloody-minded nature, her refusal to lose. Williams is nothing less than the most ferocious competitor this sport has ever seen. More than that, it's often up to Williams whether she wins or loses. To borrow a locker-room phrase, when Williams is 'on', has anyone ever been in greater control of her own destiny? And shouldn't that count for something?
Mark Hodgkinson is the author of 'Game, Set and Match: Secret Weapons of the World's Top Tennis Players' (Bloomsbury, May 2015).