Why? So is Rego. Muto is unnatural change, while Rego is natural change.
The Art of Rego allows a maga to change the state of a thing to some other state that the individual thing can naturally have. (p.78)
Considering "change" is explicitly used in the first sentence of the core explanation of what Rego is, is it really so odd to use "change" with Rego?
The thing studied can be Hermetic.
Heh, I'm not sure I want to have a conversation over whether a change of natural state applies to magic, callen. This is not water to ice. And the Rego Vim guidelines all fall pretty squarely on typical rego stuff - warding, sustaining, suppressing, containing, moving, directing through a conduit. Regardless, your disagreement seems like a strong indicator to me that an errata to clarify whether Pralicis can do this would be useful. I wouldn't have used the word change in the focus definition if Muto Vim is impossible while using it. (Notwithstanding Comprehend Magic which, yes, can be used with hermetic stuff too)
I consider active v. inactive, for example, to be a change of a natural state for an effect. ReVi seems to agree or it wouldn't be Rego. I'm not saying everyone would have made that decision from the get-go, just that that decision was made. And since "change" is the guiding term for both Muto and Rego, there is no disagreement/contradiction showing up. Clarification could certainly be added, but I don't see it as necessary since we already know for sure MuVi won't work on those foreign magics.
Do we really need an explicit statement that MuVi spells do not have their effects until they have been cast?
I mean, we might, but that is what is being asked for here, and I need to be convinced that this case is problematic enough in practice, as well as in theory, to justify including a statement that would have to start with something like "Just like any other spell…". (Because if you don't include that, people assume that because this isn't said about other spells, it doesn't apply to other spells, and therefore other spells have their effects while being cast, and so a ritual still goes off if the ritual is interrupted.)
I agree. The principle of charity applies: assume that the authors were not ignorant of the rules unless the alternative is assuming ignorance of English. (This may be generous, because the authors, and me, did forget rules occasionally, but if there isn't a clear contradiction, you should interpret things to be consistent.)
It's not that. It's the ordering that confuses people. As I mentioned, it is necessarily implied with the MuVi errata because if you don't cast the MuVi spell first, then its Duration cannot fully encompass the casting of the other spell. However, people miss that, partly because of the whole idea of targeting something that does not yet exist. Here is what you wrote way back on the Berklist:
(what you responded to) We know it's possible to cast two spells at once thanks to the MuVi guidelines.
(your response) Actually, you don't. It doesn't say that explicitly, and the Ease Factor is "slightly" easier than the Ease Factor for casting a spell while maintaining a spell. The rules do explicitly say that you cannot cast two normal spells in a single round. Thus, MuVi formulaics and the spells they affect are cast in consecutive rounds. (MuVi first.) There's a bit of blurring of the rules going on here, to avoid making MuVi completely impossible.
You can note "consecutive" and "MuVi first" that are not noted in the errata. While the "MuVi first" part is necessarily implied, people do lose track of "consecutive" because it's "consecutive rounds" and the "rounds" throws it off in the case of fast-casting MuVi and normal-casting the spell to be affected or in the case of using Wizard's Vigil. But if the two are not consecutive, what sort of intelligence is needed by the MuVi to understand what later spell it's going to affect? Things get really messy, so consecutive should be understood.
So, do we need it? No, not really due to implications of how MuVi works and that spells are not intelligent. Do I have to present this quote from the Berklist a lot because people frequently misunderstand? Yes. That's why I say it would be good, not that it is necessary.
Is there anything in the current rules stopping one from casting a Sun duration MuVi Wizard's Boost in the morning and having it affect a spell cast in the afternoon?
It seems clear to me that this is not intended, and perhaps we need to trust on SG's and players to simply know that. But sometimes it's not about trust, but about clarity.
The absence of a target for the spell when it is cast. Exactly the same thing as stops you casting a Sun Duration BoAF and then having it go off and target someone later. We have Waiting Spell and Watching Ward for that.
This is why cooperation or fast-casting are necessary; there is a very narrow window in which there is a spell to target, but you are not too late to affect it with MuVi.
Why do they misunderstand? I've just re-read the MuVi description, and there is nothing that suggests that the spells are cast simultaneously. Is it that the section about the Concentration roll does not explicitly refer to casting one spell while maintaining another?
This is a serious question, because the best way to block the misunderstanding is to rewrite the confusing bit — but I can't see which bit is confusing. A common problem with one's own text.
Verrry quickly, my question is:
(1) Doesn't a target of a spell need to be present when I start casting that spell? After all, by the InCo thread I need to identify my daughter before targeting her, right?
(2) How does a MuVi spell target another spell, whose casting was not even begun yet?
Just writing up clearly in the rules, that a MuVi spell can target spells whose casting hasn't even begun yet goes a long way. Also writing up, that the target spell must be present at completion of the MuVi spell helps further. And hopefully these addtions don't mess up target definition in other parts of ArM5.
EDIT: May be the following question helps:
If a maga wishes to target her daughter in a disguised group of people who all look alike, can she just start casting her spell and determine her target during casting - as the daughter, who sees her casting, starts jumping up and yelling "I'm here, Mom!"? Is that your idea of cooperation during spell casting?
