To the people of Athas, psionics is as much a part of life as thirst, warfare and slavery. Most creatures have some psionic spark to them, and those that kindle that spark into mastery of the mind are truly frightening foes indeed.
Only the most base creatures on Athas are without any psychic presence at all. Even those who have never manifested a psionic power have layers of instinctive, subconscious defense (the only creatures without this defense don’t really have minds to defend in the first place). We call this their psionic potential, and it’s measured much like the wound boxes in a character’s hit locations. Every character has psionic potential equal to the middle of their Body, Sense and Command stats at the very least (following a Psionic Discipline or gaining the Psychic Reserves advantage can increase this). Keep track of these as empty boxes near your physical damage; these can fill with damage as a result of psychic combat, and those who exercise psionic powers can take damage if their powers fail. If you lose all your psychic potential to damage, you may not use any psionic powers and you are especially vulnerable to telepathic powers. Additionally, once your psionic potential is reduced to zero, further psionic damage is treated as Stun.
Characters regain lost potential through meditation or uninterrupted sleep. For every hour a character meditates, roll Sense + Body with a difficulty equal to the amount of remaining potential the character still has; if successful, regain one potential. For every two hours after the first that a character sleeps, they regain one with no roll (provided the sleep is actually restful; when your enemies can visit you in your dreams, that may not always be the case). Some psionic advantages allow characters to more quickly replenish these reserves.