Sometimes the golden ages of sport come and go before too many of us have quite grasped the enduring quality of those who have fashioned them out of their talent and their character.
Sometimes it is only after they gallop off down the dusty, cluttered high road of history that we arrive at a proper assessment.
No, Muhammad Ali wasn't merely great, he was eternal. Yes, Pele had genius and humility on the field and fantastic strength. But he also understood the game better than anyone before him or since.
However, only the most strenuous blindfolding might have caused any such delays in historic perception at Wimbledon this week – or blurred even slightly the scale of the challenge facing Andy Murray.
For some years now we have discussed his potential to win the most fabled of the Grand Slam tournaments and become the first of his countrymen to do it since the brilliantly competitive, swaggering Fred Perry completed the third of his triumphs 75 years ago.
Yet this week you didn't need a shred of compassion to grant the kid from Dunblane living under all the pressure a certain upgrade in critical allowances.
It surely had to come in a week when the reigning champion Rafa Nadal, the six-time winner Roger Federer and their most exciting challenger, Novak Djokovic, on successive days all redefined superbly the nature of Murray's assignment. This is not, if we didn't know before, merely to end what has so often seemed an interminable British drought. What he has to do is produce one of the greatest achievements in the history of the nation's sport.
Lifting the Centre Court silverware has never been less than a daunting challenge for British men, and this was still eminently true even in the brief hiatus between Pete Sampras and Federer when the haunted hero Tim Henman had his best chance to placate all the local yearnings.
This week, though, we have seen some of the ground of SW19 shift under Murray's feet. It has pitched the 24-year-old Scot into an arena of tennis that has surely never known quite such a concentration of sublime quality at the top of the men's game. Certainly, it is not easy to trawl back through a lifetime covering sport and reproduce readily the certainties of performance that have made attending Wimbledon this week such an unbroken privilege.
This is not something to dream up casually, because it would be to an insult to some of the greatest champions of sport in living memory. Rod Laver, Bjorn Borg, John McEnroe, Boris Becker and Sampras are not the kind of men you would lightly shove into the margins of any sport.
Yet if we are obliged to remember and respect the deeds of yesterday – for how else can we stab at any perspective on the sensations of today? – there is another need. It is to acknowledge the glory of the moment and recognise quite how it has been composed this week. It has been created by three men who, day by day, have set an agenda of expectation that every sense suggests may well be unprecedented.
Who, for example, is brave enough at this point to assess the claims of any of Federer, Nadal or Djokovic and go to the betting window with anything more than the ghost of conviction? In America they say some fights are so finely balanced, so filled with rival possibilities, you can do no more than pick and hope. A final between Nadal and Federer or Djokovic, the evidence of this week insists, would be an ultimate "pick'um". The greatest Wimbledon final of all, most agree with respect to Borg and McEnroe, was Nadal and Federer three years ago. It was the imperishable product of the most formidable prowess and unforgettable grace and sportsmanship.
In terms of concentrated emotion, at least, there is the rival five-set collision of Goran Ivanisevic and Pat Rafter on People's Monday 10 years ago, the force of the Croat's ambition, after three final defeats and a somewhat fortuitous, rain-smeared survival against Henman, claiming almost every available scrap of support outside of the Australian Ashes team wearing their baggy green caps in the hope of inspiring their compatriot Rafter.
Ivanisevic came to mind on Thursday night when another man from the Balkans, the Serb Djokovic, accepted a brief distraction from his thrilling destruction of the big South African Kevin Anderson and discussed his burgeoning relationship with a squirrel in the garden of the house he has rented for the tournament.
"It keeps coming closer and maybe I will think of a name for it," Djokovic allowed, without suggesting for a second that he is not infinitely more preoccupied by the proximity of his date with tennis history.
In his moments of greatest pressure, Ivanisevic admitted there were times when his life was something of a horror movie, and that on the morning of the Rafter game – after a mostly sleepless night – he was calmed only by the Teletubbies when he turned on the bedroom television.
Such composure may not be so easily gained by Djokovic, for all his new authority, at the dawn of his likely semi-final with Federer next week.
The Swiss master ambushed him in the Paris semi-final but there has been little evidence of scar tissue this week. He was prepared to celebrate Federer as one of the great players but he was quick to add that never before had there been such pressure on the man who has won 16 Grand Slams. "He is challenged now by Nadal and other players, including me, and I have to tell you that when I think about tennis now I believe I can beat anyone." If he makes the final against Nadal such confidence will no doubt be reinforced by the fact that already this year he has beaten the man from Majorca in four finals, twice on hard courts in Indian Wells and Miami, twice on clay in Madrid and Rome.
This may provide another edge of intrigue but it does not intrude on the central reality that has brought such distinction to Wimbledon this week. It is that rarely, if ever, have three consummately gifted players appeared at the pinnacle of their sport in such conspicuously brilliant shape and competitive instinct.
All three have been masterful on and off the court, all three have made it clear that they are aware of the possibilities they have created. Djokovic, by some distance the most openly expressive, took some time to explain the meaning to him of the record-breaking streak which Federer brought to a close at Roland Garros.
"One thing you have to understand," he said, "is that there will always be pressure on you to win. It is part of your life, it can never be avoided. So what do you do? You celebrate when things go well, you express everything inside you, because you have worked so hard and you know there are no guarantees that everything will be positive." This was soon after a celebratory leap so high, so exuberant that it seemed to lift up the entire Court One. He happened to have produced a withering winner at the climax of a breathtaking rally, but really no one needed telling that it was about rather more than that. It was a statement of optimum self-belief.
From Federer and Nadal we have also had such moments of release, an accumulation of evidence that they are sure not only about themselves at this time but also their place in history.
Whoever is triumphant a week tomorrow, there is one certainty. He can tell himself that he won tennis's greatest prize at a time when the game had never been played with such brilliant wit and force and power. Or, if he wants to put it in a different but entirely reasonable way, he was the king of one golden age of sport that would never need the endorsement of history.