People often talk about great players and I have always thought the definition was someone who consistently delivered in major tournaments, making the difference in World Cups, Champions League trophies or league titles.
In our league, think about how Robin van Persie dominated the early part of the season, winning the games for Manchester United that provided the foundation to be 12 points clear now; think back to this time last year, when Yaya Toure took the title race by the scruff of the neck and dragged Manchester City over the line.
But, for me, greatness was re-defined on Wednesday night. I've been around a few people who have an aura and a presence.
When I was a 19-year-old professional at United, Bryan Robson would walk into the changing room and change the atmosphere. Sir Alex Ferguson can do the same.
Eric Cantona had that kind of charisma about him. I suppose I would have said previously that a sign of greatness is when someone walks into a room and everyone looks up.
But I've never seen nor been in a stadium where the impact of a player was as great as the Nou Camp on Wednesday night.
I was watching quite possibly the worst Barcelona performance of the past four years, against Paris St-Germain in the Champions League. It was so lethargic.
And that extended to the crowd, who seemed to lack any sense of the occasion. In fact, at half-time I said PSG would score, that Barcelona would become ragged and lose the game by two or three goals.
Then, all of a sudden, this 5ft 6in bloke, who has an injury which means he can't run, enters the pitch.
And you could hear, see and sense everybody in the stadium being utterly changed. From the fans, who suddenly found their edge, to the Barcelona players, who started to create chances straightaway, and, perhaps, most of all, to the PSG players, who were mesmerised.
Are you sitting comfortably? Messi was in the uncustomary place of the subs bench to start with
They changed from a team that looked as though they could win comfortably into a team without belief. It wasn't as though there was a mere tactical or technical difference. It wasn't something physically you could point to.
It was felt as though something spiritual had happened. You really had to be in that stadium on Wednesday night to appreciate the magnitude of Lionel Messi and the impact he can have.
For the first half, he was on the bench injured but in the managerial position, right at the end of the bench, as though in control. In the second half, I was watching him and he got up on his own to warm up when PSG scored after 50 minutes.
There was no instruction. He simply got up and almost brought himself on to the pitch after 62 minutes.
His influence at the club seems that great. He walked on rather than ran. In fact, he could hardly move.
He was caught offside twice early on and couldn't track back. He clearly couldn't work himself into a sprint.
Can't catch me: Messi not only turned the game with his skill, but also the whole atmosphere in the stadiumBut in one moment he attracted three players to him and performed what I can only describe as a 'Velcro dribble', where the ball sticks to his feet.
Then he went past two or three players and saw the pass to David Villa. And don't get me wrong: David Villa and Pedro still had a lot to do to execute the finish.
It wasn't as if the goal was handed to them on a plate. But it was Messi who had changed everything to bring about that chance.
Messi's presence alone, even when injured, took his team to a different level. I have never seen that before and it was special to witness.
I would say a player who can change an entire stadium and turn a match around simply by walking on to a pitch redefines greatness.
And that is what Lionel Messi did. It's not that I haven't realised how good he was before. I've watched him play brilliantly in Champions League finals to beat excellent Manchester United teams.
But I'd never seen Barcelona without Messi before. In fact, this was the first time he hadn't started a knockout game in the Champions League since April 2008.
Before the game I was saying Barcelona would probably lose 30 per cent of their effectiveness if he didn't play.
No dummies: Pedro Rodriguez (left) and David Villa celebrate after scoring against Paris St Germain Goals galore: Messi is proving unstoppable this seasonAfterwards, I would have changed that to say it would be about 60 per cent. The mental impact of not having someone that special on the pitch is enormous.
You put Messi into that team and you're transforming a very good team into the greatest team of all time.
People will say there is a lot more to Barcelona than just Messi. Xavi and Andres Iniesta are rightly considered two of the finest midfielders of all time and the club have other world-class players.
And you could point to the fact Spain have won the World Cup and two European Championships, using a team largely drawn from Barcelona.
The performance in the Euro 2012 final went down as one of the great displays from an international team.
But the reality is that, even if the World Cup draws more attention, the Champions League is operating at a higher level than international football.
Given that you can draw players for your team from all around the world and that they work and train together every day, rather than in sporadic blocks of time, it would be surprising if the best club teams weren't better than the leading national team.
That's not to diminish what Spain have achieved. They are the best national team. It's just that Barcelona - with Messi - are on a level above that.
You have to acknowledge it was Pep Guardiola who enabled Messi to show the full range of his talents and become the player he is today.
The 2008 Barcelona side with Messi, Xavi, Iniesta, Carles Puyol and Victor Valdes, with Messi pinned to the right wing, couldn't beat Manchester United.
In fact, they couldn't win a major trophy - other than the Spanish Super Cup, their equivalent of the Community Shield - for two years between 2006 and 2008.
The transformation of the team and Messi came with the arrival of Guardiola. By playing Messi centrally, Guardiola provided the platform for him to fulfil his ability.
And Guardiola added the tracking back and pressing that added new dimensions to his game.
In addition, the constant pressing and the positive style in which the team play, with two centre-halves spreading to the touchlines and full-backs pushing up, has maximised Messi's brilliance.
Thanks for nothing: Even the usually unmovable Zlatan Ibrahimovic was bowled over by the magician
Guardiola made a huge impact on this team, and it's noticeable this Barcelona side aren't bringing that intense pressing to their game at the moment.
You do wonder whether the four years of incredible football, that has seen Champions League titles, League titles, World Club Cups as well as, for many of the squad, World Cups and European Championships, is taking its toll.
There has to come a point where that amount of football at that level has to have an effect. It is too early to talk about the end of an era.
They will win the Spanish league easily and they have qualified for an unprecedented sixth consecutive Champions League semi-final.
But if Messi isn't fully fit for the final part of the season, they won't win the Champions League. Real Madrid, Bayern Munich and perhaps even Borussia Dortmund are stronger: they have more energy and aggression; they are in sharper physical shape and in better form.
On balance, I'd expect a Real- Bayern final. But Barcelona have Messi and we are witnessing one of sport's all-time great performers. His mere existence means anything is possible.
