He knows that, at least intellectually. That's why he has given every indication before and after each event over the past week that he was soaking it all in because he does not expect to pass this way again.
Maybe you don't believe him. His friend and rival Ryan Lochte apparently doesn't, and has predicted that Phelps will show up in Tokyo in 2020, but Phelps said again on Friday – rather firmly – that Saturday night's 4x100 medley relay would be his very last Olympic race.
If he shows up in Tokyo, it will more likely be in a blazer with a network television emblem on it than something he invented for his brand of swimwear.
That will be the right decision if it is indeed final, but who can really say for certain until the next Olympic cycle begins without him.
It's not that Phelps couldn't come back at the age of 35 and win some medals. He already has proven that you should never be surprised when amazing people do amazing things. He broke a record last week that had stood since 152 B.C., for goodness sake. There's really nothing more he can do to burnish his legend.
Yeah, yeah, yeah, you've heard all that before, but the fact that Phelps was in Rio at all was a product of circumstances that transcend something so simplistic as man's age-old quest for shiny objects. This was as much about redemption as it was about athletic achievement.
Phelps didn't have anything to prove in the pool, but he had something to prove to himself, his family and his sport after he showed following the 2012 Olympics London that he didn't have a firm grasp on life on dry land.
Clearly, he was searching for something as he dabbled in golf and high-stakes poker. Clearly, he had some demons, which made it easy to get lost in the funhouse when there was suddenly unlimited time and money.
When it was time to finally grow up, this final Olympic quest provided the structure and motivation to re-launch his life as a sports icon and, more importantly, as a more complete human being.
The fact that he has done so in such dramatic and impressive fashion should allow him to walk away without a second thought.
Elite swimmers like to talk about leaving everything in the pool and Phelps has certainly done that while delivering a series of made-for-TV moments that have captivated sports fans all over the world.
Who else could go viral just sitting back with his headphones on and staring bullets at rival Chad le Clos while the brash young South African tried to psyche him out with a shadow-boxing routine?
We all know how that turned out. Phelps scored a heart-stopping victory in the 200-meter butterfly and le Clos didn't make it to the medal stand.
Even when Phelps finally lost an individual race, he ended up doing something that had never been done before in an Olympic swimming event — finishing second in a three-way dead heat with le Clos and Hungarian Laszlo Cseh.
Of course, the swimming world would rejoice if Phelps changed his mind again and turned his attention toward Tokyo. NBC certainly wouldn't mind and we'd all be back on the edge of our La-Z-Boys watching to see if he could stay a meter or two ahead of Father Time and add to his already ridiculous medal count.
What would be the point? Like Cal Ripken, who played 501 more consecutive games after breaking Lou Gehrig's supposedly unbreakable record, Phelps already has raised his career medal count so high that it will probably never be challenged.
He has more than twice as many gold medals as any other Olympian in any sport in any era, and the only current Olympian with any chance to reach double figures is Lochte, if he follows through with his plan to go to Tokyo and compete at age 36.
Maybe Lochte will eventually convince Phelps to join him for another final ride in Japan, but Phelps should resist that temptation if it ever presents itself.