A Machine Spirit is a supernatural, animistic spirit believed by the adherents of the Cult Mechanicus within the Adeptus Mechanicus (and, from that source, by most other humans of the Imperium of Man) to inhabit and control any piece of machinery or technology in existence. 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 the 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. This is increased if the machine is large and/or important. Machine Spirits are believed to be bestowed by the Machine God and subservient to him. They are in every vital weapon used by those who follow the Emperor. From Bolters to Power Armour, the blessing of the Machine God and his subservient Machine Spirits allows for the Emperor's enemies to be defeated.
The term is also applied to the rare instances of machine intelligence allowed to exist within the Imperium, such as the limited artificial intelligences that guide the Space Marines' powerful Land Raider main battle tank and that also sometimes enhance other Imperial devices, particulalrly within planetary Cogitator (computer) networks.
Because alien technologies do not make observances to their Machine Spirits, many among the Adeptus Mechanicus see these devices as grave heresies that must be destroyed rather than studied, even if superior to their 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. However, the general Imperial prohibition of the use or acquisition of alien technology serves the purpose of hindering and punishing those who trade with prohibited xenos species and betray the Imperium.
Artificial intelligence (also refered to as "Abominable Intelligence" or "Silica Animus" in the Imperium) is prohibited under pain of execution by Imperial edict within the Imperium of Man unless a human mind is combined with it or serves as part of its mechanism (hence the existence of Servitors). This point was made part of the agreement between the Imperium and the Adeptus Mechanicus from the signing of the Treaty of Mars in the 30th Millennium, to ensure that intelligent machines would never again become corrupted by Chaos and rebel against their human masters, as happened during the Dark Age of Technology with the constructs known as the Men of Iron, who launched a star-spanning rebellion against their human creators.
However, some machines are allowed a limited form of independant intelligence under the guise of having an especially active Machine Spirit. An example of this is the vehicles of the Space Marines. The Space Marines' Land Raider main battle tank can continue to move and fire, albeit with lesser ability, even if its crew is unable to fight or no longer present. Imperial Titans have particularly complex Machine Spirits and can exhibit complete personalities of their own, though they still require a cybernetic interface with a human Princeps to function at their full potential.
- Warhammer 40,000 Rulebook (4th Edition)
- Warhammer 40,000 Rulebook (5th Edition)