Thanks for the interesting answer.
You seem to be saying that "naturally restore themselves" only applies to the Base 10/15 Guidelines listed in AA and that the Base 25 core Guideline is untouched. Even if it uses a Base 25, PeMe50 The Clean Slate does not "leave a person a mindless husk" once the Duration is done. Is this merely doing less than you could?
Sorry for any confusion in my previous post; I was writing in a hurry and failed to proofread.
You are right -- that was a dreadful analogy! Let's just ignore A&A for a moment. The pertinent part of the core rules is
and then there is the guideline:
We don't have an example of the Level 25 guideline in play in the core rules. However, my interpretation is that all aspects of the mind would be immediately destroyed. By the RAW, memories are gone forever, regardless of duration. Emotions disappear for the duration of the spell, since they "naturally reappear in a person." This shouldn't be contentious. By canon, Black Whisper drives the victim insane, but only for the Duration of the spell.
The elaboration that A&A adds is that there is more to a mind than memories and emotions. There is thinking (imagination), planning (cognition), and interpretation of sensory input (common sense). A&A clarifies that these things are like emotions (estimation); they naturally reappear in a person. So if you want to maintain the state of "mindless husk" in a person, then you need to extend the Duration. This stops them feeling emotion, thinking, making plans, or even comprehending anything they see.
A&A also mentions that you can destroy just one of these faculties. Use the Level 15 guideline "Remove a major/long term memory" to prevent thinking or planning, or the Level 15 guideline "Remove all emotion" to negate common sense. But like the canonical Passion's Lost Feeling, you need a Duration to maintain this effect.
Mark
Hi,
More thoughts on memory loss, this time from another source.
Houses of Hermes: Societates (Chapter 2: Jerbiton) talks about loss of Abilities; which are covered under inscribed memories (most knowledge Abilities) and procedural memories (most physical Abilities). The sample spell Dissolving the Wall of Shields removes one aspect of procedural memory, such as the ability for a group of soldiers to fight as a unit, or courtiers to remember dance steps. Only an aspect of the Ability is lost, because it uses the Level 4 guideline "Remove an important detail from a person's memory". The spell goes on to say that "[t]hese memories do not return when the spell concludes, but may be regained with simple demonstration and practice, requiring time but no Experience points."
Presumably if the whole memory was wiped, then the whole Ability would be as well (Hermetic magic is stated as not currently being able to create or duplicate these kinds of memories, but other magical manipulations still work). Extending Dissolving the Wall of Shields, a kind troupe might rule that wiping a mind of memories does remove all Abilities, but they can return if demonstrated and practiced without XP. A backup to this idea in the Agnosia spell, that can destroy all knowledge of how to use a tool, but leaves the Ability intact. So without thinking about it, a person might pick up that tool and start using it without realising it could.
A harsher troupe could say that that doesn't work when memory of the whole Ability is removed, since there is nothing else to build on.
Mark
[Edited to insert more info]
Honestly, this feels like a very strange gimping of the Pe/Me Te/Ar combo.
IMHO, this unnecessarily removes a lot of cool story-lines and plots, because Magic can't permanently destroy a persons mind. I get that this might fit with the paradigm, but get the uneasy feeling there is a sort of bias against Mentem at work here to... But that's probably just me.
Anyway, looks like it's house-rule-time... 
Having seen the power of mentem in one of our sagas, I would say that the "anti-mentem bias" is quite inexistent. It is one of the most powerful arts out there, by a large margin. it does not work so well in combat or if you live in the middle of nowhere, but in all cases where social interaction is a must, it is an extremely powerful art. Much more than (say) ignem in a lot of situations.
But no, it is not one of the "kill everything on sight arts 
It is still powerful. When facing enemy soldiers, our mentem guys tended to turn them against themselves and their master instead of just smashing their brains.
Cheers,
Xavi
The most useful and powerful spell of my fairly experienced Tremere magus was Call to Slumber. Several powerful entities with Int scores succumbed to his very good Penetration and were subsequently beheaded with axes. He may have had done a lot of work to find, bind and gain crontrol over various spirits but in the end this low level spell always did the trick.
