Let us start with that most important of philosophical beings, the Hypothetical Martian, the creature summoned to help out in every sixth-form argument. What would he think, if he watched tennis and then football on the television? In both cases he would see people doing pointless things with a ball, and doing so with a great deal of seriousness. He would conclude that they were pretty similar things, more or less the same phenomenon.

But what if he had actually been with me in Moscow, then in Paris, to mingle with the crowds going to the Champions League final and to the French Open? What would he have made of the singing, drunken intensity of the football supporters as they wended their way towards the Luzhniki Stadium, and of the cheerful crowd in holiday humour who skirted the Bois de Boulogne towards Roland Garros? Would he not have concluded that these two crowds were travelling for some completely separate purpose?

Let us not talk about social class. That is not the issue here. It is not that the two occasions, the two sports, attract different sorts of people; rather, that they elicit a completely different response. The Hypothetical Martian would conclude that one crowd was travelling for dizzy pleasure, the other out of stern duty.

If you travel from one sport to another, you mingle with a lot of sporting crowds, and it is astonishing how different they are. Indeed, you could travel with the same group to Wembley and to Wimbledon and it would be as if they were completely different people. You could all be going to watch Tim Henman contest yet another semi-final or you could all be going to watch England play in yet another desperate qualifying game, but whichever it was, you would find that the vibes were completely different.

On both occasions, your companions would be hoping for a good result; would be planning to shout for Tim, for England. But the nature of the hoping is radically different. The tennis fellow-travellers would be hoping like mad for a great performance.

They would care very much about the outcome, but it would not seem to be a matter of huge importance. The central contradiction of sport is effortlessly mastered: sport matters because it doesn't matter; sport has a reasonably deep meaning because it is inherently trivial.

This mildly dispassionate view is nothing to do with the cynicism of a sports writer who has seen too much. It is something shared by many people who watch sport for their own purposes: you can watch a tennis match and care about its outcome without feeling personally destroyed by an unfavourable ending. But in football, certainly for those who travel to far-flung stadiums, caring is essential to the experience. Without caring, it would all be meaningless.

I am reminded of the sequence in the film The Birdcage, in which the flamboyant queen Albert is being coached in conventional masculine behaviour. How did he feel about the defeat of the Miami Dolphins? “How do you think I feel? Hurt, betrayed ...” But that's how it is in football. To march with a football crowd is to be infected with the terrible illusion that it actually matters. There is no pleasure in it, only triumph. Or disaster, of course. The Hypothetical Martian would find himself concluding that football crowds were going to something important, while tennis crowds - and, for that matter, the crowds going to most sporting events - were going out to have fun.

There is a continuum. No question about that. Travelling to the Stade de France on the other side of Paris last autumn, it was obvious that the rugby crowds wanted their own team to win. The England supporters in their white shirts, many of them drinking purposefully, a number of them singing, were certainly looking for a victory rather than a good time. But there was a difference, and again, it was not one of social class. There was a feeling that this was also a bit of a lark.

Cricket always has this cheerful atmosphere. There is something of the picnic about every cricket match, even among those for whom the picnic is entirely liquid. Even at the height of the 2005 Ashes season, even on that fearful day at the Brit Oval when the ground throbbed with desperate hopes and fears, there was still the shared feeling of a great day out, a fabulous experience, win, lose or draw. There was the jocular opening of umbrellas, to summon the rain that England wanted, answered by the Australian supporters, who removed their shirts, and even by the Australian cricketers, who emerged into the murk in sunglasses.

But football has none of this. The hardcore football supporter, the sort who despises, in so far as he even recognises other sports, will see the equation of sport with pleasure, with amusement, as the most desperate emotional cop-out. Defeat has to hurt, for without hurt there is no joy in victory. And sometimes, it seems that there is not much joy in victory, only a sick relief.

In football, so much of the emotional intensity comes not from loving your team but from hating your rivals. Gloating over a rival's misfortune is at least as good as celebrating your own team's triumph. Sometimes it seems that football has its being not in love, but in hate, an emotion quite alien to most other sports. Everybody loved Tim at Wimbledon, but I don't recall anybody hating Pete Sampras, even while he was beating Henman in two of his four semi-finals.

This May, I have travelled to football, cricket, tennis and also to Badminton for the eventing. On cross-country day at Badminton, there were two or three times the number of people who made it to the Luzhniki. No one sang. No one hated the foreign riders. Most had a drink or two. We all admired performances of extraordinary skill and courage, and went home moved and elevated by the great things we had seen.

Badminton hardly seems to be on the same planet as the Champions League final, and the Hypothetical Martian might agree. Can it really be that we call both these things sport? Can it be that two activities that bring out so radically different a response in human beings are actually classified as the same thing?

If it puzzles the Martian, it increasingly puzzles me. I can see the point that non-obsessive sport is like alcohol-free lager, but I am not sure that I share it. In football, it is increasingly agreed that partisanship is all, that without the most desperate emotional investment in the result, the sport itself has no meaning. It is part of the way that football is increasingly separating itself from other sports. Or is that term still accurate? Perhaps I mean that, increasingly, football is separating itself from sport.