For years now, football experts have passionately debated theories about whether the system makes the player, or the player enhances the system.
If there are proponents still seeking circumstantial evidence to support the former stance, they might do well to consider the Indianapolis Colts defense. And, more specifically, the weak-side linebacker position and first-year starter David Thornton.
During the offseason, when Indianapolis management made no attempt to retain starting weak-side linebacker Mike Peterson, fans railed about the decision to permit a player who was regarded as an emerging star to escape as an unrestricted free agent. Peterson, after all, had been a starter since the former University of Florida standout was chosen in the second round of the 1999 draft.
But nearly seven months after Peterson signed a six-year, $20.4 million contract with the Jacksonville Jaguars, he is barely a distant memory in Indianapolis now. And it is now Thornton, a second-year veteran and fourth-round draft choice in 2002, who is presently assessed as the up-and-comer.
Despite not starting a single game a year ago, Thornton has flourished in his first season with the top unit, and he leads the Indianapolis defense with 42 tackles and also has one sack and three passes defensed. Teammates and opponents have lauded his play. And the Colts coaches speak glowingly, as well, of his early-season performance.
"(He's) good and going to get a lot better the more that the young man plays," appraised coordinator Ron Meeks of the former University of North Carolina star.
So how has Thornton, the 106th player taken overall in 2002, come so far and so quickly? And how have he and the Colts made such a seamless transition after releasing Peterson, whom the Jaguars have moved to middle linebacker?
Well, in part, credit the "Cover 2" defensive system that Colts head coach Tony Dungy developed in Tampa Bay, and which he brought with him to Indianapolis last season. The scheme is based on quickness, and in more ways than just on the field, since its basics are able to be assimilated with remarkable facility.
And, course, credit the athleticism of Thornton, a potentially terrific two-way defender who is bright and instinctive and savvy beyond his years.
A former college walk-on, Thornton didn't start for the Tar Heels until his senior year, having spent his first three seasons principally as a special teams contributor. But on a North Carolina defense that included end Julius Peppers and tackle Ryan Sims, both of them chosen among the top 10 picks of the '02 draft, Thornton was voted by teammates as the unit's most valuable player.
The Colts scouts, knowing Peterson was eligible for free agency after the '02 season and that the franchise might not be able to afford his financial demands, invested considerable time scrutinizing Thornton in person and on tape. They projected that, in time, he should be able to play the weak-side spot in the "Cover 2," and to play it well.
Through four games, Thornton, whose playing time late in the '02 campaign was limited because of a knee injury, has not disappointed anyone, except perhaps himself.
"I'm still a million miles," Thornton said, "from where I want to be. There's a lot of work still to be done, believe me. But every week, I think, I get a little more comfortable. This is a defense, I feel like, that fits well with what I do best."
Ah, yes, the system contributing mightily to the success of the player.
And a player, in this case, to whom the Colts haven't yet had to contribute a large salary. In fact, that was part of the rationale of Indianapolis management in not attempting to keep Peterson around. In the "Cover 2" scheme, the weak-side linebacker is important but not as critical to the design's success as several other positions. While the weak-side 'backer is meant to be a playmaker, the system is flexible enough to turn some responsibilities in that area to the strong-side linebacker, and the club has done so with Marcus Washington.
Said one Colts source: "We thought that Mike Peterson was a good player, don't get me wrong, but not a great one. We just couldn't justify in our minds paying out the kind of money it would have kept to keep him. The way this defense is designed, unless your weak-side linebacker is Derrick Brooks, why pay a fortune? We can play with a very good player and we felt like Thornton could be that for us."
And at a much more reasonable price.
Peterson, after all, will earn $4.53 million in total compensation from the Jaguars this year. Thornton's price to the Colts, in the second season of the three-year deal that he signed as a rookie in 2002, is a mere $304,700.
And so far, he's worth every cent, and delivering incredible return on investment.
Len Pasquarelli is a senior NFL writer for ESPN.com.