Rego Mentem is very powerful.
Ok, so the failure of Pe/Me to destroy a mind is just arbitrary then? Or purely a paradigm thing? I hold that if Pe/Co can destroy a body's eyes with a D:mom spell, then a Pe/Me spell can do the same with your imagination. But, I am now convinced this is against the RAW.
(Slightly off topic: I am really annoyed by the lack of duration "permanent" in this edition.)
Edit: And for the "it is as natural for a mind to reason as it is for a stone to have weight"-argument: No it's not. All stones weigh something. All minds can't reason.
Also, would a "Co" requisite allow for a permanent lobotomy? Or could a pure Pe/Co do it? Seriously, is there any way to use magic to make someone brain dead in AM 5th ed?
Just to play devil's advocatea bit here, not really trying to knock your ideas: I seem to remember that in medieval thinking, the brain was used solely to cool the blood so targetting that in particular should give the recipient a major fever not any mind-effect...the 'guts' did the thinking in those times! 
The reason for mentem being unable to remove the basic faculties in the game system, again if I remember my background reading, is because those come from God's gift of Reason (being derived from the soul) and therefore being beyond hermetic magic.
On the other hand, the base spell to make someone a mindless husk is pretty terminal if the hostile spell-caster feels like just wacking the victim afterwards...and there are plenty of ways to make such a spell be 'permanent' (yet fixable, if required by the plot) if you use a bit of non-hermetic magic. Curses from folk witches, Gruagachan, Vitkir, faeries ('until' duration), infernalists, etc.
Hermetic magic cannot do everything: it's the best magic tradition for doing the most different things reasonably well. Each individual spell effect can be achieved in a better way by a specialist - but that specialist will be very weak at almost anything else.
Agreed.
Agreed, but that's the part I will be changing in my campaign. I'm not entirly convinced that's how the medieval paradigm looked at it either.
Sorry, I'm looking for this to be as common as a Hermetic Mage leaving an enemy blind.
I know this, but this one limit is just to random for me to accept.
I know I'm starting to sound like a broken record, but this whole thing just annoys me.
This should probably be listed as one of the limits of Hermetic Magic if the RAW is "No Pe/Me spell can ever destroy mans ability to reason for good." It seems totally out of flavor to me that a 50th level Pe/Me spell can't reduce all mental stats to -5 or worse.
The soul can be in there, intact and blissfull, but the spirit should be "blastable" with Pe/Me.
How badly off do you want those targetted by this effect to be? They can re-learn the stuff they've forgotten, but it's your saga and they're pretty much out of commission for years and years doing that re-learning if you want them to be: no points in anything, no xp at all. The guidelines from A&A are entirely optional.
Your saga, change that rule: mindless husk=full-time intensive care treatment or die, if you want it that way. Rule 1 applies.
PeMe can already reduce mental stats. That actually has no bearing or link to the 'mindless husk' guideline. Nope, it's just as separate as using CrIg to fry them. Just because a stat is negative does not prevent the mind's faculties existing or being used, even if they are then pretty ineffective.
And that is what I have done
To me mindless husk does equal full-time intensive care. The mage has destroyed the target's spirit and the body will die, unless the spirit cured with magic, or the body cared for.
("Total amnesia" is pretty harsh, when you think about it. Altho' devastating for a mage, it's no better for a Specialist - whatever they could do, they ain't doin' it no more, and not for a long, long time - not without magic. Having a full-grown but self-sufficent tabula rasa running around could be an interesting plot element.)
Thanks for the additional clarification, M - I always prefer to know what canon is, even if I'm going to then houserule around it.
Forcing a character to re-learn (most) "everything" is harsh - but less harsh than killing them outright, which can be done with an equiv PeCo. Using the PeCo guidelines as a model for equivalence, there's lots of room for Houseruling a variety of unpleasantness in this direction (that takes as long as "serious wounds" to heal naturally) before getting to "drooling vegetable, game over" status.
