There comes a time in every dynasty where the cracks begin to show. For Barcelona, consider 17th century China. For Paris Saint-Germain, replace Li Zicheng, and his 1644 rebellion which precipitated the end of the Ming dynasty.
Nothing so big falls in an instant though - neither the 276-year rule of the Ming dynasty or the reign of Barcelona as the most feared team in European football. There are problems behind the scenes. There are small setbacks, almost negligible on their own, but which add up to something more. Something vulnerable. Something ready to be blown apart by the right wind.
Tonight's result equals Barcelona's heaviest away defeat in the competition and is the first time they've conceded four in a UCL match since 2013
The setbacks have been there for Barcelona. The early stumble in La Liga which saw them beaten by Alaves and Celta Vigo - conceding six goals across those two games - in the first month and a half of the season. Three league draws in a row going into December. Even a relative mauling at the hands of a flawed Manchester City side earlier in the Champions League, a 3-1 defeat where more goals could easily have followed.
Even at the tail end of last season, three consecutive defeats saw them come within a whisker of throwing away a massive league lead; being dumped out of the Champions League by Atletico Madrid in the middle of that same run.
Barcelona's decline - and make no mistake, it is a decline - bears all the hallmarks of a great team wearing down under the pressures of time. Gerard Pique, Javier Mascherano, Arda Turan and even the irresistible Luis Suarez are the wrong side of 30, and Dani Alves' summer departure at 33 has hurt Barca more than Luis Enrique would care to admit.
I'm about 70% sure that what Julian Draxler is doing to Sergi Roberto has actually been considered a war crime for most of the last 65 years
This isn't to say that possibly the best team to have ever taken to a football pitch have turned into a pub side overnight - Barcelona are still a team for any opposition on the planet to fear. They remain one of Europe's top five teams, without any dispute.
But they aren't that Barcelona side anymore. They aren't Pep Guardiola's side of 2008-2011. They aren't Luis Enrique's 2014/15 treble-winning side, where the 'MSN' trio broke records at a level never seen before. They're just...a really good football team. The mythos is diminishing. And not just around the team.
It's time to imagine the unimaginable. To speak the unspeakable. Lionel Messi hasn't just not been the world's best player over the last 18 months; he hasn't been Barcelona's. Injuries, and the weight of carrying the hopes of the world's most-hyped club and national sides on his shoulders, have started to weight heavy on the Argentine's shoulders of late.
The goals have still come, but in bursts. In four games against Atletico and Real this season, he's scored just once - and his trademark hatful of assists for the 'SN' who partner him up front have been nowhere to be seen. The 29-year-old has created just four La Liga goals this season compared to 16 last time out, and 18 in the campaign before that.
Messi is still an exceptional player, but he's beginning to drop down into the top tier of players rather than floating above it, ethereal in his brilliance. The last 18 months have been like playing football with your father in the back garden growing up - you catch him by degrees. You'll win once, then twice, then a third time. Countless defeats will come in between times, but the battle is changing. You're not fighting an unstoppable force anymore, just someone who's better at football than you. The mental playing field has altered; when victory is possible, you fight all the harder for it.
Messi's in that process now. He can still win, but he has to strain that little bit harder to do it. The better teams can shut him down. Against PSG, he was anonymous - notable only for being the figure standing with his hands on his hips watching his teammates track back when he lost the ball. Maybe he feels what's happening to him. Maybe he doesn't. Maybe it's a blip, an effect of injury and coincidence, and next season he'll be at his best.
But maybe not.
Maybe Barcelona will need to find a way, sooner or later, to win without his heroics. Neymar's development is, in relative terms, stalling. Suarez is beginning his own battle against the sands of time after his 30th birthday last month. Even the seemingly ageless Andres Iniesta can't nutmeg Father Time forever.
Barcelona were an incredible team. Now they're merely brilliant, in the main. Soon, they'll simply be excellent. Enjoy the autumn of the greatest football team to have graced a grassy rectangle, we'll not see their like for some time to come.