There is a core belief shared by Jose Mourinho and Pep Guardiola which helps explain why, as long as the two managers are rivals, they can never be friends.
"When we fight, we want everyone to fight together and when a problem comes up Jose always puts his face forward," said a long-term associate of Mourinho this week, when asked to explain the enmity between the two men ahead of their meeting in the Manchester derby tomorrow.
"Jose always wants to feel that behind him there is also a team," he added. "When Jose does something he wears the shirt of the club and he fights for the club. If everyone is not pushing in the right direction then he is not happy. He has to feel that everyone at the club is working with him."
The associate offered to shed light on the understanding that he would not be named. He knows Mourinho would not like it. In similar circumstances, Guardiola would not like it either. Neither man is happy if those close to them - publicly at least - speak to the media. It is about being a tight unit; a family; a team on and off the pitch. The "fight together" approach outlined by Mourinho's friend struck a chord when reading Pep Confidential, Marti Perarnau's book which provides an extraordinarily detailed insight into Guardiola's first season at Bayern Munich.
Perarnau recounts an incident in which Guardiola was "pleasantly surprised" when the Bayern hierarchy sprang to his defence the day after the European Super Cup in Prague in August 2013. Inevitably, the controversy involved Mourinho, whose Chelsea side - reduced to 10 men by Ramires's sending-off - were beaten on penalties, provoking yet another jibe from the Portuguese: "Every time I play Pep I end up with 10 men. It must be some sort of Uefa rule". "Pep was taken aback because he was so unused to getting this level of support from his bosses," Perarnau wrote of Guardiola's explosive first game in charge of the German club. "During his time at Barca he had had to deal with numerous unwarranted and serious attacks on the team and the whole institution, and his was often the sole voice raised in their defence."
The irony is that Perarnau could easily have been talking about Mourinho, who shares that "all for one" attitude. And therein lies the driving force of the rivalry, because for all their public enmity, Mourinho and Guardiola are strikingly similar. Both are alpha males, both demand intensity and loyalty from their charges, both work to unbending high standards and both see conspiracy and disloyalty where sometimes they do not exist. Memories are long and so is the sense of being slighted.
Another irony is that Guardiola could have been Mourinho's protege. Instead, he became the key player in what those associated with Barcelona claim was the "painful blow to Mourinho's pride" in 2008, when the Catalans realised that discipline had broken down under coach Frank Rijkaard and that it was time for a change.
The story is well rehearsed but the role of Evarist Murtra, a Barcelona vice-president at the time, is less well known. In 2008, Mourinho was out of work. He had been sacked by Chelsea and looked an attractive option: charismatic, if confrontational, he exuded authority and had an astonishing record of success. He also desperately wanted the job.
Board members Marc Ingla and Txiki Begiristain - now Manchester City's director of football - were sent to meet Mourinho in Portugal, where he gave them a detailed PowerPoint presentation of how he would reinvigorate Barcelona, including which players would go, which areas needed strengthening and who would make up his backroom staff. He suggested Guardiola, then coaching Barcelona B, as one of his assistants (Mourinho likes to have a club's former player on his staff). The presentation was, as ever with Mourinho, a formidable and persuasive piece of work and, surely, would seal the deal.
Ingla and Begiristain returned to Spain and it appeared the mood was, indeed, to appoint Mourinho. Then Murtra intervened and was backed, crucially, by Johan Cruyff, who had managed the so-called Barcelona 'Dream Team', of which Guardiola had been a part. The pair pointed to Guardiola and the incredible, detailed work he had already done with Barca B and warned against Mourinho. Guardiola got the job. To add further spice to the story it has also been claimed that Guardiola himself had recommended Mourinho in the first place.
In truth, the pair were never close, although there was always a professional respect. They first met more than 20 years ago when Sir Bobby Robson took his first training session as Barcelona coach. Guardiola was the captain; Mourinho the translator. The latter shrewdly identified Guardiola, a Cruyff loyalist who grew to admire Robson, as the leader of the Catalan-Spanish faction of the team - someone to stay onside with (and keep an eye on).
Mourinho left Barcelona in 2000, after four years, to forge his own coaching career. It was telling that, six years later, with Guardiola having retired as a player, one of the first people he contacted was Mourinho, who had already won the Champions League with Porto and was blazing through the football world at Chelsea as "the Special One". He asked if he could come and watch training at Cobham. Instead he was invited to meet Mourinho before a Champions League match in Valencia; Guardiola declined. Already a wariness was developing.
That grew into a gulf when Guardiola got the job Mourinho coveted at Barcelona and the plaudits started to rain down, and by the fact that they were invariably jousting for the same trophies. Mourinho's Inter Milan beat Guardiola's Barcelona on the way to winning the 2010 Champions League and then Mourinho was appointed as Real Madrid coach. While Guardiola usually bested Mourinho in their head-to-head meetings, a major reason behind his decision to quit Barcelona two years later and take a season's sabbatical was because he was worn down by the feuding.
For Mourinho the frustration will have ballooned when it emerged that Roman Abramovich had wanted the Catalan at Chelsea and wooed him several times, meeting him in Paris before Mourinho's return, and then after he was sacked for a second time. There is also intrigue around Manchester United's intentions when Sir Alex Ferguson retired in 2013. Mourinho's version is that he had already informed Ferguson he was going back to Chelsea. Senior sources at United suggest that Guardiola - who dined with Ferguson in New York while he was on sabbatical - was always ahead of him (as, indeed, was Jurgen Klopp) but he decided to go to Bayern.
The story feeds a wider narrative that Guardiola has been the preferred choice of clubs where Mourinho had wanted the job on at least three occasions - four if Manchester City are also included, with Mourinho's agent Jorge Mendes having attempted to court them on more than one occasion.
That will have hurt Mourinho, for whom any serious rival must - by definition - be an enemy. And it is why, for all the gilded footballing talent on display at Old Trafford tomorrow, the most compelling battle of all in the 172nd Manchester derby will be fought in the dugouts.