So, everyone knows HP is how much damage a Pokemon can take before it dies. That's what you all think. But have you ever thought how the amount of HP is determined for each Pokemon? In other words, why does Blissey have higher HP than a whale?
Attack is thought my many as physical strength? But why does a seemingly skinny Scizor or a not so muscular dog - Flareon - have the same attack as a Machamp?
So, let's start analyzing, first the easy ones and then move on to the harder ones.
ATTACK is not mere muscle strength.It actually shows how powerfully a Pokemon can hit, which is dependent on many things like physical strength, acceleration(or explosiveness) and leverage. Machamp may have a high Attack due to his musculature but what Flareon lacks in muscle mass, it makes up for by having very dense muscles and explosiveness. Thus, Flareon can produce as much force as a Machamp with its attacks even if its much smaller and less muscly. Scizor on the other hand may have better explosiveness, acceleration and leverage so he hits just as hard. Besides, Attack is also dependent on the means of damage that each Pokemon has. Scizor has those dangerous pincers while Machamp has his fists. So, Scizor can deal more damage easily.
All in all, the ATTACK stat doesn't only show how physically strong a Pokemon is.
It determines how much damage a Pokemon can dish out.
It is affected by physical strength, explosiveness, leverage and most importantly the natural weapons a Pokemon can use to attack(claws, fangs, pincers etc)
DEFENSE is a Pokemon's physical resistance to external wounds, injuries and internal trauma[/B], which means how well protected the internal organs of a Pokemon are from injury.
A Pokemon can be protected from external wounds and injuries by having strong armor, thick hide or tough skin. On the other hand, internal trauma and shock is further prevented by having a sturdy built - dense muscles and heavy, strong bone structure.
Defense is actually what protects HP from running out. So, Pokemon with good Defense can absorb damage better and are better protected from bleeding wounds(which seriously deplete HP). When a Pokemon is attacked, its Defense absorbs the damage and prevents it from seriously harming the Pokemon. Of course, some damage is always done because 1. the damage may be so big that the defensive armor can't absorb it whole, thus some penetrates the armor and hurts the Pokemon and 2. even if no shock/damage manages to penetrate the defensive armor of the Pokemon, the armor still takes damage and when too much damage is done then the armor fails and the Pokemon is injured easily.
All in all, DEFENSE is resistance to wounds and both external and internal injuries and damage.
SPEED is certainly not just how fast a Pokemon can run/fly/swim/move. Pokemon stats are only valued for combat and fight and do not display general traits of Pokemon as creatures. For example, as seen in ATTACK, just because a Pokemon has high ATT doesn't mean it is physically strong and it can lift a car. Similarly, with SPEED, just because a Pokemon has good speed doesn't mean it is a fast runner.
SPEED indicates movement speed, reflexes, movement acceleration and explosiveness. Being quick on your feet is certainly a big help but you can't have good speed only with that. You have to pair it with fast reflexes, explosiveness and rapid acceleration. Movement speed is more about dodging while reflexes, explosiveness and acceleration determine who is going to go first.
For SPECIAL ATTACK it's a bit more complicated. It's basically magic.
SP.ATT is mostly about concentration and focus - that's also why most Psychic Pokemon excel at this stat. A Pokemon with high SP.ATT is able to concentrate hard so as to generate enough energy to produce a powerful attack while a low SP.ATT Pokemon can't generate as much energy to summon an attack of similar power.
Example : An Alakazam can generate more psychic energy than a Drowzee because the way his brain is wired allows him to concentrate better and produce more energy. Same for a Roserade. She can concentrate harder so as to produce much more energy than a Meganium and use a more powerful Leaf Storm.
SPECIAL ATTACK is purely about brain complexity and the Pokemon's ability to exert itself mentally and concentrate hard to summon energy. Having high SP. ATT doesn't say much about intelligence and brain capacity. It's about how a Pokemon's brain is wired. Psychic Pokemon just tend to have very complex, well developed brains.
SPECIAL DEFENSE, is the natural resistance of the Pokemon's cells, external tissue and immune system against energy and chemicals. It's pretty much the counterpart of DEFENSE but for energy related damage.
Special Defense is determined by two factors :
1. Skin resistance to energy and harmful chemicals.
2. Immune system and it's ability to fight said harmful particles.
Higher Special Defense means that the cells and external tissue of the skin are more resistant to energy thus they absorb and diffuse it more easily. High Special Defense means the cells are more resistant to energy which means that fewer energy and chemical particles penetrate the cell membranes and enter the bloodstream to hurt the Pokemon. When the energy is too much or too strong for the cells to diffuse, the cells die and the particles enter the bloodstream. Then the second line of defense kicks in. The immune system of the Pokemon will try to fight and diffuse the harmful radiation, chemicals and energy particles that hurt the internal energy flow of the body - thus hurting it.
