One of the original appeals of The Fantasy Trip's three stat system (ST, DX, IQ) is that these three stats are pretty near orthogonal. You can see combinations of high and low in each of them as being about equally credible.
Intuitively, the six stats of D&D are a harder sell.
Wisdom, like Intelligence, tends to be a slippery subject. I really tend to define those more as "intuition" and "learning" to avoid the confusion between character and player abilities. They are filters through which the player receives information in character, rather than fetters placed on the player's common sense and problem solving ability. As such, it is easy to think of a weaker link between the two, making for dotty professors with low WIS and high INT or canny urchins with low INT and high WIS.
Charisma, probably can exist with any level of mental or physical aptitude, and the charismatic moron makes for good comedy if nothing else. (I suppose by now you can tell why I hate Comeliness, quite apart from its tendency to lead games down the path of the drool-soaked adolescent hornfest.)
I'm sure we've all resorted to these explanations but they just seem kind of forced. STR and CON, as defined, don't really seem orthogonal.
But let's take a look at what CON does. In almost every case, it increases the survivability of the character from immediate threat. Its role in hit points, saving throws, and system shock rolls over various editions speak to this. So why not make it a metaphysical rather than physical trait?
Fortitude, or Fortune. Someone high in this ability possesses a strong will to live, almost a sense of invulnerability at high scores. They are fortunate - not in the sense that they can lift up their shoe and find a gold piece stuck to the sole, or roll random encounters with wish-giving unicorns. No, they are fortunate where it counts, at the thin edge between life and death.
|Snails: Low Fortune from minute 1
Fortitude/Fortune then takes its place as Charisma's sibling - two physical, two mental, and two metaphysical stats. All more or less orthogonal. All I have to do now is settle on a name .. Fortune ... Fortitude ... maybe something starting with Con that calls back more readily to the original stat ...?