As I noted a few weeks ago, the introduction of widespread instant replay into major league baseball threatened to do serious damage to how the game is played and enjoyed. That damage arrives in ways that replay's proponents simply failed—or refused—to countenance.
Baseball is a game of rules both written and unwritten. And even written rules have been shaped and influenced (as all written rules are) by the real world that they're intended to govern. For example: most baseball fans probably haven't spent much time pondering, "what is a catch?" At least, they surely did not stop and consider it at the level of detail long set forth in Major League Baseball's official rules:
A CATCH is the act of a fielder in getting secure possession in his hand or glove of a ball in flight and firmly holding it; providing he does not use his cap, protector, pocket or any other part of his uniform in getting possession. It is not a catch, however, if simultaneously or immediately following his contact with the ball, he collides with a player, or with a wall, or if he falls down, and as a result of such collision or falling, drops the ball. It is not a catch if a fielder touches a fly ball which then hits a member of the offensive team or an umpire and then is caught by another defensive player. If the fielder has made the catch and drops the ball while in the act of making a throw following the catch, the ball shall be adjudged to have been caught. In establishing the validity of the catch, the fielder shall hold the ball long enough to prove that he has complete control of the ball and that his release of the ball is voluntary and intentional.
Rule 2.00 (Catch) Comment: A catch is legal if the ball is finally held by any fielder, even though juggled, or held by another fielder before it touches the ground. Runners may leave their bases the instant the first fielder touches the ball. A fielder may reach over a fence, railing, rope or other line of demarcation to make a catch. He may jump on top of a railing, or canvas that may be in foul ground. No interference should be allowed when a fielder reaches over a fence, railing, rope or into a stand to catch a ball. He does so at his own risk.
If a fielder, attempting a catch at the edge of the dugout, is “held up” and kept from an apparent fall by a player or players of either team and the catch is made, it shall be allowed.
Perhaps you skipped that long, tedious explanation—and for good reason. By and large, a catch in baseball was like Justice Potter Stewart's famous definition of obscenity: "I know it when I see it."
That is not to say that a "catch" is completely undefined, but rather that in real-life, with players in motion—especially in lightning quick double plays—the umpires, the players, and the fans were comfortable with a basic sense of what a catch looks like.
And even more importantly, the umpires, players and fans had to be comfortable relying on that basic sense, because umpires had to call baserunners "out" or "safe" based on their best judgment of fast-moving plays, on the spot.
At least that's how umpires used to make their calls. But now, thanks to instant replay and high-definition cameras, these plays are now being adjudicated after the fact, in microscopic detail.
Which means that when an umpire tries to evaluate whether a player, in the words of the official rule, "secured possession" of the ball in flight and "firmly held it," demonstrating "complete control of the ball" until "voluntarily and intentionally" releasing the ball, a lot of plays that seemed like (and thus were called) "catches" in the pre-replay era are now the subject of intense discussion and debate ...
In the middle of games.
As umpires stand around with headphones on, waiting for the official instant replay decision from a headquarters in New York.
And while the players stand around on the field.
To its credit, Major League Baseball anticipated that the introduction of replay could cause these sorts of fundamental problems, and so league officials formulated new guidance to try to help define a "catch' more objectively, so that umpires could call runners out or safe in real time.