It was high school lunch time, and as usual I showed up to the library's back room ready to game. We were starting new characters (AD&D 2nd Edition as we played at the time), so I figured most of the session would belong to character creation.
The DM was already there, so I sat down and got to rolling my stats.
"Don't worry about that," he said. "I've already done that for you."
Uh, pardon me? I thought. I'm all for DM authority, but surely if one part of the character creation process is sacrosanct, rolling your 3d6 for ability scores is it. Nevertheless, I sat there like a spineless jellyfish and accepted this violation, and the other guy playing did likewise.
"Just tell me what order you want them in, highest to lowest," he continued. I was all set on playing a ranger, so I went with Strength, Constitution, Wisdom, and whatever else those guys needed to qualify. The DM started assigning these numbers in secret, then he started talking. "Okay, you're really strong and healthy, and rather wise. Your intelligence is about average, as is your agility and hand-eye coordination. You don't have many friends, though - people don't like you for some reason."*
Okay. So I didn't know my stats. I knew I was a human ranger, and I was allowed to buy my own equipment, but as far as the concrete numbers and game mechanics went I had only vague descriptions to go on. Sort of like real life, I had a rough idea of how good I was at stuff.
So the other player and I ventured into the dungeon, and there was an actual genuine sense of fear and the unknown. We were pretty reckless hack-mongers in those days, as high-school students tend to be. So when the goblins showed up, we normally would have waded in and kicked ass. But a funny thing happened - without the security of knowing the math, we got cautious. My ranger refused to enter melee, and spent the entire battle keeping out of range and peppering the goblins with arrows. We treated combat like it was potentially deadly, not like a grand old bit of fun.
I can't remember the rest of the game (I'm betting that lunchtime ended and we never went back to it) but the experience has stayed with me. There was something really scary about not knowing the numbers. When you get hit by a spear and the DM tells you that you are "lightly wounded" what does that even mean? How many hit points do I have left? You don't know. You could have 1 hit point to begin with and feel as fit as a fiddle, and you'll never know that you'll be unconscious and dying the first time you get hit.
There's a lot to be said for this as a gaming experiment, but it's not going to work in all groups and all systems. I wouldn't want to try it with more than four players, and I certainly wouldn't want to apply it to rules heavy games like D&D 3e or 4e. 2e was about the right level of simplicity for the DM to manage it.
So if your players are getting a bit cocky, know everything in the Monster Manual, and don't treat their adventuring as a dangerous activity - try taking their character sheets away. There is security in knowing the math, and fear in uncertainty - take away the numbers and see what happens.
* My memory is vague, and I'm making this up. So if that description doesn't match the ability score requirements for a Ranger, it's only because I don't have a Player's Handbook in front of me.