Next candidate explanation of HP is energy and stamina. At first, it sounds logical. But then again, what does energy reserves and stamina have to do with the pure act of enduring trauma and injuries?
Also, if HP is stamina then Blissey and generally Pokemon with high HP should have high stamina. Then again, if stamina is the ability to last a long time, how can Pokemon with high stamina(high HP) get OHKOed? I mean, how is it possible to lose all of your energy in one hit and get knocked out in a matter of a few seconds since stamina is the ability to last a long time? So, stamina doesn't stick either for those two main reasons :
1. Stamina is purely about fatigue not tolerance/endurance against injuries. If taken otherwise, then its like saying that all marathon runners can take a hell of a beating and survive better than anyone else just because they ... don't get tired.
2. Pokemon with huge HP(thus hypothetically stamina) can faint in 2 turns only. That's .. not what stamina is.
Stamina is about how long you can keep attacking. But if you can't stay alive for a long time, then no matter how much stamina you have, you won't be able to keep going.
For example, a Pokemon may have full HP but zero PPs in all its moves. This means that it is completely drained of energy and has no stamina left to attack. But it still has full HP which means that it can still endure damage even though it doesn't have any energy left. Similarly, it may have max PPs but 1 HP left. This means that it can't take any more damage but it still have a lot of energy and stamina to keep attacking.
The above example explained everything and I think you won't have any doubt after reading it.
So, what is HP in your opinion? I think body mass and stamina are pretty much ruled out. And please explain your opinion sufficiently with examples(if possible) and make sure it is logical.
Oh and if you are going to say HP is life force then please explain it very thoroughly because it really is hard to comprehend. I mean, how can one Pokemon(or any creature anyway) have more 'life' inside? More vitality? I don't get it.
Another thing that clearly shows that HP isn't energy is when a Pokemon is put to sleep. When the opponent uses Sleep Powder the victim of the attack doesn't regain any HP. Yet, when you sleep you replenish all your energy back. So, HP can't be energy.