Because by fate even the gods are cast down

Oh boy....

The machine spirit is a ghost in the machine, a benevolent spirit, sanctioned by the machine cult of Mars.

AI is free willed, it thinks and acts on its own in ways we can not predict. A machine Spirit needs manual input, an AI does not.

This is my horrid explanation.

40K Pre-Context: Most members of the AdMech don't know nearly as much about how technology works as they aught to, because they memorize everything.

IRL Pre-Context: Everyone who maintains machines (of any kind) professionally will inevitably humanize the little inconsistencies and mystify the obtuse elements of operation. The machine becomes some sort of Beast of Burden to them.

Theology

The Machine Spirit is a theological concept in the Cult Mechanicus. It's not literally real, in a direct sense. The Machine Spirit is believed to be the soul of a machine (typically any machine of sufficient complexity to break in ways which are not immediately obvious) which imbues a machine with ability to operate. It is believed that adepts of the Cult Mechanicus may interact directly with a Machine Spirit. This includes learning about the machine directly from its Machine Spirit, soothing a Machine Spirit to improve operational reliability, healing a Machine Spirit to fix a broken machine, and even destroying a Machine Spirit to break a machine.

Interacting with Machine Spirits is believed to grant some kind of advantage in repairing or maintaining a machine. This advantage goes beyond the physical acts involved in repairing or maintaining a machine. A tank repaired in the field by a few Guardsman is considered "less repaired" than if the exact same repair was performed by a few AdMech adepts in a garage, because the adepts know how to interact with the tank's Machine Spirit.

Bigger, older, and more complicated machines are assumed to have Machine Spirits of greater sapience. Think of the difference in complexity between a Titans with a multiple neural interfaces versus a lasgun: naturally a lasgun will have fewer problems with it, so its Machine Spirit might be seen more like a pet bug, but the titan's Machine Spirit may be seen as close to a human in terms of sentience.

"Real" Machine Spirits

When the belief in the Machine Spirit becomes more grounded in reality is with the introduction of computing systems (cogitators) and the Warp.

Imperial computing systems are a vast hodge-podge of systems old and new, digital and biomechanical, known and unknown. Computing systems of the Imperium include networks of human brains, holographic crystalline neural duplicates of dog brains, undercover Abominable Intelligences, and worst of all some fucking communication standard that nobody documented well enough to be understood 5000 years after it was implemented. Now these really complicated, expensive, and mission-critical machines are genuinely acting on their own in unpredictable ways. Treating these machines like an old and cantankerous ox that could trample you to death is probably a good place to start.

The Warp adds further complications to the mix, because it can fundamentally alter reality. There is a somewhat unstated phenomenon in Warhammer 40k lore whereby machines that interact directly with the minds of sapient beings can begin to develop their own consciousness, on a long enough timeline. It's happened with basically every Titan (except Ork Gargants, that I know of). This is likely because of minute amounts of psychic backwash from the operators getting into the thinking bits of these machines, and marinating that backwash over hundreds or thousands of years. It's also possible that neurological systems within Warhammer 40k just start developing souls at some threshold, and most of the cogitators run on artifical neural networks that grow more complex (or sapient-like?) over time, until they hit the ensouling threshold. This would grant a cogitator with a genuine will and/or personality, however subtle.

Abominable Intelligences

The ban on "Abominable Intelligences" goes back to the Men of Iron incident (at least), wherein humanity fought off a robot uprising. The wisdom behind the dogma is simple: don't intentionally make a machine smart enough to self-determine or self-improve. The scifi concept to note here is "Artificial General Intelligence", or" AGI".

The Cult Mechanicus want to prevent an AGI uprising from ever happening again. But they also don't have extremely precise tools to determine what is and isn't an AGI on a nuanced level. So it's mostly guesswork and vibes. If a Knight Titan keeps fighting for a while after its pilot is probabaly dead, but it doesn't do anything traitorous and there's some plausible deniability as to when the pilot died, then the AdMech aren't going to be motivated to destroy the Knight. They might run several extra diagnostic tests before giving the green light for a new pilot, but as long as the vibes are fine there's no need to waste good war materiel. The logic goes like this: "The Knight wasn't built to be sapient, it wasn't sapient 200 years ago, and the adepts always used the best holy oil on the joints regularly! It's been blessed too regularly to harbor and Abominable Intelligence! Plus, it wasn't fighting the forces of Chaos, so it's probably not posessed either! It's just the Machine Spirit feeling wrathful toward the enemies of the Emperor, and mournful of its pilot. Nothing to worry about."

"AI" is not given a pass. There's wiggle-room in the creed of the Cult Mechanicus, because nobody has time to administer 144 Turing Tests on every cogitator and debate existential philosophy with other adepts when there are 80 gajillion wars going on everywhere.

The father of Warhammer 40k art direction and the man that has inspired me as an artist, down to inspiring my current artstyle (and I am sure will continue to inspire me, even in death). 40k just wouldn't be 40k had it not been for the foundation of grimdark sci-fi that he laid.

Farewell, you absolute legend.

A. That which is readily apparent by looking at standard figurines, or reading about chains of command and,

Primarch, and their opinion about tax/tax season.

Horus: Tought that like. Tax happened to other people. He was his dad favorite lil guy! You wouldn't tax a lil guy like him, right?? Was eventually hit wih many years of late fees and unpaid taxes, and the fallout that ensued may have helped in the start of the heresy.

