My Magus wants to make a spell that pulls the spirit out of someones body, leaving the body alive (but deprived of its motive force and will) and the spirit free (so that he can do other nasty stuff to it.
Rego Mentem is obviously the art to be used and the closet spell to what i'm after seems to be the mindswap spell (serfs parma, lvl 55 ritual, swaps too intelligences).
What kind of guidelines would this use? would it even be possible?
I would say no, it is not possible, I think Hermetic limitations say one can not afect the soul. And if one could I think the art would be Vim. However one can surly send somone into such an vegtable state whit mentam and doing nasty stuff to his or her mind.
Can't effect the soul...with out a major breakthrough....
Depends what he wants to do with this power...make a lord look a fool?
permenantly remove the will/emotions?
have it reversible?
care if the person remembered what happened after?
try creo-corpus to create a soul less being with no motivation or free will (can put in magical sleep if you want no recolection of events to occure) then transfer the minds. this would be reversable where a perdo-mentem isn't. The targeted would have a blank spot in his memory (from being asleep) when his body was plodding around doing what ever it's told
Possibly you could affect the spirit with a very high level spell, as even church doctrine (I think) divides the ephemeral 'you' into a soul (untouchable by Hermetic magic) and a spirit. If it could be done at all, you would use Rego Mentem. The proof behind this is that many believe the soul is the essential you, the spirit though could be left behind after death and become a ghost. That's why Lay to Rest the Haunting Spirit has a chance of working. If it were an actual lingering soul the spell couldn't work.
aha, but i don't want to effect the soul, i want to affect the spirit. i.e. the bit that makes up a ghost.
Since hermetic magic CAN affect ghosts, could it not wrench that part of the person out leaving the soul and body behind. Effectively you are stripping out the conscious mind, leaving the body in a coma. You don't touch the soul.
maybe i should be ripping out his mind rather than his spirit.
Well then your getting in an issue of symantics.
By 'ripping the spirit out' do you mean pulling out his mind and 'holding' it in a manner that allows you to 'place' it some place else. Be it another person or a garbage can?
Rego Mentem
I agree that there is a difference between spirit and soul, but I also think that spirit is an ambigious word in Ars Magica. I've mentioned this before in discussions concerning controling ghosts vs. spirits. Is there a difference? If so what is it?
Your suggesting that living people have spirits? These spirits are seperate and different than their minds? What's the difference? I ask these questions only to understand what your trying to do. If the end result is you want to blank someones brain, then maybe you want to use Perdo Mentem. But if you want to rip their spirit out and you think this is different than erasing their memory, we need to understand what the difference is. What ever that difference is, I suspect 'Ripping a spirit from a living body' is significantly more difficult than "erasing the memory of a living target'.
There has been innumerable doctrinal discussions concerning the separation between the body, the spirit and the soul. I cannot at the moment recall the first time the church decided to identify the three separate parts of a human, but I can tell you that one Valentius, an early Gnostic (mid second century) was the first to propose the triple nature of man; body (known as soma), soul (psyche) and spirit (pneuma). (it should be mentioned that St. Paul makes the same division earlier without elaborating) his division is of course not exactly according to doctrine (seeing as he was both a Gnostic and also some 200 hundred years before the Cult of Mithra was incorporated into Christianity), but his views are still the basis for all later discussions on Spirit-Seed and Angel Christ.
Soma: (being our physical form) controls the animal urges, as well as instincts and base fears. It is the intriscinctly "evil" part of man. (Gnosticism defines the world as a creation of the devil, not god.)
Psyche: the so-called "immortal soul" which, after death, resides in what we could call Purgatory, it will there be purified (much like the burning away of sins we now have, only without the burning) to finally be able to be resurrected. It is not entirely certain what he meant by this, because he also says that true resurrection can only happen at the end of time. (not the End of Ages as we had some years ago). The Soul carries memories, thoughts and the personality. This is the bit that Hermetic magic cannot touch.
Pneuma: Every person possess something known as the spirit seed; Sperma Pneumatikon, which can combine or merge with the Pneuma (here, and later, defined as/equated with Angel Christ) itself, thus upon death enabling it to return to its source; true resurrection. If the Sperma Pneumaticon merges with the Pneuma it is free of this world. I would guess that when a person dies without having fulfilled said potential, they become a ghost. Thus, as the Soul is the true bearer of memories and emotions, a ghost is naught but a pale copy of the true original.
Sorry for the extremely incoherent rant here, am a bit tired. 
Hope it's at least a bit enlightening.
Personally I'd say that Soma is controlled by Corpus and Mentem, while Pneuma is controlled by Mentem alone and Psyche cannot in any way be affected. Thus changing a persons emotions conflicts with Essential Nature, and in effect making any change temporary, exactly like with shapechanging.
Just my two Aureus...
Thanks d31m0z, thats almost exactly what i meant.
What i'm aiming for is to pull a victims concious spirit(or Pneuma) out of his body. This spirit would still be concious and aware of what was going on. The body and soul would still be alive but deprived of the sentient force behind them, would probably flop onto the floor. The spirit, well aware of what has happened to it (or at the very least terrified and aware that something terrible wrong has occured) should then be easy bait for my magus to barter with (i.e. "tell me where the jade monkey is and i'll put you back in your body").
The spirit, once yanked from the body and soul should be roughly analogous to a ghost since its the Pneuma that forms a ghost when a person dies (the soul whizzing off to heaven, hell or puragtory, and the body going into the ground).
My character is a necromancy equally adept at manipulating physical undead and spiritual undead and this spells seems like a fun (albeit horrific) spell for him to use once he has his art scores a bit higher.
Based on the definitions above, emontions would be split between the three, and several, like lust and hunger would be easily affected by hermetic magic... How one reacts to these is another matter entirely...
Would it not be possible to make a rough copy of somones mind and take that out of the body and then subdue the real mind whit mentem in the body you wish to control?
I reluctantly suppose this is possible, but it seems terribly complicated and excessive. Now you need two very specific spells of likely high level. To pull this feat off. Again depending on how you interpret 'spirit' to me at least determines what sort of spell will be used.
If your talking some aspect of the soul, then no this can't be done at all. If spirit is 'mind', then we have something we can talk about. That's my take.
I can see no reason why that shouldn't work... apart from y'know, common decency 
As for the ghost bit; I'd say "roughly analogous" is about right, as I can't see the ghost having neither Might, nor vis. (else you'd have a new and interesting way of harvesting) Nice idea btw... Suddenly thought of a new trap for my Sanctum 
Quite correct... I can't recall the exact defination, but emotions and feelings are also split in three categories. Animal Urges, Base Emotions and Higher Emotions. Animal Urges are, well, anything an animal feel. Lust and Hunger. Anger is not one though. Base Emotions are Greed and Anger, possibly some positive emotions that are just not rated high enough. Higher Emotions are things like Loyalty and Love, fear of god etc... Persoanlly I'd say that the Animal Urges could be manipulated by Corpus.
All emotions can easily be AFFECTED by hermetic magic, how long this last on the other hand... And of course one's reaction to said change.