Michael Phelps is a phenomenon whose records may never fall

My view on Michael Phelps' career is double-edged. You could build up his feat in becoming the most decorated Olympian of all time, although you could also play it down. He has been able to pursue multiple golds at his last three Olympic Games, and one might argue that this diminishes his place among the greats.

But how do you rank greatness? With 15 gold medals he already has to be listed as one of the finest Olympic athletes ever, and yet I still believe his feat would have been more impressive if he achieved it over six or seven Games.

No one should doubt the power of Phelps' achievement to inspire. My son, Zak, was with me in Beijing on the night when Phelps won his eighth gold of those Olympics in 'The Cube'. Zak was just 10 at the time and did not fully appreciate the significance of what he was witnessing. But a little while ago he said to me: "Dad, I want to be a professional sportsman." That is the impact the Games, and the finest accomplishments, can have. They are about dreams.

That evening in Beijing, Phelps broke Mark Spitz's record of seven golds at the Munich Olympics in 1972, a standard that I never thought would be beaten in my lifetime. To be honest, I was a little disappointed that he did break it, because Mark has become a good friend of mine. I asked Mark whether he wanted to watch Phelps win eight and he replied: "No. I don't want to see my record taken down." But I am still pleased to say that I was there when Phelps made history, for I do not believe those eight golds at a single Games will ever be bettered.

There are 300 medals awarded at the Olympics, and there are so many champions that we are constantly looking for the truly outstanding performance. In 2008 Phelps, almost unbelievably due to the tight margins he was working against, delivered it. He took that step up to greatness, because he was taking on specialists in each of those individual races and beating them.

Even though his disciplines are very close to each other, he has still been racing against people who have trained for most of their young lives for just one event. And Phelps' ability to leave them trailing in his wake is what makes him special. Take the other extreme, and look at the decathlon.

I do not want this to be seen as a rift between Daley Thompson and I, because one does not exist, but some people claim that decathletes are the ultimate athletes. I hardly think, however, that a child wakes up in the morning, finds a sport and thinks, 'I want to be a decathlete.' You want to be a sprinter, a discus thrower, but ultimately you do not quite make it. Instead, you have the capacity to take on lots of different events.

It is extremely impressive to be able to do that, but your rivals are not specialists - they are generalists, just like you.

You are never going to be competing against Usain Bolt in the 100 metres, or against Liu Xiang in the high hurdles. In one or two of the disciplines you might be exceptional, and capable of rising to the top of the Olympic field, but otherwise you are defined by the breadth rather than the depth of your talent. You are pitched against people doing precisely the same.

Phelps is different. He has taken on the best in their chosen fields and won. He possesses a perfectionist streak, just like Mark Spitz did, and just like I did in winning five Olympic golds in rowing. Whereas Matthew Pinsent would always shout, "Don't tell me when the boat's going badly, tell me when it's going well," I would tend to say, "This is not good enough. We have to do better." I saw that in Mark's make-up but I loved him immediately as a character: he was an inspiration to me, and his seven swimming golds in Munich encouraged me to pursue my Olympic dream. He tried to make a comeback in Barcelona in 1992 and discovered, rather like Phelps has found in London this week, that the old magic was not quite there.

But we are all very competitive people. It is immensely difficult for somebody as driven as Phelps to call time on his search for the thrill of an Olympic medal. People say to me, "You went to went to Sydney to win a fifth gold." I can assure them that was not true: I went there to win an Olympic gold for its own sake. And in the men's four, I was trying to win a race that I had never won before at that level. I was not going out trying to make history.

Being so passionate, you want to keep winning golds, to keep winning medals, to keep doing the best you can. Some disagree, insisting: "You should have retired. You are never going to do any better." But who are we to give the right to tell great champions when they should stop?

Only Phelps, quite rightly, has taken the decision on his retirement. For the pool has been his life.