D&D-style games traditionally have 6 ability scores, but those 6 scores actually represent 8 different abilities. Those 8 abilities, in turn, are simply the combination of three different dichotomies - physical vs mental, force vs grace, and attack vs defend.
This is something I've thought about before, but my immediate inspiration for writing about it now is something that Jack from Tales of the Grotesque & Dungeonesque posted on Google Plus, and the outpouring of responses and ideas he received in return. I'm going to miss G+. The conversations that happen there can only happen because of all the people who are there. And for now, at least, the conversations that happen there inspire me more than conversations happening anywhere else.
Recognizing the 8 underlying abilities does a couple things. First, it points to the direct parallels between D&D's mental and physical ability scores - Charisma, for example, is mental Strength; Intelligence is mental Dexterity. Second, seeing the underlying abilities gives us some insight into the ways the can be re-combined to make a smaller number of scores. (Jack argues, and I agree with him, that it's more interesting to have a smaller number of important scores than to have a larger number of unimportant scores - which is why I wouldn't suggest expanding out to 8 ability scores, although you certainly could if you want to.)
The 8 Underlying Abilities
Physical force attack - The ability to interact with objects on a large scale. Force open stuck doors, batter down brick walls, bend iron bars, break chains, lift weights, pull yourself up, climb. Break things, smash things, crush things, throw things. Overcome obstacles by flattening them, get through defenses by overwhelming them. Be the hammer. Do damage by hitting hard.
Physical grace attack - The ability to interact with objects on a fine scale. Pick the tumblers on a lock, disarm a trap, do tricks with your fingers, perform sleight of hand, disarm. Touch sensitive things without triggering them, manipulate one thing without affecting another, go around obstacles, get through small openings. Get behind them, out-flank them, out-maneuver. Aim well, be precise, find the hole in their defenses, slip through the crack in their armor. Be the scalpel. Do damage by striking in just the right spot.
Physical force defense - The ability to survive being hurt by being tougher than the thing that's hurting you. Endure extreme temperatures, remain unbowed by crushing weight, flex your muscles so they break the hand punching you, toughen your skin so it dulls the knife trying to cut through it. Be your own armor, be your own shield. Let their attacks hit you, break against you, wash over you. Survive poison, survive disease. Endure.
Physical grace defense - The ability to survive being hurt by avoiding the thing that's trying to hurt you altogether. Duck out of the way, dodge, deflect, riposte. Stay in the shadows so they can't see you, step quietly so they can't hear you. Stop short, jump back. Roll with punch, fall to the floor, slide out reach. Twist to escape their grasp. Move so their attack never touches you, move so that the force of the attack simply pushes you along in the direction you're already traveling. Take cover from the bomb blast, hide from the dragon's breath. Evade.
Mental force attack - The ability to affect the emotions of people an animals. Give a rousing speech, make a call to arms, impress them with your faith, show them the courage of your convictions, demonstrate the strength of your principles. Inspire, incite. Tell a story, sing a song, make them weep, make them thunder with applause, make the wicked cower at your feet. Make a good first impression. Calm an animal, train an animal. Cook a meal. Gain trust. Impress.
Mental grace attack - The ability to manipulate people and objects. Tell lies, wear disguises, negotiate contracts and sales. Deceive, defraud, trick, fool, feint, scam. Draw mazes, make puzzles, tell riddles, set traps. Ambush them, take them by surprise. Diagnose illness, forge documents, scribe scrolls. Uncover clues, unravel mysteries, decipher codes, translate and speak languages. Outwit.
Mental force defense - The ability to avoid having your emotions affected by hardening your heart. Don't be afraid, don't run away. Ignore compulsions and commands. Know history, remember family trees, understand loyalties and relationships, know your enemy, understand who they work for. Do things even though they're boring, chop firewood, fetch water, pick berries, make camp. Endure trauma, bear witness, hunt monsters, stare into the abyss, live to tell the tale, never forget. To thine own self be true. In a game with sanity loss, this would be the ability to witness horrible sights without going mad as a result.
Mental grace defense - The ability to avoid having your thoughts affected by looking in the right place, or by looking away at just the right time. Avoid ambushes and surprise, notice architecture, find hidden doors, search rooms, spot traps. Find tracks and follow them. Appraise the value of objects, discern lies. Search your own memories, remember details, recall lore. See the Medusa in the mirror. See through illusions. Out-think. In a game with sanity loss, this would be the ability to avoid looking at a maddening sight, even though it tempts you.
