Zaruthustran said:
Given that the game uses the term "damage" whenever an attack lands, I think the rename should be "Damage Threshold".
Instead of having a HP total that's reduced by damage, the paradigm is that you start with 0 damage and accumulate damage as you're hit. Surges and other healing reduce your damage. Temporary hit points are renamed to temporary threshold increases.
So if you're hit in combat and the DM says "take 5 damage", you don't reduce your total HP by 5. Instead, you literally take 5 damage--you add 5 to your total amount of accumulated damage.
This is more intuitive, and negates the whole "what are hit points?" question. "What is damage threshold?" is self-evident; damage threshold is your threshold for soldiering on despite accumulated damage. Exceed it, and you drop.
The question "what is damage" is answered by the game itself. The game already uses the terms "weapon damage", "radiant damage", "poison damage", and so on. Take fire damage and you're burned. Take poison damage and your brain is fried. If you take too much damage, you drop.
When you're sorely wounded, you don't call out "Guys! I'm low on hit points!" Instead, you yell "Guys! I can't take much more damage!"
Makes sense to me!
OK, but reversing all the math doesn't change the core mechanic.
Whether you calculate 0 + x < y (0 is no damage, x is damage taken, y is damage threshold) or y - x > 0 (y is HP with no damage, x is damage taken, 0 is damage threshold), the end result is still the same mechanic with the signs reversed.
The problem is that it is still possible to be killed by taking lots of morale losses, or be nearly killed by morale losses then have that damage repaired by using healing spells. And it is still possible to take real damage, then be "healed" by inspiring words.
But, you are on to something.
What if we expand on your idea by adding up real damage, and subtracting down your non-physical damage (morale, exhaustion, fatigue, etc.)?
When the two meet in the middle, you have run out of HP (just like going to zero in the RAW).
But, magical healing that heals wounds can only repair the physical damage, and inspirational words that inspire you to overcome your fatigue can only repair lost non-physical damage.
So, if you fall out of a tree and smack your head, you add physical damage. A warlord then could not heal you with his encouraging words, but a cleric or paladin could heal you with their magical healing powers.
Of course, there are probably very few things in the game that actually say "This monsters special ability doesn't damage your flesh, but causes you to lose HP due to fatigue". So, probably, just about all damage you are ever likely to take is from crushing objects, or sharp pointy things, or flesh-destroying fire or acid, etc. So almost everything that damages a character
looks like physical wounding.
So we would need some mechanic for determining how to apply certain damage as wounds and certain damage as fatigue.
When you're first level, and a kobold stabs you with a spear, you probably have a wound. But when you're 20th level, and a kobold stabs you with a spear, you probably have a tiny scratch that is barely even bleeding (otherwise it would take 30 wounds from kobold spears to kill you, which we've agreed isn't what is happening at all). But, in both cases, that spear did 6 HP damage. So why is it a wound at level 1 but a scratch at level 20? What mechanic says the damage is HP when you're level 1, but fatigue at level 20? And what if you're level 3, shouldn't some of that damage be wound and some be fatigue?
This ends up complicating the damage model. The combat gets bogged down as players and the DM have to calculate what percentage of each wound is damage, and what percentage is non-physical. It also bogs down healing (et. al.) when each potential healer has to first figure out what kind of healing the wounded individual needs.
Probably too much of a hassle. And if we want this kind of hassle, there are better systems for handling it, anyway - we could just use one of the good ones.
But if we're not doing all that, then what do we gain by reversing the math (other than the fact that most people are better at doing addition in their head than they are at subtraction)?