Ever have one of those moments where one of your lifelong assumptions is turned on its ear?

That happened to me Friday, at least on a baseball level.

I'm writing this still somewhat in a state of disbelief, but I wanted to share with you the discussion that's going on.

The inspiration for the discussion is ex-Dodger Eric Karros, whom Oakland signed to be a part-time first baseman. Rob Neyer wrote about the deal on ESPN.com. The justification for the signing is that, although Karros' numbers against right-handed batters are poor, he hits lefties very well, and is ideal for a platoon.

The numbers are there.

Karros vs. LHP, 2001-2003: 207 AB, .904 OPS
Karros vs. RHP, 2001-2003: 991 AB, .672 OPS

So, here's the revolution.

The best way to look at how a right-handed major league batter will perform against left-handed pitchers ... is to make sure you give heavy emphasis to his stats against right-handed pitchers.

I am really going to try to give you the shorthand version of this. For one thing, a carpal-tunnel like fatigue is taking over my typing fingers, and for another, I'm not looking to convince you in one shot. I'm content to let this simmer for a while.

Studies performed by Bill James and other sabermaticians - MGL on Baseball Primer is another notable source - indicate that over time, the ratio between a right-handed batter's OPS against righty and lefty pitchers is consistently 1.09 to 1.

In a given season, or even in given seasons, there will be aberrations. Because most pitchers are right-handed, players get relatively few at-bats against southpaws, thus skewing the sample sizes.

But the stats guys are trying to assure us that overtime, right-handed batters will regress to this Platoon Golden Mean.

Thus, while there's no denying that Karros enjoyed a dramatically better performance against lefties in recent years, the odds are that his platoon gap will revert to normal in coming years. And since he has more at-bats against righties, his stats against righties - as poor as they are - represent the norm.

So, if you're picking a hitter to go against a left-handed pitcher, you'd be better off picking a player who had proven success against righties than a player like Karros.*

Finding this hard to accept? I did. It's intuitive, for me at least, to assume the human frailties involved in picking up a piece of wood and swinging at small ball coming very fast would affect different players differently. The arm that the opposing pitcher uses to throw that ball is clearly a variable, so certainly, some batters would respond better to left-handed pitchers than others.

My instincts battled what I was reading all afternoon, with the result being that I pumped the guys on Baseball Primer with questions. You can read more some of their responses in this thread, but let me boil them down for you.

No one wants to demean my intuition, but the numbers just don't back it up. Over time, a right-handed batter with a .500 OPS against righties will do 109 percent better against lefties (.545), and a right-handed batter with a 1.000 OPS against righties will do 109 percent better against lefties (1.090). The fact that the first guy might be coming off a year in which he posted a .800 OPS against lefties, and the second guy might have gone .700 against lefties - to borrow from Meatballs, it just doesn't matter! At least as far as predicting the future is concerned.

Digesting that? Here's something else: It doesn't work the same in reverse. If you want to predict the performance of a left-handed major league batter against right-handed pitching, you can rely a little more confidently on his personal platoon split - if you are looking at several seasons worth of at-bats. To make a prediction based on one to three seasons of at-bats, instead of multiplying the player's overall OPS by 1.09, multiply it by 1.20 - the league average platoon ratio for lefty batters.

Steve Treder's explanation on this matter was particularly helpful (and note that it applies to major leaguers only).

"The best explanation is this: there are many fewer LHP than RHP," Treder wrote on Baseball Primer. "So while all RHB must learn to hit RHP reasonably well even to make the majors, it is possible for a LHB who hits RHP well to make the majors even though he never masters hitting against LHP. The demographics of handedness allow many LHB to practically never face LHP."

"Here's another way to think of it: there are RHB who can't hit RHP, but they never make the majors," he added. "LHB who can't hit LHP often do make the majors. So RHB and LHB at the major league level aren't exactly comparable populations."

I really find this extraordinary. Think about the impact this revelation, if one accepts it, would have on your player personnel decisions - both as you plan a season, and as you plan for a pinch-hitter to face a southpaw in the bottom of the ninth. Where you might have picked the Karros-like hitter with the gaudy platoon split, you now have more reason to want with the guy whose past numbers against lefties are a little softer, but whose numbers overall are stronger.

Counter-arguments to this theory may well rise, and we should keep an eye on them. But when this kind of evidence is presented, even if it counters my way of thinking for 20 years or more, I have to take notice.

*MGL adds, in reading my first draft of this post:

"What you should encourage people to do if they want to predict how a RHB will perform versus a RHP or a LHB is to use his overall stats and do the appropriate adjustments (for a RHB who faces 65% righties and 35% lefties, multiply his overall OPS by 1.05 to predict his OPS v. lefties and divide by 1.03 to predict their OPS v. righties)."