Aura theory

Aura theory is a theory that states how Pokémon can attack and battle each other without killing one another, with the exception of critical hits.

The theory
Each Pokémon has a hit point (HP) aura, a form of health, which is how much stamina a Pokémon has. When their HP aura reaches 0, then a Pokémon cannot attack or battle. Their HP aura can be affected by battle effects such as poisoning, paralysis, etc. But these effects wear off after a battle. HP can be healed by either resting, using a Berry, or another healing item, or by going into a Pokémon Centre.

Pokémon also have a move or attack aura, which is an aura that can affect a Pokémon's stats or HP. Moves cannot kill a Pokémon but can cause them to faint. Critical (crit) hits can cause a Pokémon to bleed, because of the way the move affects an aura, but the reason is unknown. A Pokémon can die if their HP is 0 and is attacked after fainting.

Pokémon, despite having attributes related to nature, can die from real ice, fires, rockslides, electrocution, etc. because there's a difference between a Pokémon's attack aura and nature. Pokémon can also die from starvation, falling, infections, etc., as they are part of nature and no different to humans.

Pokémon that can sense auras
Very few Pokémon can see and sense auras, but the exception is and, as they are able to sense auras of Pokémon and humans. Psychic-types can also see and sense auras, such as the line. It is unknown as to why Riolu and Lucario can sense auras, but it is possible their evolutionary ancestors may have been psychic-types.