In the fictional universe of Warhammer 40,000, a Machine Spirit is a spirit believed by the Adeptus Mechanicus (and, from that source, by most other humans from the Imperium of Man) to inhabit and control a piece of machinery. The functioning, or lack thereof, of any machine is believed to be the result of its spirit's disposition. To keep their machine spirits happy and cooperative, operators therefore make numerous ritualised advances to their machinery. These depend on the machine, but in most cases include various maintenance rituals and prayers of function and occasionally also an offering or libation. The more so if the machine is large or important. Machine spirits are believed to be bestowed by the Machine God and subservient to him.
Because alien technologies do not make observances to their machine spirits, many among the Adeptus Mechanicus see them as grave heresies that must be destroyed rather than studied, even if superior to human-manufactured counterparts in operation. This view is not universal, however. Indeed, many human colonies near the boundaries of the Tau Empire were found to be using Tau technology for agricultural purposes during the Damocles Gulf Crusade, and the offending colonists were punished with penal sentences.
Artificial intelligence (or Abominable Intelligence, as it is known in the Imperium) is generally prohibited in the Imperium unless a human is combined with it or part of its circuitry. This is to ensure the intelligent machine does not rebel against its human masters. However, some machines are allowed such an intelligence under the guise of having an especially active machine spirit. An example of this is the Land Raider tank, which can continue to move and fire, albeit with a lesser ability, even if its crew is unable to fight. Titans have particularly complex machine spirits and can exhibit personalities of their own.
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