Football is a rolling examination of the character and ability of managers and players. On Tuesday night, Arsene Wenger's team was set a test by Everton and they flunked it.
Last week, I wrote about Arsenal as a title contender and ventured the opinion that Wenger had, after many failed attempts, finally found a balance which would help sustain the club through this difficult coming period of games.
Shkodran Mustafi was a big part of my reasoning and there is now a statistic which illustrates my point perfectly.
Arsenal have lost three times this season - against Liverpool on the opening day, Southampton in the League Cup quarter-final and now Everton.
Mustafi wasn't at the club when Jurgen Klopp and Wenger traded seven goals at the Emirates. He wasn't picked for the Cup tie and he was injured for the Everton defeat in midweek.
It's not hard to make the connection here. Arsenal lose when he doesn't play.
He has quickly become very important to Arsenal's defence and in a more general sense, a crucial part of the team's backbone.
I'm not being facetious when I point out once again hat Wenger has a terrible record picking defenders and indeed goalkeepers, so much so that any half-decent centre-half was always going to be an improvement.
But there is more to Mustafi than just good, basic defending.
Without him, Arsenal folded like a deck of cards against physically committed opponents, themselves under quite a bit of pressure to perform in front of home fans.
When they needed to, Arsenal were not up to the job of matching Everton physically and that shook my faith in Wenger and his ability to deliver another title for the Gunners after such a long time trying.
He seemed puzzled after the game when an interviewer suggested that this was an occasion when Arsenal needed to fight for the points but didn't.
I think it was obvious to anyone who watched that Arsenal were out-muscled by Everton and no matter how much distaste Wenger shows for that side of the game, it is ridiculous to ignore it on the premise that your passing game will always be strong enough.
For nine seasons it hasn't been strong enough for Wenger and Arsenal.
Players like Mesut Ozil and Theo Walcott will not put their bodies on the line for you, for all their talent.
Think about all the title winning teams of the past few decades. Every one of them had warriors who created the environment for the fast lads with skill to do their work.
Wenger didn't have to look too far to see one in action at Goodison.
Seamus Coleman is exactly what I'm talking about.
I'm not sure what's happened the lad since June but what we are witnessing is the blossoming of a personality to match the ability Coleman shows on the pitch.
There was nobody more committed, mentally and physically to winning that game than Coleman and when he scored and ran the length of the touchline pumping his fist to get the Everton supporters going, he showed in 20 seconds what can be achieved by a simple emotional response.
The crowd lifted, Everton lifted and suddenly Arsenals pattern was disturbed and their composure threatened.
Wenger saw that too and must have admired Coleman's spirit and leadership qualities but he doesn't seem to recognise the need for players like that in his own team which is the great, enduring mystery about this Frenchman.
After the game, he was moaning about the referee and that was the worst feature of a bad, bad result.
In his complaints lie the root of his problem. He blamed Mark Clattenburg for calling a corner which should have been a goalkick.
As it happened, he was right. The referee made a mistake but it took two or three angles and slow motion to confirm that a goal kick should have been the decision.
My problem with Wenger is that he didn't acknowledge the real cause of the defeat. Ashley Williams was allowed wander into Arsenal's penalty area unmarked and had a free header for the winning goal.
So it wasn't the referee's decision which cost Wenger the game. It was bad defending.
There was one other moment in the after-match interviews worth mentioning and it came when Ronald Koeman was giving his match summation.
His report on Everton's first-half described a nervous opening half hour with "every ball being passed back".
In that moment, he made my week and I had James McCarthy in mind when he said it.
Unlike Wenger, who chose to focus on the fiction of what might have happened if the referee had given the 'right' decision, Koeman wanted to deal in cold reality.
He doesn't spare his players and I suggested a while ago that he could well be facing some resistance to this approach in his dressing room.
But judging by the response to the win and the way Coleman galvanised the whole stadium, I think his message has been heard.
It would be no surprise if Everton's season takes off from here.