Before we talk about Kyle Schwarber (and a little Bryce Harper), let's point out that in the old days, a defender built a reputation and kept it more or less forever.
Bernie Williams was a good example of that. For the first few years of his career, Williams was a good outfielder. He had above-average range. Williams was as solid as they come. He was legitimately good, maybe even Gold Glove Award good. Then in 1997, with the Yankees right in the middle of their dominance, people began noticing. Williams began getting acclaim for his defense. It was nice.
The trouble is that by 1997, Williams was no longer a good outfielder. His range was well below average. The early attempts to rate defense suggested that Williams made many fewer plays than the average center fielder. The gurus at Strat-o-Matic -- the extraordinary baseball strategy game -- noticed and dropped his defensive rating dramatically.
Williams still won four Gold Gloves Awards in a row.
Once you get the reputation, you keep the reputation.
This is a story that repeated itself often in baseball. Back in the day when errors were pretty much the only way to judge a player's defense, that reputation was everything, there was no other way to keep track. A player who was good defensively would always be good. And a player who was bad defensively would always be bad.
Enter: Kyle Schwarber.
Schwarber is famous, at least In part, for his defensive deficiencies. He was a college catcher, and after the Cubs drafted him with the fourth overall pick in 2014, they made noise about keeping him behind the plate.
Two things prevented that from happening. One, Schwarber needed a lot of work as a catcher. Two -- and more importantly -- he was too good of a hitter to keep in the Minor Leagues. cblitzed through all the levels in a year and a half, and then he came to Chicago and blasted the ball, and there was no time to mess around with his receiving skills. You don't hesitate with a bat like his. The Cubs put Schwarber in the outfield.
And, well, it was rough. The thing about Schwarber was that not only was he not a good outfielder, but he also had a knack for making the particularly ugly defensive play, the one that stuck in your mind. He would run in on a ball that went over his head, or he would dive for a ball that landed 10 feet away or something. Whatever Schwarber's true defensive value, people assumed he was even worse, because of those frightful moments.

People wrote off Schwarber as an outfielder.
And so it's unclear how to deal with the fact that, by the numbers, Schwarber is playing adequate -- even good -- defense in 2018.
Quickly, what do those numbers say? Well, Baseball Reference ranks Schwarber five runs above average. John Dewan, who has been studying defense forever, has him plus-one on his plus-minus scale (which estimates how many plays above or below the average player are made). Fangraphs has Schwarber five runs above average. The Statcast™ folks have him slightly below average but show him holding his own.
What do you make of it? Are the numbers just wrong? Probably not. Even basic numbers show that Schwarber is just playing better defense. His range is way up from last year, and it is right abound league average. Schwarber already has a career-high eight assists and has made just one error.
"Kyle's been terrific out there and his arm has been a huge weapon," tweeted Cubs broadcaster Len Kasper. "He has passed the eye test too."
And yet, it's hard for people who don't watch the Cubs regularly to accept that Schwarber is playing better defense. I mean, it's Schwarber. Part of the issue is how geared we are to thinking about defense a certain way. We tend to think of defense as a static thing, unchanging.

We do not naturally think that players can have good defensive years and bad ones. We fully understand that happens with offense, that one year someone might hit .311 and the next .260, one year hit 29 home runs and the next year 13, this makes sense to us.
But we think of defense as constant. And it isn't. There are countless variables to defense, just like hitting. Players absolutely have good and bad defensive years, no question about it. We've have just never had defensive numbers that we trust the way we trust batting average.
In Washington, a sort of counter-Schwarber thing is happening to Harper. He came into the season with a solid reputation as a defender. Harper worked hard, showed athleticism, went all-out after balls, smashed into walls. Nobody was going to give him a Gold Glove Award, but he helped his team defensively. Everybody thought so.
This year, Harper is having a disastrous season defensively, utterly disastrous. The numbers suggest he has been one of the worst defenders in the game. Harper has minus-1.6 wins defensively at Baseball Reference. Dewan has him making 13 fewer plays than the average outfielder.
So what's the deal with that? What's happening with Harper? Is he hurt? Harper is playing like he's hurt. Is he frustrated? Harper is playing like he's frustrated.
Then, you go back and ask what's happening with Schwarber? Is this real? It seems real. He seems to be getting comfortable in left field. Schwarber is showing off his above-average arm. He's impressing people around the game.

The final point must be made: We're talking about a half of a season. It's nothing. It's such a small sample size that you can't make conclusions, not about a moving target like defensive skill.
Still, these defensive numbers -- imperfect as they might be -- are great, because they allow us to follow the story in real time rather than relying on outdated reputations.
For years now, everyone assumed that the Cubs would have to deal off Schwarber, because he is destined to be a designated hitter. That assumption might be changing. He might be coming into his own. A Schwarber who plays average defense -- even slightly below average -- can help the Cubs an awful lot.