Sanguinius: His forms are always, always flawless. Not a mistake in sight. What people don't see is the absolute panicked meltdown that those forms create evey years. By the end he's a disshevelled mess and his room is covered in feathers, tears and blood, but the forms are done.

Vulkan: Give a good, honest try every year. Somehow always end up with a small return, wich makes him very happy. He actually has been making small mistakes about it, but it's within he margin of errors and the Tax Organisation gives it to him anyway.

Perturabo: Libertarian. Refuse to give taxes to the Imperium, insist he has a provincial local systhem that work beter and that HE can manage the expense for his legions through it. Forrix had almost a mental breakdown trying to fill it.

Fulgrim: Super torn. On one hand, he UNDERSTAND why tax are important. On the other hand... What's a little creative accounting? Some light embezzelling? Surely this won't blow up in his face, right?

Ferrus: Did not know that tax were a thing until Fulgrim told him. Asked him if he could do them for his legions, and his boyfriend accepted. He pays him back by spending a few days a year with him. In bed. He LOVE tax season.

Jaghatai: Does his taxes, but grumble every years that they are VERY frustrating for not accepting horses as a valid payment form! He even offered to give extra, if they just let him pay in damn horses! The conversion rate gives him headaches. Somehow always just shy of being late.

Alpharius & Omegon: win at taxes. Somehow get a returns that is bigger than their entire spending and earning combined for a year. They just hacked into the systhem and gave it to themself. They are not ready for the Horrors that will soon follow (a full audit).

Magnus: Insist that divination is a valid way to get to your tax statement. When it was refused, tried numerology. His forms spontanuously combusted when completed. He then makes Ahriman do it.

Mortarion: tried the first year to pay in a percentage of a grain harvest. It was refused, and he threw quite a big tantrum. Make the process as long and as painful as possible for whichever poor soul is sent to talk about his tax with him. Contest everything.

Rogal: Oh he fill them so, so good. Actually enjoy the process, it pleass his brain so fucking much. Fuck yeah, give him more tax papers baby! Sometime, if he's feeling spicy, he will intentionally put in a mistakes to see if the tax agents catch it. One time they even sent him extra forms to fill once it was spotted, and he was shaking in excitment.

Angron: All tax forms of the WE are redirected to Lotara. She send them back with a red scribles "FUCK OFF" and a pile of teeth. It's marked as paid.

Lorgar: Does his tax very well! On time and everything!! But his papers keep having the most unlucky bullshit happen to them. One time they were literally dropped in a lake. Constantly has to redo them, and its stressing him out!!! The chaos god are in full effect to garante that hr never has an easy tax return.

Leman: is "conveniently" unreachable every tax season. Constantly on the move in random corner of the galaxy for it, and laugh nervously when tax are mentioned. Actually does not know what tax are and at this point he's too afraid to ask.

Konrad: Insist that he made proper payment, and that his tax are ABSOLUTELY exact, without a single fault or flaw, because that would be a Crime. The fact that he paid in humans skins and organs, corresponding to their exact monetary value, is disturbing.

Lion: Stare at the paper. Stare at whoever gave him the paper. Make a simple grunting noise, and slides all the paper toward Luther. This was the start of the deterioration of their relationship.

Corvus: Tax are good. Tax are greats. She love and respect an equal tax burden, and will publically respect a good tax burden. In private tho, she bitch and moan about her tax funding things she hates. Daydream about witholding them, but never do so.

... And Roboute: The one that instaured the tax systhem in the first place. Know every single rules, regulation, and how exactly to break them. Unwilling to admit that the entire database is staying on a house of card that are 3 excel sheets. He has one machnicus guy keeping it alive.

Bonus round:

Malcador: Has been doing the tax for Terra, some-fucking-how. He actually just imput bullshit numbers and people assume that he did the calculation correctly, because there is no way that they are doing the 3 millions page audit.

Imagine this guy. rational, brave, an independent thinker, brilliant and erudite. He is fiercely against the theocracy and all its superstition. so, as his first shot fired against the corrupt dogma of the ecclesiarchy, he writes a biography of Guilliman. He pieces together every scrap of fragmented historical material he can find, desperately trying to strip away the saint, the demigod, the son of god, and reconstruct him as a historical figure. a real, flesh-and-blood human being.

His work starts circulating in the underground scene, and somehow, against all odds, a copy makes its way into Guilliman's own hands.

And Guilliman is profoundly shaken. Because the book captures him perfectly. it sees his monumental achievements, but it also calls out his flaws, and the author managed to deduce all of this despite the suffocating censorship of the 41st millennium. Guilliman is desperate to meet this author. He needs someone like this—someone who has managed to stay so incredibly lucid and sharp in this era. someone who actually, truly understands him.

He tracks down the author's identity and goes to meet him.

And then it happens.

This rational, brave, independently thinking, brilliant scholar takes one look at guilliman and is instantly, utterly crushed by the primarch aura.

He drops to his knees, sobbing hysterically, shaking all over, repenting for his past ignorance and arrogance. he used to doubt the Emperor's divinity, but now, seeing the son of the God-emperor standing right in front of him, he realizes how incredibly wrong he was. If the Emperor isn't a god, how could he have forged such a transcendent primarch? How dare he ever perceive him as a mere mortal who laughs and weeps? The absolute blasphemy of it!

He curses everything he ever wrote. He begs for forgiveness, swearing that if the great, holy, immaculate, and sinless son of god spares him, he will dedicate the rest of his life exclusively to singing hymns and writing psalms for the god-emperor and his son.

And Guilliman just stands there.