It is always inspiring when a great sportsman defies the march of the years, announces that he still believes he can find the best of himself. But sometimes there is an extra force, the resonance of a special conviction.
Such certainly was the impact the other day when it was Brian O'Driscoll announcing that he was far from done. At such a moment you have the essence of the man who has turned away from the mirror and said: "I still see someone who can compete at the highest level – and I will not quit while this is still possible."
Roger Federer is the latest to shake his fist at the tyranny of time but then the great tennis player has always been separated from his opponents by a net.
When O'Driscoll makes a similar commitment the effect is maybe rather more elemental.
When the Irish captain does it you are bound to think of all the punishment he has taken, how much extraordinary physical commitment has accompanied the most sublime talent.
Gareth Edwards, arguably the greatest rugby player ever born in these islands, explained the motivation behind a decision to play on quite unforgettably when, as he hit his thirties, he was agonising over whether to play on one last year or get on with the business of the rest of his life in those days when the financial rewards from the game were so much less generous.
"My head says one thing," Edwards revealed, "but my heart says another. You know the prudent course would be to say, 'I've had some wonderful years but now it is time to start to organise my life after rugby.' Then you think, 'This chance to play another year at the top, do the thing that draws from the best you are ever likely to have, will never come again. If it is still possible, why not do it? Why not run out the string while you still can?'"
Edwards played until he was 31, and it is easy to hear now the echoes of his resolve in the declaration of O'Driscoll, surely a candidate for nomination as the greatest northern hemisphere talent since the Welshman finally walked away.
Driscoll's incentive is, of course, huge. He has the chance of winning a second straight Grand Slam, another statement of extraordinary resolve that would follow hard and gloriously on last year's achievement – Ireland's first such triumph in 61 years.
He could scarcely have been more emphatic that when he does retire the decision will be dictated by no other factor than the promptings of his own mind and his own body. He said: "I still have more to give. I have seen players leave before their time and I would hate to do that. So why bother with an end date? I want to set records. I want to do everything on the rugby field within my means."
When he looks back to his first international for Ireland, a mauling by the Wallabies in Brisbane in 1999, he says it as though it happened only yesterday. "I was really looking forward to my first cap and didn't feel nervous until the fireworks went off when Australia came on to the pitch. I wasn't expecting that. Once the whistle went all those nerves evaporated. I still have those nerves. If you don't, the mind isn't sharp.
"The butterflies focus the mind and you get ready for combat. Ireland is in the fortunate position of having a lot of great leaders around so the workload is shared. But it would be strange playing in an Ireland international without being captain. When I last did that it was an eternity ago."
An eternity, this is, of astonishing physical commitment – a willingness to put his body on the line so frequently that his father Frank, a Dublin doctor, has confessed to feeling nothing less than trauma in the wake of the consequences. "I worry," he said some years back. "Brian is risking serious injury with his level of physical involvement, but nothing I can say will deflect him. He says if he plays the game, he can only do it one way."
O'Driscoll Snr's trauma reached one of its highest levels a few weeks before the start of the 2007 World Cup, when in a warm-up game Bayonne's Tonga international Mike Tewhata left his son's face covered in blood in an off-the-ball incident. The relief was that O'Driscoll had merely sustained a fractured sinus and a wound beneath his eye. He would not be kept from the action too long.
What is most stunning about O'Driscoll's career, the way it has combined so consistently a beautiful talent and relentless combative instinct, is its sheer durability.
Inevitably, listening to the still flinty ambition of O'Driscoll the other day you were carried back to the rain- and wind-scoured night five years ago in Christchurch when O'Driscoll's nerve and appetite for the game were tested as never before – or since.
He will always believe that he was the victim of one of rugby's most callous and cynical acts, the spear tackle, when his Test leadership of the ill-starred 2005 Lions lasted less than a minute. The controversy lingers on with no dissipation of O'Driscoll's belief that the assault launched by the great Tana Umaga and his team-mate Keven Mealamu was an offence beyond his powers to forgive.
A few months later his account of the incident was still so searing it might have been delivered from the stretcher on which he was carried into the dressing room.
He said: "It was the biggest game of my life and it lasted 41 seconds. You should have seen the injury when the surgeons opened up the shoulder, reset the joint and tightened it up with 16 staples. It looked like a shark had tried to tear my right arm off. There I was captaining the Lions in a massive Test and suddenly I'm up in the air and things are turning bad.
"It's the first time I've felt powerless on a rugby field. If you're caught in a ruck and your leg's bent horribly you feel uncomfortable and you just want everyone to get off quickly. This is different. This felt like I was in the lap of the gods. I knew I had to get my head out of the way. My neck, also, would not have easily withstood the impact. When you consider the fact that the fall dislocated my shoulder it's safe to assume my head and my neck would not have fared much better.
"I've never felt such sustained pain. If you get friction on an ankle injury it feels like torture. But that lasts five seconds. The pain in Christchurch stayed at that level for 20 minutes before they finally found some morphine."
Five years on O'Driscoll's rage to compete remains as intense as ever. In any circumstances, it would be a phenomenon of modern sport. But in those that have seen him come back from another witheringly physical Lions tour, the unremitting battle in South Africa, to launch himself on still another pursuit of the Grand Slam, he has staked out for himself terrain all of his own.
He is rugby's supreme artist-warrior, a man who continues to push back all limits that might be place against his name. Ireland once again is a huge beneficiary. So, too, every corner and an embattled game.
Who will shine? Other greats
*Ronan O'Gara (Ireland)
Scored most Six Nations points.
*Adam Jones (Wales)
A ferocious scrummager, arguably the best tight-head prop in world.
*Chris Paterson (Scotland)
Highest scorer in Scottish rugby.
*Imanol Harinordoquy (Fr)
Powerful No 8 who can dictate matches.
Spotlight on Ireland
Brian O'Driscoll and Gordon D'Arcy, the classiest midfield pairing around.
*One to watch
Leinster loose-head prop Cian Healy. He sure can run with the ball in hand, but can he scrummage?
*Can they win it?
Certainly, but much depends on their visit to Paris in week two.