The Classic 6-Ability Division
D&D's 6 ability scores mostly take these abilities individually, but a couple of them double up. Strength represents physical force attack. Dexterity combines physical grace attack and physical grace defense. Constitution is the physical force defense. D&D's mental attributes are basically mirrors of the physical ones, but there's a slight asymmetry. Charisma combines both mental force attack and mental grace attack. Intelligence is mental grace defense. Wisdom is mental force defense. The broken symmetry, I think, is the result of the organic nature of the way D&D has grown over the years. Yes, in some moments it has been designed, but in-between those moments, it has simply grown by accretion.
There's nothing particularly wrong with this set-up, although as Jack notes on Google Plus, it does make Dexterity unusually important, especially in versions of the game where physical combat is more common or more important that social and/or skill challenges. Reducing the number of scores would make each attribute more important by making each ability do more work. Reducing the number of modifiers might also make them easier to use, even if they apply to a larger number of situations. It's easier to remember +1 on this, +2 on that, -2 on THAT, than it is to remember 6 or 8 different bonuses and penalties.
The other possible problem is their names. If terms like dexterity, constitution, and charisma were ever common outside of gaming, they certainly aren't anymore. DCC renames Dexterity as Agility and Constitution as Stamina. Personality replaces Charisma, and arguably adds mental force defense to its repertoire. The Luck attribute from DCC serves a little like a mental grace defense, but it's also something new, a kind of all-purpose defense against misfortune, as well as a resource that can be used up to improve any situation, like a generalized version of the specific forms of "Effort" in Numenera. When it comes to renaming, I also kind of like Daniel Davis's suggestion to call the abilities Puissance, Celerity, Obdurateness, Supercerebrality, Perspicacity, and Pulchritudinousness, if only because it leans into Gygax's old timey naming conventions and does him one better, and because they're so silly I find them kind of charming.
Two Possible 3-Ability Divisions
In the 3.0 ruleset, D&D introduced new Fortitude, Reflex, and Willpower saving throws, representing essentially the physical force defense, physical grace defense, and mental force defense. When other people have tried to simplify the D&D rules by reducing the number of ability scores, the most common reduction mirrors these saving throws.
Although they give them different names, both Into the Odd and Numenera make the same decisions to arrive at 3 ability scores. There's a physical force attribute (combining attack and defense), a physical grace attribute (combining attack and defense), and a single mental attribute (combining attack and defense, force and grace).
It feels worth pointing out that in the original version of D&D, Strength, Intelligence, and Wisdom didn't represent what they do now. They only things they modified were the XP you received from playing certain classes, which means that they were more like measures of Fighting-Man-ness, Magic-User-ness, and Cleric-ness, respectively, than they were like the abilities that familiar to us using those same names today. And that means, when you take those three away, that in playing OD&D, you're left with only 3 real ability scores, Dexterity, Constitution, and Charisma, which is just what you get in I2TO and Numenera.
There was another suggestion though, that came up on Jack's G+ thread a couple times. Adam Thornton, Tim Other, Paolo Greco, Jay Murphy, and Joe Coo all spoke up to call for something like Mind, Body, Soul as their preferred division. Paolo uses Physique, Craft, and Spirit in his Adventure Fantasy Game. There's also something similar in Torchbearer and Mouse Guard (and possibly in other games based on Burning Wheel), which use Will, Health, and Nature.
What all these other approaches have in common is that they invert the Fort/Ref/Will division by creating a single physical attribute (force and grace, attack and defense) while allowing for 2 mental ability scores. Mind (AFG's Craft) is mental grace (attack and defense) while Soul (AFG's Spirit) is mental force (attack and defense).
In Torchbearer and Mouse Guard, however, I think the division is different. If I understand correctly, Will is a single mental attribute (combining force and grace, attack and defense), while Nature, like DCC's Luck, is something else entirely, something new. It represents a certain self-ness, or you-ness, or perhaps species-ness (human-ness, elf-ness, dwarf-ness, or mouse-ness) of the character, something outside of the traditional abilities, that isn't represented by any of the traditional ability scores. Whether you call this Nature or Soul, I think, just depends on your preference ... or you could call it Alignment.
