I recently read a really nice piece (by Timothy Ferguson) in Sub Rosa about Divisible Men, magicians who can "detach" (and later reattach!) hands, eyes, ears and hearts, essentially to have them function independently (an eye can roll around and spy, etc.). It seems something that hermetic magic should be able to do out of the box, as it violates none of the limits of magic. How would you do it?
The most natural way seems through Muto, but at least in the corebook there seem to be no guidelines that even remotely approximate the change. "Change someone to give them a minor ability" seems a bit weak. With a lot of imagination, "Turn (part) of the target into an insubstantial object" could perhaps fit the bill, but it's a long, long shot.
A surprisingly good candidate seems the Perdo guideline "destroy one property of a person, such as their weight or solidity". In this case, you'd destroy their ... "essential oneness". Of course, it's a really, really high guideline, but a magus casting at R:Per could still manage a D:Sun without ending in Ritual territory.
Any other ideas?
EDIT: I see now that Marcus of Criamon (from MoH) does something similar, starting with a Mu Co Level 3 baseline (mmm, what could it be?) with a slew of requisite Arts (Intellego, Imaginem, Rego). I am ... not sure I find the solution very elegant. In general, I find a lot of the interpretations behind Marcus' spells questionable, or even outright against the rules set out in the Corebook (e.g. a Creo spell that creates a human limb with T:Part...)
Simple approach would be to use MuCo with some complexity modifiers but I think you want detail. I don't see it as Perdo, as nothing is being permanently destroyed. It might have Intellego involved in you are going to transfer your perception to the detached limb, and the InCo guidelines are good for that especially as you'd certainly have an AC to the "object".
If you want the limb to speak use the Grant Speech effect from the Familiars section of Ars p.105, same-same-but-different.
I'd do it with a few spells, and alter the spell Durations according to what your magus can handle, or if they want to use a Maintain the Demanding spell each time instead.
So for the de/reattach:
MuCo10 - Base 3 for an "utter change" seems reasonable, +1 concentration, +1 for allowing de/reattachment, +1 part.
I'd maybe consider a complexity +1 here too, however it is no more stranger than making dirt flow like water and separating the dirt into two puddles. Just have your hand on-hand when the spell ends. So MuCo10 is reasonable.
To grant the severed limb a dull intellect and allow it to move:
ReCo/Me 25 - Base 10, +1 Me, +1 touch, +1 conc
Which means it can follow orders just like an animated corpse that can follow your instructions independently.
To grant speech
MuCo15, Base 5, +1 Touch, +1 Conc,
And I guess a small mouth appears on the hand. Its getting weird.
I mucked about with some parts of this when I was creating necromancy spells (link) which is where some of my reasoning comes from - I cannot even imply with a straight face it is as well though out as TF's Sub Rosa content.
Remember: high level Perdo can destroy properties of bodies, such as weight or solidity. It need not destroy them permanently; in fact, I'd argue that it cannot destroy them permanently, in that the result would be unnatural. In this case the property destroyed would be the fundamental "indivisibility" of living human bodies - the fact that if you cut a human body in two, at least one of the two pieces is going to be dead.
I must say I find this is a far more elegant approach that Marcus Tauros five-art "Eyes from the Wizard Torn", for which, among other things, I'm not sure I can understand the base guideline. It seems to me that the level 3 base guideline "Utterly change the appearance or size of a person (though they must still remain human in form)" isn't quite applicable: you are not changing size or appearance, but instead giving the target a major ability of "detachability". I think in this case the most appropriate way to use Muto would be to start with the base 2 guideline "change the target as to give them a minor ability", and add a certain number of magnitudes to account that the ability is not, after all, so minor.
A consideration would be the difference between the "eyes from wizard torn" approach and my detachment approach is I saw the MuCo or your Perdo approach to create a limb which had no capability for motion or intellect. The EfWT allows motion and intellect straight away, which is kind of cool (and cannon). I suggested a few effects too as my thinking started from thinking about enchanted items too, so many ways to skin a cat.
As an aside, I believe you could simulate it with faerie magic. I've written about prosthetic parasitism on my blog. It's quite long so you'll pardon if I don't quote it here.
AFAICS the MuCo base only detaches the eyeballs in a nonviolent manner that is not immediately destructive. Any ability to move them slowly comes from the Re requisite (likely ReCo base 4, R: Pers, T: Part, D: Conc). Any return of the eyballs into their sockets is just the usual result of dropping a Muto effect.
The handling of the In and Im requisites to see through the detached eyes looks like an experimental benefit to me, though. There is neither a Range parameter nor a Vision Target parameter given for it, so some undefined sympathy in the eyeballs detached by Muto is likely exploited. That's just the kind of liberty a SG or troupe may take when a magus character experiments, and instead of a Discovery a Side Effect with some - likely major - side benefit comes up.