Sanity represents an investigator's level of mental health and stability. The higher an investigator's Sanity, the better they can handle the horrors of combat with monsters, strange encounters, and other harrowing events.
At the beginning of the game each player receives a number of sanity tokens equal to their investigator's maximum sanity, which is depicted clearly on the investigator sheet. Each time the investigator loses sanity they discard a token. Sanity can be lost when encountering horrifying creatures during combat, when sealing gates with an Elder Sign, or through casting spells. Sanity can be healed in several ways, such as through encounters; by staying at Arkham Asylum; or through items or special abilities.
If an investigator's sanity is reduced to 0 through any means that investigator is driven insane. If this happens while the investigator is in an Other World, he is instead lost in time and space. If the investigator is reduced to 0 Stamina and 0 Sanity at the same time, they are devoured.