City value Kevin De Bruyne at £36m more than Chelsea wanted
Is Kevin De Bruyne brilliant or brittle? Having failed once in England, does he warrant the second biggest transfer fee in English history to bring him back?
Is he closer to Eden Hazard or Thorgan Hazard?
The question of whether De Bruyne is worth it - the money, the time, the fuss - divides and defines the two best teams in the country.
It will go some way to determining the destination of the Premier League title, as well as revealing something about the difference between Chelsea and Manchester City.
The numbers themselves tell the story: 19 months ago Chelsea, with no regular first-team place for De Bruyne, sold him to Wolfsburg for £18m.
This week, City reached a deal with Wolfsburg to buy him for £54m, three times that fee.
De Bruyne has certainly performed very well for Wolfsburg.
He starred in and settled the German Cup final against Borussia Dortmund in May.
Two months later, he made the equaliser and scored a penalty as Wolfsburg beat Bayern Munich to win the German Super Cup. Overall, this was a good spell for a good player in a good side: but De Bruyne did not treble in speed, size or skill during his time at the Volkswagen Arena.
Ultimately, this is the story of how Chelsea and Manchester City value De Bruyne, and how a player discarded by one side could be so valuable to the other.
Not much separates Chelsea and City, so how can they value this one player so differently?
The tearful deterioration in De Bruyne's relationship with Jose Mourinho, leading to the midfielder's departure, was remarkably swift.
When Mourinho returned to Stamford Bridge in 2013, De Bruyne, just 21 years old, had completed an excellent loan season at Werder Bremen. Jurgen Klopp had tried to sign him for Dortmund earlier that summer, texting him on the day of the Champions League final at Wembley, only for De Bruyne to decide to try his luck with his parent club.
At Mourinho's re-unveiling, the Chelsea manager said how important it was to get the best out of loaned players like De Bruyne and Romelu Lukaku.
"They are the type of players Chelsea have invested a lot into," Mourinho said, "and it's my work to extract the best from those investments. Both are ready for my squad."
So when Mourinho began his second spell in charge, De Bruyne started the first Premier League game against Hull City and the third, as Chelsea went strikerless, at Manchester United.
Then the Belgian was replaced by André Schürrle. Rather than inspiring him to improve, the opposite happened - his head dropped, as did his level in training.
When he was brought back for a trip to Swindon Town in the Capital One Cup, he struggled and Mourinho made clear how unimpressed he was.
"The next time Kevin is on the pitch," he said, "he has to think he's playing for his next appearance."
The player's final outing was in a Capital One Cup defeat at Sunderland in December 2013.
Mourinho likes to test his players' mentalities, to drop them, criticise them and see how they react. This is what he calls "confrontational leadership".
"It is when you are ready to provoke your players, to try to create some conflict," he explained last season, "with the intention to bring out the best from them".
When asked last week why he sold De Bruyne, Mourinho said the player simply could not cope with competing for a place.
"If you have a player knocking on your door and crying every day he wants to leave, you have to make a decision," he said.
"He was an upset kid, training very bad. He needs motivation to train well, by playing every game."
There is a sense, watching Manchester City over the past six years, that they have lacked the relentless intensity that Mourinho has inculcated in Chelsea. Ultimately, De Bruyne will walk into an environment far more amenable to him than the one he struggled with at Cobham.
Mourinho said that De Bruyne "needs to know that he is important" and a £54m fee is certainly a powerful way of conveying that.