IT is a debate that will rage for years to come. When Michael Phelps helped the United States' 4x200m freestyle relay team win gold on Tuesday evening, he claimed his 19th Olympic medal, more than anyone else in the history of the modern Games.

He is the most successful Olympian of all time, eclipsing Soviet gymnast Larisa Latynina, who won 18 medals at Melbourne, Rome and Tokyo in the 1950s and 60s. But does being the most successful inevitably mean that he is the best?

Defining greatness is a terribly thorny task. Success can be quantified; greatness is rather harder to pin down.

It can be argued that you don't even have to be particularly successful to be great. Qualities such as longevity, impact, talent and an ability to defy the odds also have to be taken into account. So where does that leave us with Phelps?

His achievements in the world of swimming are certainly incredible. If Phelps was a country, his tally of 15 gold medals, two silvers and two bronzes – a total that with three days of swimming still to go is not finished yet - would leave him 55th on the all-time list. So that's more than 150 countries that have won less medals in the history of the Olympics than this remarkable swimmer from Maryland.

He is unquestionably the greatest swimmer of all time, even if the current Olympics are witnessing him at a level slightly below his peak, and his eight gold medals in Beijing still rank as an achievement that is unlikely to be matched.

It is not just at the Olympics where he has excelled either. An unwaveringly committed competitor, he has also won 26 world titles and set 39 world records.

But does all of that make him the greatest of the greats? It seems churlish to pick holes in Phelps' record so soon after his record-breaking feat, but it can be done.

For a start, swimming is a sport that lends itself to multiple medallists. The fact Phelps was able to contest for eight gold medals in Beijing is held against him.

If you're a shot putter or a middleweight boxer, you can only compete for one medal every four years. If you're Phelps, or even his US rival Ryan Lochte, you can challenge double figures in one week.

So on that measure, the weight of medals should not be enough. More is needed for greatness, and perhaps it is here where Phelps slightly falls down.

Leaving aside the supreme dedication he shows in training, it could be argued that Phelps has achieved his success from a position of privilege.

The United States is widely acknowledged to be the strongest swimming nation in the world, with the best facilities, coaches and development structure. He has not had to overcome too many hardships to reach the top.

Has his success transcended his sport? Possibly. He is a globally recognised figure and there is no doubt that excitement levels tend to increase when he is in the pool. But he will never be a Jesse Owens, whose success at the 1936 Olympics provided a powerful rebuttal to Adolf Hitler's Aryan vision, or a Tommie Smith, whose Black Power salute in 1968 helped transform the way millions viewed the world.

He doesn't have to be of course. He just gets himself into the pool and gets on with the task of winning medals. But on a stage as influential and far-reaching as the Olympics, is that sufficient to be lauded as the greatest of all time?