Sanity is completely removed in A Machine for Pigs, possibly due to Oswald's much more stable mental conditions compared to the two previous protagonists, as being in darkness, witnessing disturbing events and looking at enemies have no detrimental effects.
The lower sanity goes, the more disturbing, surreal and dangerous the environment seemingly becomes. The loss of sanity is detrimental, but inevitable, as some unavoidable scripted sequences cause sanity to drop. The same concept applies when the player must hide in the dark to not be seen by enemies, as the dark also lowers sanity.
Daniel's and Justine's mental condition can be observed by bringing up the inventory screen. Sanity is divided into four levels and has a color associated with each level. Listed from most to least sane:
- Crystal clear (Green)
- A slight headache (Yellow)
- Head is pounding and hands are shaking (Orange)
- ... (Red)
Sanity can be recovered by solving puzzles and progressing through the game. Standing in lit areas conserves a bit of sanity. If in addition, no further disturbing events occur, some sanity may be regained. The highest level of sanity will grant the ability to see enemies glowing in the dark. The last two stages of sanity will cause Daniel's and Justine's vision to have a dragging blur effect and disorienting warping.
There are visual and audible events that can occur sometimes to give the player a hint of sanity changes, assuming the player has all types of video effects enabled in their options.
- Screen warping/zooming in and out while hearing teeth/rocks grinding. (Standing in darkness)
- Screen doing a vertical outward stretch then returning to normal, with a wind-like noise. (Witnessing a sudden disturbing event)
- Center of the screen zooms outward as the edges are pulled inward. (Staring at enemies)
- Shockwave ripples the screen and Daniel gasps in fear. (When the Shadow roars at him making progress, or Alexander psychically assaults his mind)
- Screen suddenly fades to black, sounds are heard, then vision is restored. (Some scripted/disturbing events or in the event of injecting vaccinated blood into himself)
- Screen will distort and shake if any of the torture devices Daniel can interact with are used. If Daniel repeatedly touches the two Octagram seals that require his blood in the Inner Sanctum his sanity will quickly deplete and he'll temporarily collapse.
- Edges of screen zoom in while a noise sounding like a distorted church bell is heard. (Standing in the corpse-filled rooms in the Morgue or in the torture rooms)
- The screen shakes violently at different angles, with a gasp being heard. (When the torch ignites itself in the Storage)
- Screen giving a blue flash while an exhale sound is heard. (Completing a puzzle, finding an item or using a Sanity Potion)
- Staying in the light radius of a candle or torch. (Sanity will very slowly rise from "..." to "Head is pounding and hands are shaking" if one waits long enough)
- Standing in direct sunlight (Sanity will very slowly rise to "A slight headache" if it was lower than that, but no further. Also requires a long waiting time).
- Simply turning on the lantern. (Typically, that's enough to stop the teeth's chattering and hallucinating cockroaches to fly)
- Making progress by simply exploring further into a new area. (Providing Daniel does not witness unsettling events while doing so)
- Obtaining all the Orb pieces in the Transept and the Choir. (Sanity is fully replenished)
- Simply entering some rooms. (Example: Sanity is restored to full upon entering the Cistern Entrance)
When Daniel or Justine reaches the lowest ("...") level of sanity their vision is very warped and their breathing is panicky and erratic. Should any further substantial sanity loss occur it'll decrease to the point where they will collapse on the ground and crawl for several seconds. A continuous high-pitched whine will be heard when this happens.
If an enemy is nearby while Daniel or Justine collapses, they will go into a "searching" state for a short time, leaving them very vulnerable. Eventually, Daniel/Justine will stand up again, their sanity will return to "Head is pounding and hands are shaking", they cannot run and their vision is blurred. They will lose some health after this recovery. Their movements thereafter are slow and delayed, making the exploration even more dangerous than before.
Low sanity also makes Daniel/Justine more exposed to enemies, which are able to sense them even in the dark. Daniel may also be heard speaking randomly about the environment. This is probably caused by Daniel's inability to control himself due to his mental stress. Furthermore, if Daniel is in a low sanity state for some time, he will hallucinate. He will begin to hear disturbing non-existent sounds such as "slime-covered" footsteps, a monster running or distant cries of pain of other people and small children crying. He'll even imagine at several points he's being followed by someone or something, though there is no one behind him should he turn around.
Daniel can also hallucinate enemies, and both characters can see cockroaches crawling or flies buzzing on the screen. On rare occasions, doors approached by Daniel will slam shut in front of him; giving a startling jump-scare.
When Daniel reaches the sanity state of "Head is pounding and hands are shaking" and worse, various aspects of the game will appear different, such as:
- Alexander's portraits, among other paintings, will change into horribly distorted ones.
- Dead bodies will appear hanging from the ceiling in the Choir Entrance.
- A cage in the Transept will contain a corpse.
- The large stairway in the Nave will be covered by Spiders.
- A corpse will be leaning against the pillar in the Cistern Entrance.