And speaking of that, it should be noted that there is no Base 20 effect listed for PeMe - if one is going to houserule, there may be room for a gentler progression from losing a specific memory to total amnesia to brain-dead, or whatever "most severe PeMe effect" your Saga is going to adopt.
Don't forget that, as at least some magical entities aren't susceptible to deprivation, they just don't need to sleep.
The same thing probably also applies to most Angels and Demons, although not to faeries: In you exact exemple, I can very well see a Faerie sleeping and being beheaded.
YMMV, of course, and this may be fine for you, no problem. But if you see this as a problem with Mentem and entities, the solution in just out there.
Try duration: Until (and similar faerie tricks)
Well, we'll have to disagree, then. All minds with Intelligence can reason. If "intelligence" doesn't include reasoning, it isn't intelligence, then, but cunning.
Can you really picture an intelligent person being naturally incapable of reasoning?
That's because the fine distinction between Man's Faculties wasn't written down at the time.
Note, though, that, as Mark Shirley kindly pointed out, the roots of Mentem being unable to destroy these faculties is already right there in the corebook:
A&A basically just tells "the other faculties that we present you here are also immune to destruction".
This is not a restriction, just an extension of the core rules, that has the advantage of allowing you to custom-tailor your effects by targeting a specific mental faculty.
Your saga, your rules, but you may want to think before allowing that guideline to do what you say: The RAW currently already allows you to do this, be it for an instant or a constant wipe (although the duration must be sustained), as well as allowing you to fine-tune your effects. By using appropriate durations and effects, you can have your lobotomy, the classic amnesia victim, the person cursed with the loss of one's specific faculty, or the person reduced to the state of a newborn infant (from which he can grow again). Do you really want to lose all that?
IMO, you'd just lessen your options if you just made that guideline "permanently reduce someone to the status of a drooling idiot".
Note also that you can very well have all mental stats at -5 and still learn (and gain XP) normally. You'll just suck at what you do, whereas a little-trained (skill 1) genius (Int +5) will beat you any time of the day.
Statistics have nothing to do with this or this spell, it's just your "natural aptitude" for something, just like some people, given the same training, will be better than others at soccer, driving or maths.
Once again, and for the record: I have come to agree that the RAW does not support a Pe/Me spell that destroys the mind of its target forever. I just find that completely backwards from a balance, paradigm and consistency standpoint.
I personally don’t see how a D:mom spell is going to put anybody who is standing upright, in what may or may not be a noisy environment, to sleep. As soon as they hit the floor they would wake up. The spell is, as its level suggests, of limited use.
We are talking about Hermetic Magic, not borderline exploits.
I can picture a “spirit” that comes from a soul and manifests in a body that is incapable of reasoning, yes. And that’s what I’m talking about. Mentem spells, IMO, target the spirit. In the medieval paradigm that I have come to understand, man is split into three parts: His body, his mind and his soul. Only his soul is beyond the reach of magic, the rest is fair game. No matter how god-given mans physical form or spirit is, it is still a part of the mortal realm, and can be messed with by daemons and magic.
Further, brain damage to the point of permanent coma or different levels of vegetative states, were well known in the Middle Ages. Whether they recognized it as damage to the brain or thought it was a failure of the spirit, makes no difference here. Both can be targeted by magic; one with Corpus and the other with Mentem.
The problem here is one of consistency. We seem to all agree that Mentem can mess with the mind/spirit of the subject. The thing we disagree on is if it should follow Imagonem or Corpus guidelines for duration. I tend towards Corpus while the RAW seem to pull towards Imagonem. For example:
Pe/Co – Can destroy a natural feature of the body like the eyes and they will not grow back or return.
Cr/Co – Can improve the body with a D:mom ritual.
Mu/Co – Can change a feature of the body for the duration, and never permanently. It goes against nature to change a man to a toad, so you need a duration.
Pe/Im – Are never permanent.
