Team chemistry is such an elusive thing and, once you finally find it, such a fragile thing.
What was the difference between the 2004 and 2005 Eagles? It wasn't talent. It was a solitary wide receiver who was intent on destroying the chemistry of a Super Bowl team because he wasn't happy with his contract.
No sport requires this level of teamwork, and no sport requires this level of unselfishness. With very few exceptions, you really need a special group of people for sustained success in the NFL, and that's why figuring out what kind of person you're getting in the draft or free agency has become just as important as figuring out what kind of player you're getting.
Which brings us to the 2017 Eagles, who by any measure are the best team in the NFL as we arrive at the midpoint of the season.
They're rolling along at 7-1 with a six-game winning streak, with those six wins coming by an average of 11½ points. You know all the facts and figures. The NFL's fourth-ranked offense and 10th-ranked defense, the hottest quarterback in the league, the stingiest run defense in the league in seven years.
Here's another number: 22.
That's how many new players are on the roster.
And that may be the most remarkable number of all.
GM Howie Roseman and vice president of player personnel Joe Douglas took a 7-9 team with a rookie coach and quarterback and instead of just hoping that another year together would turn them into a winner, they gutted the thing.
Some 42 percent of the 53-man roster that will face the Broncos on Sunday was not part of the 2016 Eagles.
Think about all the key guys on last year's team the Eagles cut ties with.
The leading rusher and leading wide receiver. A former first-round pick. Both starting corners. Two defensive line starters who had played at a high level here. And so on.
And think about the new guys. How many key contributors on this team weren't even here last year?
Derek Barnett, Mack Hollins and Rasul Douglas came in the draft. Alshon Jeffery, LeGarrette Blount, Patrick Robinson, Corey Graham and Chris Long signed as free agents. Tim Jernigan, Ronald Darby and Dexter McDougle arrived via trades.
Corey Clement was an undrafted rookie. Jake Elliott was plucked off the Bengals' practice squad. Kenjon Barner was home in California out of work.
We've seen the Eagles make wholesale changes in the past and it didn't go so well. But this group is different. The Eagles managed to rebuild the roster and truly reshape the franchise while also creating a remarkable chemistry and maintaining the unselfish, team-first culture that Doug Pederson has been crafting.
To turn over nearly half the roster and emerge with a singular unit in which new guys and old guys are all working together for the common good is an incredibly difficult thing to do. But Howie and Joe pulled it off virtually overnight. And Doug certainly deserves a ton of credit for taking all these disparate parts he was given and helping mold them into a unified whole.
One thing most of these guys have in common is that they're winners.
Blount and Long won a Super Bowl last year. Torrey Smith and Graham won a Super Bowl with the Ravens. Tennessee went 25-14 when Barnett was there. West Virginia was 18-8 with Douglas on the field. Wisconsin was 40-9 with Clement. North Carolina went 19-8 in Mack Hollins' two years as a starter.
This was not an accident. Roseman and Douglas wanted a locker room full of players who not only are talented but also have tremendous character and understand the commitment it takes to win.
Put enough guys like that together and you have a pretty good chance to build that winning culture all coaches and front office executives talk about but very few know how to build.
It's happened here and it's happened faster than anybody anticipated. Even the people that put this all together.
The Eagles went out and found 53 seemingly random puzzle pieces, and halfway through the season, it sure looks like they all fit together perfectly.