Investigators begin the game sane and rational; however, during their investigations they will encounter otherworldly horrors, alien creatures and the terrible cosmic truths of the Cthulhu Mythos — all of which will challenge their state of mind and sense of normality, and push them to very edge of sanity. In some cases, the experience will be too much and the unfortunate investigator will descend into periods of temporary, indefinite or even permanent madness.

Each investigator has Sanity points that can rise and fall during play. Sanity is used to determine whether the investigator can withstand the horrors of the Cthulhu Mythos, as well as stand up to fear-inducing situations without turning tail and running away. Loss of Sanity points can lead to insanity. When Sanity points are reduced to zero, an investigator is permanently and incurably insane, and ceases to be a player character.

Sanity in the game is modeled after the behavior of protagonists in H.P. Lovecraft’s fiction who, more often than not, faint or go mad when confronted with mind-bending physics and monstrous entities from beyond space and time. Sanity is the game’s register of the investigator’s flexibility and resilience to emotional trauma. Characters that start with a high number of Sanity points find it easier to rationalize traumatic events or to repress horrific memories.

Those with a lower number of Sanity points are mentally fragile and more susceptible to emotional upset. While the cosmic horror of the Cthulhu Mythos is the fundamental assault on the mind of an investigator, other gruesome and comparatively ordinary sights and events can also cause emotional disturbance.

In an unnerving or horrifying situation the Keeper will test the resiliency and emotional strength of the investigators.

This is done by calling for a Sanity roll for each investigator entangled in the situation. Investigator insanity is triggered when too many Sanity points are lost in too short a time, causing temporary or indefinite insanity.