It is one of those things where the math and the history do not quite bear each other out.
First, the math. It is held to be a universal truth in football that it is harder to retain a league title than it is to win just one. The calculation is simple. It is hard enough to win one championship. Double that, then, and you have the level of difficulty required to win two.
History, though, does not quite agree. In the 22-year history of the Premier League, only two sides have managed to repeat their title win of the previous season. Jose Mourinho's Chelsea did it in 2005-06 -- "It's definitely harder to retain it than to win the first," Joe Cole said -- and, obviously, Sir Alex Ferguson managed it with Manchester United.
If all that seems to support the mathematics, then bear in mind that as with so many things, the Scot rather gives the lie to the statistic: In the course of more than a quarter-century at Old Trafford, he did it on six occasions. Twice, he went one better, and produced what is known in American sports -- and, no doubt, will soon enough feature in England -- as a "three-peat": three title wins in a row.
Suddenly, it does not look quite so difficult. Indeed, holding onto the league title seems to be rather easier -- in England, anyway -- now than at any point in 70 years. In a little more than two decades, the reigning champions have defended their title seven times. Between the resumption of the Football League in 1946 and the start of the Premier League in 1992 -- not far off 50 years -- it only happened seven times in total.
This same phenomenon, of course, applies to the domestic double, that achievement that was once the a hen's teeth of a rarity but, ever since United started doing it with increasing frequency -- three times in all -- in the 1990s, it has been devalued to the extent that there have been years when finishing as league and FA Cup champions has been seen as a disappointment.
There are many reasons for that, including the loss of competitive balance in English football, the concentration of wealth in the hands of the elite and the emergence of the super-club.
The most crucial, though, is probably the presence of Ferguson, by most measures the greatest manager the British game has ever seen, and a man who made the impossible seem ordinary. Ferguson was an outlier, in countless ways. Just because he has done it does not mean others can easily mimic his success.
When Manuel Pellegrini, the Manchester City manager, said this week that defending the championship they won last season is a "great challenge" for his players, he was perfectly correct and for a whole raft of reasons. Some of them linked to the team trying to retain the title and others that apply to their opponents.
There is the fact that it is far more complicated for champions to strengthen their side than it is for their rivals. Winning the league is a veil; it means that even a flawed team has a legitimacy, that even those players whom a manager might believe might be improved upon most easily have a right to keep their place. Football has always believed that you do not change a winning formula, that if something is not broken, you do not fix it, even when actually it might be rather more broken than it first appears.
That was certainly a source of frustration -- and perhaps intimidation -- to David Moyes last year. He felt that moving Manchester United players on who had won a league title the previous season was impossible.
The best example, though, may well be Barcelona, who have found it almost impossible to alter the structure of their side despite the fact that the weaknesses in their squad are many and varied.
Then, of course, there is the desire of your rivals to shoot you down, to catch and overhaul the team that beat them the previous year. That is a powerful emotional impulse, particularly when combined with a freedom to change your side to address their problems.
Jose Mourinho's Chelsea have not just been more active in the transfer market than City this summer; the Portuguese's team talks and the atmosphere he forges in his squad, is probably more conducive to ambition, too.
But the most interesting (and probably the most crucial) is the ability of the players and the manager to -- borrowing Steven Gerrard's phrase -- "go again." This is a fascinating psychological phenomenon, one of which Ferguson was undoubtedly the master.
Managers often talk about a hunger for trophies. Brian Clough always said that the most important triumph in his time at Nottingham Forest was not the league title or the European Cup, but the 1977 Anglo-Scottish Cup, because that was the one that set the team on the way to further glory, which included two European Cups. That first gave them a taste for success.
This is called the "Champagne Effect." Mourinho felt it, too, ruthlessly targeting the Carling Cup in his first season at Chelsea to give his players a sense of what it was like to win tournaments. He believed it was that victory in the early part of 2005 that led immediately to two successive championships.
Ferguson's approach is almost diametrically opposed. To listen to him, it is almost as though the teams that are best-placed to repeat their victories are the ones who do not particularly enjoy the taste of success, who are prepared to forget all they have done almost as soon as they have done it.
"Winning a trophy does not really mean anything to me after it has gone," Ferguson once told FourFourTwo. "At the time, it is the most cherished thing. As soon as it's over, it's forgotten. Not forgotten, but it evaporates. Your next step is the important one, and the mentality here is of that nature. The players are brought up to go for the next thing."
Ferguson believed he could teach his players to be hungry, but that to do so he needed a careful balance between youngsters brought up through his club and strategic reinforcements from outside. The former taught the latter the mentality that had been instilled in them; the latter ensured that the former did not rest on their laurels.
He gave his players champagne, in other words, but he denied them the chance to pick up a hangover. That was the key to constructing not just victorious campaigns but dominant dynasties, doing what Arsene Wenger, for example, never managed to do. Maybe his wonderful, beautiful Arsenal teams were rather too taken with their status as champions; maybe they enjoyed it a little too much, forgetting that they still had something to prove. They exulted in their success.
Ferguson was different. His teams practised a form of almost wilful denial. He did not want addicts to success. He wanted players for whom the next time was always the first time. That was what enabled him to do what nobody else has done, to make the impossible seem ordinary.
Rory Smith is a columnist for ESPN FC and The Times. Follow him on Twitter @RorySmithTimes.