What's less easy is putting it all into immediate historical perspective.
Phelps earned a $1 million bonus today from Speedo by tying Mark Spitz's record for most gold medals in a single Olympics. He'll have a chance to win his eighth tomorrow in the medley relay. With 13 gold medals in his career, he stands alone among Olympians. The debate on who is the greatest Olympian will rage for decades, but not when it comes to his coach.
"I think if it was over today, he's the greatest Olympian who ever lived," said Bob Bowman, not needing to pause and think about the question yesterday. "I do think it's difficult to compare [different sports], but in terms of just sheer dominance in his events and the times he's putting up and what he's doing now in two Olympics … I think it's hard to argue."
What is more definitive is how Phelps stacks up against Spitz, who won seven gold medals in 1972, a mark that stood unchallenged for 36 years. The two swimmers are not close and have met and exchanged words only a few times, but they will be forever linked in some people's eyes.
"Being compared to the greatest Olympian of all time, that's not bad," Phelps said this year. "But I want to do something that no one else has ever done before. I want people to look at me as being one of a kind."
Spitz, who this week told Agence France-Presse he was upset at not being invited by anyone to come to Beijing and chose not to come on his own, was gracious about Phelps on The Today Show on Thursday morning.
"He is the single greatest Olympic athlete of all time now," Spitz told Today co-host Matt Lauer from California. "He will be probably the single greatest athlete compared to anybody in any century, the 20th, the 21st century - whatever.
"You know, you judge a person's character by the company you keep, and I'm glad I'm keeping company with Michael Phelps. I'll be happy to stand aside and let him carry the torch. He's just one great athlete, and it's very exciting to watch him."
Most people in the world of swimming feel that Phelps' performance this week easily exceeds what Spitz did in Munich, and there isn't much to debate.
"Mark was a very talented swimmer, and very dominant in his time," said Mark Schubert, director of USA Swimming and head coach for every Olympic team since 1992. "I think that the difference with Michael is that he has a much more challenging program, and I think that the competition throughout the world is more challenging."
In 1972, when Spitz swept his events, there were only 21 countries competing and only 10 countries that medaled in swimming. The United States won 43 medals between its men's and women's teams, 17 of them gold. The rest of the world combined to win 44.
On the men's side in 1972, at least two American men medaled in every event except the 200-meter breaststroke and the 400-meter individual medley, and the United States won all three relays by more than three seconds. The Americans actually swept the 200-meter butterfly, something that isn't even possible now because the rules were changed limiting each country to two competitors per event.
At the 2008 Games, the competition is much fiercer. Twenty-four nations are competing, and through the first six days, 16 different nations had already medaled. "We can't even compare Michael to Mark," says Jon Urbanchek, an assistant coach on six Olympic teams. "It's apples and oranges. It's a different ballgame. … Technology has changed, their personalities are different. I'm not saying anything negative about Spitz, but his time is gone. It's time to turn the reins over to somebody else."
In 1972, Spitz was the best in the world in two strokes - freestyle and butterfly. Phelps is an elite freestyler, the best butterfly swimmer in the world, and he's probably the third-best backstroker in the world behind Aaron Piersol and Ryan Lochte.
Additionally, Phelps is the most dominant individual medley swimmer ever, which is probably the best measure of overall swimming talent because it requires competency in all four strokes. Spitz did not swim the individual medleys.
"Michael is the best in the world in three different strokes," Urbanchek said. "It's totally unusual for anyone to have a talent like that in those three different strokes. Michael Jordan was talented in basketball, but he couldn't hit the baseball, could he?"
Competitively, Spitz and Phelps are also different creatures. Spitz also wanted to drop the 100-meter freestyle at the last minute in 1972 because he didn't think he would win, worried it would ruin his perfect Olympics. A teammate and coach talked him out of it, and he won by 0.4 of a second.
Phelps took the exact opposite approach in 2004, choosing to swim the 200-meter freestyle even though he knew he probably could not win. He took third behind Ian Thorpe and Pieter van den Hoogenband.
"When I do lose like that, it just motivates me to swim faster," Phelps said.
While Phelps is very much one of the guys, Spitz was a loner, distant from his teammates. He has a complicated relationship with the world of swimming, even today. He helped increase the popularity of swimming, but he has also rubbed people the wrong way.
Other than 41-year-old Dara Torres, no member of the U.S. swimming team was even alive when Spitz won seven golds in Munich. But for those watching the ride this time around, it's a daily thrill.
"It's unreal," said Matt Grevers, a silver medalist in the 100-meter backstroke. "I never thought I'd see this type of swimming in my life. It's absolutely going to be something for the ages. A hundred years from now, people are going to remember these Olympics for what Team USA and especially Michael did."
Phelps said he hopes what he's doing can have enough impact to increase interest in his sport. He laughed this week when a reporter informed him that there were trading cards of him selling for $500 on eBay.
"That seems like a pretty good deal," Phelps joked. "[Swimming] is definitely on the way up. I heard that the ratings are the highest back home that they've ever been for the Olympics and watching swimming. More and more people are starting to get interested and get more involved. So, I think my goal of raising the bar in swimming is coming along, but it's going to take time to really get it where I want."