Cr/Im – Are never permanent.
Mu/Im – Are never permanent.
Pe/Me – Are only permanent when attacking a specific kind of memory. All other destruction is limited by duration.
Cr/Me – Can improve the mind with a D:mom ritual.
Mu/Me - Can change a feature of the mind for the duration, and never permanently. It goes against nature to change a man’s mind to that of a toad, so you need a duration.
Mentem then, seems to follow the same guidelines as that of Corpus for all other techniques than Perdo. And that’s what annoys me. I half expect someone to tell me Ghosts destroyed with Lay to Rest the Haunting Spirit, come back to unlife on the next turn because the duration is “Mom”. And something tells me that, if the spell hadn’t been in the core book, some of you would have reasoned as such against that spell as home brew. :-p
And finally, to readdress the question of “unintelligent intelligence”: There are people with learning disabilities so severe that they will never function above the mental level of a five year old, and I wouldn’t call them “unnatural”. Aren’t we just splitting hairs here though? The question is if the Pe/Me can permanently damage its targets ability to function to a point where it can never recover in a meaningful way.
Giving guidelines for splitting the mind into target areas the same way we al know the body can be split into target areas is nice. But it has nothing to do with the debate over whether or not the damage to them should be a kin to destroying eyes and limbs, or removing the smell from a freshly cooked pie.
No. Just no. Saying that High Level Perdo Mentem CAN destroy a targets mind forever does not reduce my option as a SG. It gives me one more option. Low level spells can disrupt the mind. That option is still there. I’m not saying that I’ll house rule in mentem-lobotomy in at 25th, I’m saying it should be an option at SOME level.
That’s why I added “or worse”. At some point a spirit is so destroyed it is incapable of doing more than breathing and swallowing, and that’s what I am aming for.
Anyway, thanks for answering my ramblings.
-A
I think your PeCo description is partial. For instance:
If you see the 5 wits as limbs, you may decide that the PeMe Base 25 Guideline permanently servers them. I am not sure this fits what was believed in the Middle Ages, but who cares. It all comes to how physical or how innate those wits are for your Saga.
The point is that there are NO permanent Im effects at all, but there are permanent Pe/Co effects. That's why comparing Pe/Me to Pe/Im is, IMO; wrong. Pe/Co, Pe/Im and Pe/Me all have temporary effects, nobody is denying that. The reason why I like to compare Me to Co is that the Mu and Cr sides of the forms are so similar. The kind of things that can be Cr permanently with D:mom rituals are the same that can destroyed by D:mom Pe spells.
But, you are correct; it does come down to how innate Wits et al are in my campaign. And in my campaign they are a part of the spirit that animates the body on behalf of the soul, and that spirit is just as real as the body and the soul. Unlike the soul however, the spirit (like the body) is not protected by God.
For what it's worth, I disagree with the view that under RAW Hermetic magic cannot permanently destroy a mind. On the one hand, it can turn a mind into a bird, and kill that bird (check out the MuMe guidelines). On the other hand, the examples in the core book and in A&A only prove that you cannot have a mind "naturally" without reason, emotion etc.; they do not prove that you cannot destroy a mind in full-- again, just like Hermetic magic cannot permanently destroy a body's weight, but it can permanently destroy a whole body.
I think that there are too many examples of this in genre to ignore. Even ignoring the "until" duration, taking away someone's mind - or most of it - is too good to use. And the search for a magical cure likewise - the ability to restore a mage to his "mage-dom" is also an in genre classic.
Whether or not you are left with a literal empty shell, or just a self-supporting blank slate depends on whether you think that Mentem can destroy "the mind" itself or just "all memory" (pretty clearly the latter, at least) - how you deal with "magical cures" is clearly outside the RAW.
Another interesting thing - in the main rulebook, there are no examples of CrMe spells or guidelines to heal a mind, only a vestigial mention in the guideline box.
How about diseases affecting the mind? Is that in paradigm? Momentary PeCo can inflict leprosy...
Seconded