DG Chapman at the Graverobber's Guide proposes Attunement instead of Nature. Characters start with an Attunement score ranging from 1-4 depending on their race. If they receive a favor from a fairy, their score goes up by 1. If it reaches 7, they're so attuned to fairy-land that they go off and live there. I propose a similar idea to make Alignment an ability score. Roll 1d6 (or 1d4+1) for your starting alignment. 1-2 is Lawful, 3-4 is Neutral, and 5-6 is Chaotic. Over the course of your adventuring career, you can occasionally receive help from agents of elemental Law and Chaos to decrease or increase your Alignment score, respectively. As with Nature, if you ever drop to 0 or rise to 7, you cease to be a playable character and become an NPC Agent of Law or Agent of Chaos. Or follow DG Chapman's lead, and use the Seelie and Unseelie Fairy Courts instead of Law and Chaos.
Possible 4-Part Ability Scores ... and Beyond
Of the two possible 3-part ability scores, my own preference leans toward 2 physical, 1 mental - but if I were planning to write a set of rules with fewer ability scores, I think I might want 4. My current preference would be for a physical force ability (combining attack and defense), a physical grace ability (combining attack and defense), a mental attack ability (combining force and grace), and a mental defense ability (combining force and grace).
I recognize that I've paired the physical abilities differently than the mental abilities. I also recognize that some of the skills or abilities that I've assigned to each ability score differ somewhat from the way D&D assigned them - although in part, that's because I'm assigning them more systematically and all at once, while D&D's were assigned organically over time. After all, as I mentioned Strength, Intelligence, and Wisdom started out solely as XP modifiers for fighting-men, magic-users, and clerics. The 8 abilities / 6 scores system I've laid out wasn't anyone's initial plan, it's my interpretation of where we've ended up after years of adding on to that initial framework. (Which is probably why the physical and mental abilities are divided asymmetrically - someone might plan to have Dexterity and Intelligence mirror each other and encompass two abilities each, but I don't think anyone would decide to break symmetry by doubling up two un-matched ability scores like Dexterity and Charisma as part of a plan - I think that could only happen organically.)
I could see someone else wanting to make their 4 ability scores by creating a physical attack ability (combining force and grace) and a physical defense ability (combining force and grace) to match the way I've paired the mental abilities ... or a mental force ability (combining attack and defense) and a mental grace ability (combining attack and defense) to match the way I paired the physical attributes ... or, like me, they could prefer mixed doubles, just the opposite of the way I arranged them. Shadow of the Demon Lord also uses 4 ability scores, Strength, Agility, Intellect, and Will, as does John Stater's Tales of the Space Princess, Strength, Dexterity, Mentality, and Knowledge. One thing that breaking down the original ability scores into 8 abilities based on 3 dichotomies does is let me imagine other possible ways to combine them, and thus other possible ability scores that can be derived from the original abilities.
I find the creation of new abilities like Nature and Luck to be interesting, because they represent truly new additions to an old system. Over the years, I've seen other abilities, but none that have felt nearly as appropriate. Comeliness is supposed to measure physical beauty, but that feels useless to note if it's divorced from Charisma, from the ability to influence the emotions of others. I've also seen Social Standing and Wealth represented by ability scores. That's okay, I guess, although for Social Standing, I'd rather know my character's specific pre-adventuring occupation, and have a sense of which tier of society that job fits into. (I also think there should be fewer than 18 tiers, and I also also think that there's little point in playing a truly high-caste character who already possesses real wealth and worldly power - why would someone who can command armies go knock over a goblin's liquor store or carjack an orc?)
For Wealth, if an ability score is going to replace actually counting gold and treasure, I think it would be more appropriate to have just the modifier, not the score, and I think the range should probably be more restricted than the usual range of ability modifiers. (Replacing found treasure with a Wealth attribute implies plenty of other changes about the way you play as well, so this is certainly not a one-size-fits-all-games kind of idea, notable, both D20 Modern and Torchbearer include abstract Wealth mechanics. There's probably an argument to be made in favor of dispensing with ability scores entirely and just using modifiers, as is done in Mutants & Masterminds.)
But Nature and Luck are different, and they point out the fact that although I can imagine a simple system that encompasses all the original ability scores, skills, and modifiers, and maps well onto 3- and 4-attribute rule system, the original abilities don't exhaust the all the possible abilities you could want your character to have. In particular, Nature, Luck, and the way Numenera uses ability scores as pools of points that are both spent to pay for abilities and lost due to damage suggest new ways of using ability scores that go far beyond what original D&D envisioned, and far beyond the 8-ability system I've laid out here.