Piecemeal System Review: Sanity in Chaosium's Call of Cthulhu

Everyone knows that in the Call of Cthulhu RPG, when you see something sanity-blasting: a murder, a giant severed head falling down from the heavens, a fairy, or even Great Cthulhu himself, you lose sanity from your Sanity stat. When you drop down to zero, you go insane. Kind of like hit points, right?

Not quite.

Starting Sanity and Hit Points
While Sanity and Hit Points operate largely the same (you can pretty much operate at full capacity while you have some points in either). However starting values are different. In the RPG, your hit points are based on the average between two stats -- which normally top out at 18, while your Sanity is based on a single stat (POW) multiplied by 5. In general, you have way more Sanity Points than Hit Points but jumping to the conclusion that it's easier to die than go insane is misleading; there are extra rules governing one's sanity.

When you take physical damage, you roll the damage dice, total it up, and subtract it from your current hit points. If you have any left, you're alive and can keep on keepin' on.

The first Call of Cthulhu
book I ever owned 

When you see something sanity-threatening, you're asked to make a percentile roll against your current sanity value. If you make it, you lose sanity equal to the lower sanity loss dice of whatever you saw. If you don't make it, you lose sanity equal to the higher sanity loss dice of whatever you saw.

For example, if you saw something truly nasty, the ratings are (1D10/1D100) Sanity loss. Make the sanity check, and you only lose a maximum of 10 sanity from your total. Blow the sanity check, and you could lose 100 sanity. That's bad because...

Permanent Insanity and Temporary Insanity
The maximum possible value for your sanity is 99 (more on why later). Lose 100 sanity points means instant permanent insanity and NPC-dom for your former Player Character.

But even if you do make your Sanity roll, and lose only 10 Sanity... there a chance you could go temporarily insane. You compare your total Sanity loss in a short time period against one of your stats (which doesn't go higher in normal characters than either 18 or 21 -- I forget which -- and averages around the 10 to 12 range) and if it exceeds... you must check if can FAIL an Idea roll.

An Idea roll is an interesting old school mechanic meant to reflect that while the Player may not be able to think of something, his or her smarter character might. If after losing too much Sanity a PC FAILS the Idea roll, he or she does NOT understand the full import of what was seen and is okay. If the PC succeeds the Ideal roll, the he or she does and goes temporarily insane.

In other words, remaining sane in this game is not only a mixture of prudence and cunning game play, it's also a matter of luck and some timely (or inherent) character stupidity.

Maximum Sanity
The maximum possible Sanity for a PC is 99 minus the amount of Cthulhu Mythos knowledge the character has. This means that the more a character learns about the secrets of the mythos, the less likely he or she is to be sane. To be an expert in Cthulhu Mythos is to threaten one's sanity.

I bring this up, not only because I'm fascinated how the mechanics have been able to echo the logic seen in the stories, and how being too smart and too brave and too knowledgeable in the game can threaten one's character. All these things are important to successful completions of adventures, but in a game where you really can die quite easily, there's a mechanic that makes you question whether or not you really do need more information, and whether or not you can handle it -- or if some other character should shoulder the burden for at least a little while.