Sometimes a whole career, and a reputation that will stand for the ages, comes to a single moment of decision, a time when all doubts are put on one side and a team are sent out to play to the very limits of their potential.
Jock Stein, who many still feel was the greatest manager British football is ever likely to see, faced such a challenge in Lisbon 39 years ago. He coaxed a performance from the team he had raised entirely in the environs of Glasgow that not only beat the great power, Internazionale, but ran them off the pitch. Such is the example Arsène Wenger must follow in Spain next Tuesday night.
His potential reward could scarcely be more bewitching: springtime in Paris, a Champions' League final probably against the team of fantasy Barcelona, and the chance to join Stein and his countrymen, Sir Matt Busby and Sir Alex Ferguson, at the peak of the European game. But first he must engage that test of will.
Wenger cannot permit any kind of compromise from his players in the heat and tension of the Spanish night. He cannot tinker with containing tactics to defend a solitary goal. He cannot give random fate that much rope. He cannot sweat his way to the glory. He cannot go off half-bore.
He has to produce the best of himself, and of course the beauty of Wenger is that the best of himself as a football man has always been his team, the way they play, the way they fulfil the dream of how the game should be.
Villarreal do not carry the reputation of the old Internazionale as the masters of the catenaccio - "bolted door" - defensive system. They do not have such a luminary coach as Helenio "the Black Magician" Herrera, who acquired his reputation with larcenous "rope-a-dope" tactics and the habit of draping a black cape over his shoulders. But, under the sophisticated Chilean Manuel Pellegrini, Villarreal have become today's masters of the smothering game - and the stealthy strike. If there was any doubt about this it disappeared at Highbury on Wednesday night, when the game Arsenal so brilliantly employed to dispose of Real Madrid and Juventus was stifled almost to the point of extinction. All that was missing was the stealthy strike, but Arsenal may fear that at least one is still in the works.
Pellegrini's men fell not to the rapier thrust of a Thierry Henry or a Robert Pires but the magnificent commitment of Kolo Touré. He and Gilberto Silva took up the slack left by the relative failure of such as Henry, whose supply lines were critically cut, Pires, and an often agitated Cesc Fabregas.
Arsenal's football, even at its best, will always involve higher than average risk, and when it slipped a notch on night it was their good luck that Touré and Gilberto, particularly, were full of moral courage, Gilberto for once displaying it nearly as much on the ball as when he was off it.
These lessons, and others, have to be imprinted on the Arsenal subconscious when they make their last stride to glory next week. Number one is the kind of appraisal which shaped their superb victory over Real at the Bernabeu. That was the fruit of Wenger's understanding that his opponents were a great team only in the death throes of their reputation. He knew that Henry could terrify their defence. He knew that the fluency of his team's movement could undermine a Real who had shed so many of the basic demands of true professionals.
Now he has to be equally clinical in his assessment of the quite different Villarreal. They are what they know they are: functional, pragmatic and adept at stretching relatively modest resources to their very limits. No one should have been surprised they offered much greater resistance this week than did Real; earlier they had confounded the attacking might of Manchester United.
They also beat Everton, whose manager, David Moyes, offered the advice that in Juan Roman Riquelme Villarreal hold arguably one of the world's outstanding players, someone up in Ronaldinho territory.
No doubt Riquelme is a beautiful operator... on his terms. At Highbury his influence was negligible. He waited for the ball to be served to his feet. He pined for the time granted to him by the tempo of the football of his native South America. Ironically, the Brazilian Gilberto was one of the architects of his failure this week. The big message here, surely, is that Wenger cannot afford to give Riquelme any time to settle on to his game in Spain. Initiatives have to be taken away, and that means a restatement of the qualities that brushed Real and Juventus aside.
In the second leg at Juventus, Arsenal were less positive than they had been in Madrid, a fact which may have escaped too much notice in the light of another inept performance by the champions-elect of Italy. But the seeds of disappointment might easily have been sown. An early strike by Juventus, however fortuitous, might just have thrown into doubt an Arsenal superiority so profound at Highbury it should really have been reinforced from the first kick in the Stadio delle Alpi.
Pellegrini and his men will surely pray for a relatively passive Arsenal. They will want to nag themselves back into this tie, maybe from a cunningly struck free kick by Riquelme or a sharp turn and quick strike from Juan Pablo Sorin that might remind a sceptical Arsenal that he once kept Ronaldinho on the bench when they played together for Paris St-Germain.
Perhaps Wenger would do best to recall Stein's response to such pinpricks of concern. What he did was order his men to engulf such worries as the break-out flair of the brilliant midfielder Sandro Mazzola in a football equivalent of napalm, and it was something they did despite the potentially devastating setback of conceding a penalty. Celtic ran Internazionale dizzy on the strength of Stein's conviction that man-for-man he had the better, braver team.
His pre-match thinking still thunders down the years: "Inter will play it defensively, that's their way and their business... but we feel we have a duty to play it our way - and our way is to attack."
That should be no great reach of ambition for Wenger. Already his team have produced entrancing football in Europe this season, and one step from the mountain top is no place to lose your nerve. This week Wenger said that Henry's decision to go or stay should not be based on the circumstance of winning or losing a Champions' League trophy. It should be to do with the values he most cherishes, and his desire to be involved in the football he loves to play.
Those values, and that football, are on the line in Spain next week and Wenger maybe has no better lead than the imperative of Jock Stein. It is a call to duty, one to light up the football sky.