Twelve inches and half a second. That's all. That's it. That's the difference for Barcelona between beaten and brilliant, heroic and has-been. When you think of these margins in sport, these inches and seconds by which reputations are forged and history made and men remembered and nights recollected, it's unnerving. Welcome to pressure.

In tennis at least the comeback is not defined by a fixed time. You can hunt a man down through many hours, but in football the clock is like a bully. Tick, tick, tick. Barcelona have no more minutes and they need another goal. Neymar may not know it but he probably has one final chance as he slips past a defender and chips with his left leg.

Neymar's toes have a PhD in feel and his metatarsals have a Masters in accuracy. Still he must be tired and emotional and right now his chip has to be perfect. Perfect spin, perfect trajectory, perfect velocity.

If he hits it too hard it will be one foot too far from Sergi Roberto, who will not score and headlines won't be You Guys Have Become Legends but End Of An Era. If Roberto doesn't break at the right time and moves half-a-second too fast he will be offside and there will be no goal and TV pundits will spend the evening asking, Is Messi Done?

It's that simple. It's that frightening. It's that beautiful.

This is why we love comebacks. Because so much rides on so little. We love it because this is exactly what Winston Churchill meant when he said: "Never give in, never give in, never, never, never, in nothing, great or small, large or petty."

We enjoy it because this is the real-life, more-dramatic version of what Matthew McConaughey says in the film We Are Marshall: "When you take that field today, you've got to lay that heart on the line, men. From the souls of your feet, with every ounce of blood you've got in your body, lay it on the line until the final whistle blows. And if you do that, we cannot lose."

We relish comebacks because they're always accompanied by a stylish backstory. Like the one from the America's Cup in 2013, which was recounted in The Wall Street Journal. Team USA are down 1-8 and bring it back to 8-8 and on Sept 25, on his way to the last race which he will win, their skipper has a song playing in his car. What else but Pearl Jam's version of Immortality.

We love comebacks even though they reveal the utter ruthlessness of sport, for the world embraces the winners while the defeated players stand there with the stunned faces of people who've been told, sorry, we made a mistake, you didn't win the lottery. When Steve Davis led Dennis Taylor 8-0 in the final of the 1985 world snooker championship and then lost 17-18, he was - Taylor told The Telegraph - "in shock for about three years".

We fancy comebacks because we don't want sport to only be logical and form to dictate everything and everyone needs to believe that magic is not a myth. That some days there are some things just out of this world. That early one African morning Muhammad Ali will get brutalised by George Foreman in Zaire and then bounce off the ropes and send the young bull to the canvas.

We adore comebacks from the first time we witness one because we can't really comprehend it. It's our first introduction to the human spirit and obstinacy. It's our first appreciation of the fact that athletes are not like us, for their will is overdeveloped. It's our first understanding of the truth that forget art, technique, muscle, nothing beats desire.

My first comeback involves Jimmy Connors in Wimbledon in 1987, 10 years older than Mikael Pernfors and down 1-6, 1-6, 1-4. He wins and once his bag is packed he lets Pernfors lead the way off the court, because as he, the old lion, exhausted, says: "I didn't want him to see me pass out as I walked out."

We prize comebacks because they are a demonstration of mankind's refusal to be constrained by limits. An act of defiance against a "no chance, can't win" world. So boxer Vinny Paz breaks his neck in a car accident and fights again. And Lasse Viren falls during the 10,000m final at the 1972 Olympics and breaks the world record. And Rocky Bleier, a college football player, is shot in Vietnam and injured in a grenade blast and wins four Super Bowl rings.

We like comebacks because they offer us hope and make us grin and leave us shaking our heads. Whatthehell? Didyouseethat? They restore faith in a sporting universe of too much money and too little manners. They make us a little dizzy till we start thinking of outlandish stuff.

Like this:

In the matter of comebacks this year it's been Roger Federer in January and Tom Brady in February and Barcelona in March. So in April if that 41-year-old guy with the temperamental back and Major resume shows up at the Masters and everyone says no chance, maybe we should very quietly put $10 down on him. Just in case.

The impossible occurs.