In Keegan's case it is not enough to conclude that pressure imposed by the responsibility of massive investment proved intolerable. He was not simply overwhelmed by specifics. Nor, like some in the profession, could he lean on rage.
Volatile, the inability to conceal his innermost feelings manifest in emotional outbursts, finding it difficult to compromise, Keegan probably suffered most from a purity of spirit. "If we can only win the championship by changing the way we try to play, I'll get out," he said last season after a debilitating 4-3 defeat at Liverpool. For him the joy was not only in victory but in the manner.
That Keegan even entertained management astonished many who knew him. There was no question that he wanted the job at Newcastle. A larger question was how long he would continue to want it. This season he has often seemed moodily uncertain.
Following a loss at Leicester, Keegan denied BBC television an interview on the grounds that he had been let down last year over the reporting of an incident at Manchester City involving the Colombian international, Faustino Asprilla. After a recent defeat at Coventry, he left without a word to waiting reporters and was seen staring blankly from the windows of the darkened team bus.
Keegan was an expedient footballer rather than gifted. He had good feet, a strong body, quickness, drive and the determination to make the most of every opportunity. Management required different things from him but not a different set of values. His way or not at all, which led to an early conflict with Newcastle's ambitious benefactor, Sir John Hall, and maybe was at the root of his resignation.
The appointment, unique in English football, of a former Liverpool team- mate, Mark Lawrenson, as defensive co-ordinator carried the suspicion of interference. Even when Tottenham were crushed 7-1 to end a run of seven matches without a victory, Keegan was curiously uninspired, perhaps already convinced that it was time to end his association with the club.
One night some time ago, on the eve of the Grand National and a match Newcastle had at Manchester City, I was invited to dinner with Keegan and his coaches in a hotel just outside Warrington. I reminded him that he had frequently argued against the idea of becoming a manager. "This was different," he said. "I couldn't resist the challenge but I don't expect to be doing it all my life." On television recently, interviewed while playing golf with Alan Shearer, I heard him say something similar. The difference was that he did not appear to be thinking very far into the future.
Football tells anyone who watches intelligently about the times in which we live: about managed news and corporate politics, about what the process of pressure does to strong men. Television makes every living room an arena. If that is what you want for your living room, televised sport can be thrilling, but the focus on managers has become so intense that we are left in no doubt about their emotions, lips moving in profanity, a winner's joy, a loser's despair. The handshakes perfunctory.
Talk is another kind of mask. Some managers beat away questions with other questions and hold people off with small pronouncements. Others become lovers of silences. Keegan has always worn his heart on his sleeve, sensitive to criticism as he was revealingly last season when needled by Alex Ferguson at a critical stage of the championship.
We can only imagine the effect of Hall's statement - subsequently denied - that Newcastle must win something this season, on Keegan. But it surely prompted his offer to resign following a Boxing Day defeat at Blackburn.
By then it was perfectly plausible that Keegan was at odds with a policy alien to his nature. Once he accepted grudgingly that a more secure formation was necessary, he was lost. Adrift of the leaders, his team was neither one thing or another. Every day, as autumn deepened into winter, he must have thought seriously about turning his back on it all.
The exhilarating boldness that characterised Newcastle's best football under Keegan, boldness in denial of principles that most modern coaches hold sacrosanct, was of another time. It was of Real Madrid in the 50s and 60s, the Tottenham Double team, the best of Manchester United under Matt Busby. "Too much mind will make the game less attractive and may eventually destroy it," Busby once said.
Mind did not figure greatly in Keegan's philosophy. Adventure, football as the glory game, did and the thrilling sight of it filled Newcastle's fans with optimism. Last season's disappointment when Manchester United made up 12 points to take the championship was overtaken when pounds 15m was spent to acquire Alan Shearer. But, with that one record purchase, Keegan put his foot in a bucket. He could not afford to fail again.
Was it simply disenchantment that prompted Keegan's resignation? Or did he turn away from the man who, more than anyone else, represents the idea of football as big business? The reason is less important than the fact.