Same with DEFENSE though, there is always some damage done ( ranging from 1% to 100% of HP according to the resistance of the external tissue and the strength of the immune system). This is because : 1. The energy or chemicals may be so strong that the cells and tissue can't absorb and diffuse it whole thus some penetrates the cells and goes in the body - this leads to external wounds. Then, the immune system may not be able to deal with the harmful particles so the Pokemon gets damaged internally - internal wounds. 2. Even if the external tissue and immune system are strong enough to deal with the damage, they still get damaged each time and when too much is piled up then the cells die - so its easier for harmful particles to enter the body, while the immune system gets weaker - thus it can't handle much more.
Last but not least, Hit Points.
HIT POINTS don't indicate how much damage a Pokemon can take before fainting or dying.
HIT POINTS indicate the amount of damage a Pokemon can sustain before it is rendered unable to keep fighting. It's not necessary to die but rather to be incapacitated. If an injury is too much for a Pokemon to sustain and it's still in condition to fight then it still has HP. If it's limbing on the ground unable to stand up then its over.
HP, or the capacity to sustain damage, both external and internal injuries and wounds, is determined by :
1. Pain Tolerance. How much pain and discomfort a Pokemon can endure and fight through while still capable of functioning normally - Ability to keep fighting under extreme physical pain or the amount of pain its body can take before it shuts down. It's both physical and mental. Physically, it's how much pain the Pokemon's body can handle before it automatically goes into shock and shuts down while mentally it's how much pain the Pokemon can endure before it is completely overwhelmed and incapacitated by the pain - where it falls down and can't get up or move.
2. Cardiovascular fitness and physical endurance. A Pokemon needs to have excellent endurance/stamina so as to last in a long battle and stay strong, since when a Pokemon is completely out of breath it can be rendered unable to battle. Aside from that, having good endurance and cardio means the Pokemon's heart, blood vessels and lungs are very efficient at transporting oxygen and nutrients to its body and muscles to give it energy. It also means that its heart and vital organs are very robust and they can function under lots of strain, fatigue and lower oxygen levels. Since, most injuries bleed, either externally or internally, the muscles and organs won't get a normal supply of oxygen and nutrients, thus being robust and capable of functioning under those circumstances adds vitality. Also, good endurance prevents Pokemon from losing their breath and getting the wind knocked out of them when they deal with blunt trauma like punches.
3. Body mass. Large Pokemon tend to have higher HP than smaller ones. This due to 2 reasons :
1. Bigger body mass means there is more flesh(muscle, fat) as well as bigger bones to absorb the force of a blunt hit. A punch will put a dent on the internal organs of a cat but it won't hurt a seal because there is much more tissue to absorb the kinetic energy before it reaches the organs so damage is reduced. *This is not to be confused with defense. HP is how much bulk surrounds the organs while Defense is how dense the flesh surrounding the organs is because the denser the flesh(muscles and fat) the better they absorb force. So for example, Snorlax and Blissey are very meaty and have a lot of flesh as well as big bones - high HP due to big bulk. But their flesh is soft and their bones aren't very tough so their defense is low. Umbreon has much less muscle mass and fat surrounding its organs - low HP - but its muscles and bones are very dense thus high DEF.
2. A larger body has more tissue. Therefore, you have to penetrate deeper to reach vital organs or arteries . A large Pokemon has more flesh surrounding its organs which means they are better protected from fatal injury.
*Possibly, one of the reasons, many fat or rotund Pokemon have high HP.
3. Larger Pokemon with more living tissue contain more blood than small ones. Since wounds cause bleeding, having more blood to lose puts them at an advantage because they can lose more blood(quantity-wise not percentage) before their body gets so weak that it can't move.
So, basically it's because of their large body surface and their bigger blood volume.
*Some large Pokemon like Steelix and Aggron don't have nearly as much as others of their size. This is due to their body composition. They are mostly rock or steel which is not living tissue. You have an advantage on HP if you are large but only if your body mass is living tissue. This is mainly due to living tissue not having blood, thus very large Pokemon like Aggron have much less blood in their body than it seems.
4. Healing capabilities. The ability of the blood to clot and reduce bleeding at a fast rate as well as the rate as which blood regenerates. The efficiency of cells and tissue at recovering from damage and tear. It's how robust the Pokemon's constitution is and how well the body can handle injuries. It's basically how well the living tissue can recover from damage and reduce bleeding. Rock types for example tend to have low HP because they are mostly rock, not living tissue. When rock gets teared or breaks it can't really recover well. Same for steel and Ghosts since they don't have much living tissue to begin with.
Now, Healing moves replenish HP in various ways. Some healing moves like Softboiled or Milk Drink or Synthesis provide lots of nutrients and energy that supply the body's tissue and muscles so the Pokemon gets invigorated and the body can recover from its injuries much faster. Others like Recover and Heal Pulse accelerate rapid cell regeneration so wounds heal. Slack Off, Rest and Roost on the other hand, rest the body thus the brain provides much more energy instantly so as to heal the injuries faster - kind of how sleep helps with disease, your body saves energy from other functions which it can use to heal wounds.
So, anyone disagrees with my theory? :D I know it's long, sorry.