- As with Alexander's portrait, it is highly likely that the Gatherers' distorted, frightening appearance is caused in large part by Daniel's low sanity whenever he sees them up close and face-to-face, and that they do not look quite as terribly inhuman in real life. Indeed, when Daniel first came to Brennenburg, on August 3, all he wrote about the Grunts is that they were "strange", "quiet", and "skulking".
While some causes and effects of sanity change are hinted at in-game, most specific reasons behind this go unexplained, which has confused many players. The hints about sanity loss and gain can only be seen if the player has said hints enabled via options.
Because sanity drains when witnessing disturbing events, standing in darkness and staring at enemies - it is easily known that the key factor is fear. As sanity drains, the player's vision and perception of reality diminish more and more, as well as delaying motor functions, showing that the fear gets the best of the player character to a point where simple body control becomes cumbersome.
Daniel's crippling nyctophobia is also an unexplained phenomenon, but owing to the fact that Daniel is completely defenseless, no longer fully realizes where he is (after taking the amnesia mixture) and that he is being followed by what appears to be an intangible mass of darkness, it is apparent that the dark is worth being afraid of. Justine, the protagonist of the expansion Amnesia: Justine, also seems to suffer from this phobia, due to being in a different type of nightmarish situation.
In both the game and the one expansion, staring at enemies reduce sanity and warp the player's vision. This is most likely due to the frightening and grotesque hideousness of the enemies; coupled with the fact of increasing fear in the player character that they will be spotted. The lower the sanity, the more likely an enemy will spot them, showing that the intense fear within the protagonist makes them more visible to enemies (this is likely to be either the result of the protagonist becoming panicky and erratic and thus being easier to spot, the enemies sensing the increased fear, or both).
When standing in any of the six torture chambers found in both the Transept and Choir, sanity is drained there as well. These rooms will trigger a continuous sound similar to a church bell. The sanity loss here is most likely caused by both the presence of the torture devices and the bad memories related to them - which would make sense, as Daniel feels horrible guilt for his wrongdoings to the captured victims during the events of the game.
Some areas of the game will produce enemies in the form of a hallucination, which will vanish once close enough to the player. Some hallucinations are caused only by low sanity while traveling in specific zones. This shows that once the player character's mind reaches a low sanity state, thus subject to increased fear, the character's mind is unintentionally capable of creating experiences which in themselves serve to increase the level of fear.
The gaining of sanity is much less explained than the loss of it. Sanity can only be gained by solving puzzles, making progress and standing in lighted areas with no disturbing events occurring for some amount of time. The most likely explanation is that the player character regains confidence of their situation, knowing that they have stepped further toward intended goals and temporarily taken control of some fear because of that.
There is an item called a Sanity Potion that can raise sanity by one level for each potion bottle consumed, but was cut from the final release of the game's levels, but can still be obtained by using a map editor and in some community-made stories.
- The concept of sanity meters "controlling game effects, audio effects, creating hallucinations and the like" is patented by Nintendo, the publishers of Eternal Darkness: Sanity's Requiem. Amnesia's sanity system is implemented in much the same way as ED's, with the characters' sanity decreasing upon being spotted by monsters and witnessing disturbing events.
- Daniel has nyctophobia, an intense fear of the dark, and cannot endure even a short sojourn in a dark room or passageway. The darkness will drain his sanity even when there are sources of light in the distance.
- When the sanity is depleted, Daniel will start losing health as well (for which the only explanation can be disregulation of adrenaline in his body).
- As stated, this is caused by too much adrenaline in Daniel's system, as high amounts of adrenaline are toxic.
- Daniel's sanity will not decrease when he experiences Flashbacks, so use this as an advantage and turn off the Lantern to conserve oil until the flashback ends.
- Despite the nightmarish situation Justine finds herself in, she doesn't lose sanity. Only when looking at a Suitor or in the end of her test in the Crypt, whilst walking in the blood-ridden corridors, will she start to lose sanity, but even then at a very slow rate. This may have been purposely put in by Frictional Games, as there is no oil to be found in the Cabinet of Perturbation, nor does the lantern have any oil to begin with, and Justine's options to light anything are limited to five tinderboxes, which would make certain areas where being stealthy in the darkness (such as escaping from Basile) much more difficult if she lost sanity at Daniel's rate.
- If Justine goes into "..." sanity, her heavy breathing and teeth chattering aren't different from Daniel's.
- Walking at least 4 "steps" from a candle lit table will cause sanity to decrease.
- If Daniel's sanity is quite low, in rare occasions the screen goes red and he can hear the Shadow roaring at him at several places in the game.
- A previous game series also made by Frictional Games, Penumbra, had a similar game mechanic when staring at enemies. Though the Penumbra games didn't have a sanity meter, staring at enemies would increase the player's chance of being detected, whereas staring at them too long would eventually cause the character to panic and stand up after being stealthily crouched.
- Oswald's ability to hide in darkness without any repercussions in A Machine for Pigs heavily implies he does not have nyctophobia like Daniel. He also is not mentally disturbed by staring at enemies, disturbing events, his children's ghosts, or the Machine's taunts, implying he has a much more stable mental condition compared to the former two, which is probably the reason why sanity was removed in his game.
- The sanity mechanic from Amnesia: The Dark Descent has also become an internet meme.