- "The Machine Spirit guards the knowledge of the ancients. Flesh is fallible, but ritual honours the Machine Spirit. To break with ritual is to break with faith."
- —The Warnings, Tenets of the Adeptus Mechanicus
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 rituals depend on the machine or technology at issue, but in most cases include various maintenance rituals and prayers of function. They may also occasionally include a sacrificial offering or libation. The number of rituals or the size of the offering is increased in value if the machine is large and/or important to some specific purpose. Machine Spirits are believed to be bestowed by the Machine God and subservient to Him. They are believed to exist in every vital weapon and device used by those who follow the Emperor. From Bolters to Power Armour to voidships, 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, particularly 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 T'au Empire were found to be using more advanced T'au technology for agricultural purposes during the Damocles Gulf Crusade, and the offending colonists were punished with penal sentences. However, the general Imperial prohibition on 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 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 Mechanicum from the signing of the Treaty of Mars in the late 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 independent artificial intelligence under the religious 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), pg. 25
- Warhammer 40,000: Rulebook (5th Edition), pg. 81