You’ve heard the theme, not once but a thousand times: “You need defense to win in the postseason.”

Why in the world, then, are the Boston Red Sox up 2-0 in the World Series?

To recount the weekend’s action, Boston, a notoriously spotty defensive team, appeared to come completely unglued in the first two games of the Series. In Saturday’s game, throwing errors by pitcher Bronson Arroyo and first baseman Kevin Millar and botches of two simple balls by left fielder Manny Ramirez helped turn a game that was at one point a 7-2 Boston rout into a nail biter that was tied as late as the top of the eighth inning.

Last night, things got even worse. In the top of the second, Jim Edmonds, leading off the inning, popped an easy fly ball behind home plate on the first pitch. It somehow fell between catcher Jason Varitek and third baseman Bill Mueller, for whom it would be a long night.

In the fourth inning, Albert Pujols scored from third and Reggie Sanders reached first after Mueller was handcuffed on an easy grounder. Then, in the sixth, Scott Rolen reached base with two outs on yet another Mueller muff, this one promptly followed by a bobble by second baseman Mark Bellhorn that put Edmonds on again.

None of these were anything but routine plays, so far as such a thing exists in the major leagues. (I once saw a ball hit fairly softly into the outfield stands in batting practice knock a woman’s front teeth out; the idea of the routine play, like that of the soft-tossing lefty, is in some ways comical.)

The Red Sox’s inability to turn them had potentially disastrous results: Curt Schilling was, as in Game 6 of the ALCS, on the edge of catastrophe with every pitch last night, and these were extra outs being given away not to just any hitters. Edmonds, who had an out turned into a mere strike, was third in baseball in slugging percentage this year. Rolen’s 5-3 groundout to end the inning was turned into a situation that left a man on first with Edmonds coming up, which was turned into a situation with two men on and Reggie Sanders, a perfectly decent hitter, coming up.

By all rights, the Red Sox’s ham-handed defense should have cost them the last two games. But it didn’t; last night, only Pujols’s run resulted from all the misplays. Is there anything special to make of this?

The received idea is that defense counts for more in October, and intuitively this makes sense: When the value of an out is magnified, so should the value of giving one away. If this is so, though, it doesn’t explain why the Minnesota Twins and Atlanta Braves, probably the two best defensive teams to make the playoffs, failed to last past the first round. Nor, if defense is so important in the postseason, is it easy to explain how the Yankees won six pennants in eight years.

To look at it from a different angle, the Red Sox’ defensive misadventures, while embarrassing, haven’t been particularly bad. They were all one-base errors, the equivalent of a passed ball, a walk, and a hit-by-pitch. Had Schilling done all these things with the same end results, hardly anyone would have noticed.

This isn’t to say that the Red Sox are a good defensive team; they actually aren’t very good. Their worst flaws aren’t killers, though. The absentmindedness of Ramirez and the tendency of Millar, Mueller, and Bellhorn to compensate for a lack of athleticism by rushing routine throws and scoops manifest themselves as egregious-looking errors from time to time. But for the most part, the team catches the balls it needs to catch.

Furthermore, the Red Sox boast several good bench players who minimize the damage in late innings. Pokey Reese came in for Bellhorn in the seventh inning last night when the Sox shifted priorities from getting ahead of the Cardinals to holding them down. You could fairly say that the sort of errors the Red Sox make are the kind least likely to hurt them, and that the time when an error is most likely to hurt them is the time they’re least likely to make one.

The kind of bad defense that hurts a team is the kind that shows up on play after play, from the first inning of a game to the last: lack of range, compounded by bad positioning, at key defensive positions. That’s the kind of bad defense the Yankees have with Derek Jeter, who was miserable in the ALCS, and Bernie Williams. It’s one thing to have a mutton headed left fielder; it’s another to have a center fielder who turns half the balls hit his way into doubles.

All this said, the Red Sox will pay if they keep giving away outs to the heart of the Cardinals’ lineup, and this is especially true at Busch Stadium. The greater size of its outfield makes range much more important, and potentially increases the consequences of plays like those Ramirez botched on Saturday. For the Sox to keep the pressure on the Cardinals, Terry Francona will need to go to the defensive replacements early and remember another old truism: As Curt Schilling reminded us last night, good pitching can not only beat good hitting, it can overcome bad defense.