There is a reason in my Sagas neither me or my players ever played a MuVi specialist despite the potential of interesting magic.
I find confusing the bit of maintaining concentration for a spell with duration Mom for example.
It is nowhere else explained. And for longer duration spell, "it is not possible... to affect a spell after it has been cast".
As I am re-reading the MuVi section, I don't know in which order I should cast spell - but since MuVi needs a target, it seems the base spell should be cast first, then the MuVi spell.
Second, I cannot figure out if it takes two rounds for the mage casting both spells to perform the magic or if it is a special case where a mage can cast two spells in a round (which I don't like because it is opening a can of worms).
It is cannon that spell can be cast over two rounds (Slow caster flaw), so it should be possible for a mage to "drag" a spell casting over a few rounds. It is never done because why would a mage makes his life more difficult and risk misfiring a spell because of a bad Concentration roll. But MuVi spell on his own spell would be a case of entwining two spells together, taking two rounds: the mage start the base spell just enough for the MuVi to take have a target and take hold, cast the full MuVi spell, then complete the base spell, releasing the effect.
The issue is, it takes a lot of assumption/deduction from various rules bit for me (I like to emphasize this "Me" bits, because if I am the only one, that's my problem, not yours to fix) to understand the exact process.
Of course, such shenanigans are not needed when two mages cooperate.
It's as OneShot just said. People get confused because, if the spell-casting to be modified doesn't exist yet then you shouldn't be able to target it so they figure you cannot cast the MuVi first.
Consider what OneShot and I have said is the common problem: that MuVi always has an absence of a target. If that is to be ignored for MuVi, then the absence of a target doesn't explain why you cannot cast the modified spell a long time later. That's where your "consecutive" comment comes in, though the "rounds" part needs to be interpreted as intent rather than taken verbatim.
Careful. Cooperation or fast-casting are not necessary. You can normal-cast the MuVi in round 1 and cast the spell to be modified in round 2. That is actually what the default for doing the MuVi yourself has been for a long time, since your Berklist clarification. Fast-casting allows you to do it in one round instead of two.
As I am re-reading myself, this option opens another small can of worms:
If it is possible to stretch a spell casting over two rounds, would it be possible for a mage to slow-cast on purpose a spell, so it takes effect quickly on the second round, like a spell with Fast-Cast mastery ?
Why would you do that ? In situation where you are pretty confident what the opponent is about to do, instead of having to spont a half-baked spell, you go for a formulaic one. Example: an archer is pulling his bow, waiting the order from the officer. A mage "pre-cast" a ReHe Repel of the Wooden Shaft, but hold it until the arrow is fired, the release the spell.
Or faced with a famous Fire mage, an enemy pre-cast a Unravelling the Fabric of Ignem. By doing so, he can counter much higher Ignem spell.
Then of course, this is where the can of worm start to get "wormier": for how long can you old a pre-casted spell ? Can you maintain it with a Concentration check ? Can you maintain it with Maintaining the Demanding spell or another specific variant ?
I don't like where it is going as I am pulling on the string of a yarn that looks bigger than I initially thought.
Thus, the need for clarification. I do not believe it is a topic that should be left to the troupe to decide (contrary to requisit for Teleportation/Fast movement for example), because it touches the core mechanic of magic and spell casting.
I acknowledge, it is a rather narrow and specific point, but I have seen some interesting Vim specialist mage concepts on this forum, however, I always refrained from going there because I cannot wrap my head around on how it works.
Look at Lords of Men p.118-119.
I don't have it.
Oh. 
I'm not going to copy two pages of copyrighted material. But I can say that it goes into optional rules for holding actions and for using held actions to interrupt other actions.
That one virtue that makes your sun duration spells expire at noon and midnight I think. Reminder that people are valid targets of vim spells (p157) unless that's been errataed out.
Actually, I think I'm going to leave this all as-is, apart from the Rituals clarification. Muto Vim does not actually make sense, but as long as we do not draw attention to the place where it breaks down, we can all muddle through. A total rewrite to make it make sense is outside the scope of errata, and because fudging it for magi casting MuVi on their own spells works perfectly well, it isn't urgent.
WONTFIX
This was an issue i asked about waaaaay back, concerning the Concentration rolls and which of the spells - the MuVi or the target spell - benefited from Imperturbable magic mastery.
Our house rules/clarifications says:
Clarification of Mu Vi metamagic: The MuVi spell like 'Wizard's Intensity/Reach/Extension/Expansion/Growth is cast the round before the spell it will affect. Even D:Mom is sufficient for this, as it only needs to last until the target spell is started to cast. In the next round the target spell - to be affected by metagic - must be cast, or else the MuVi fizzles. Remember the Stm+Conc of 9+ to hold onto the MuVi spell while casting the target spell. Therefore it is Imperturbable Casting of the MuVi spell that affects the Concentration roll.
This is to clarifu that you don't cast two spells at once, and that there is a certain timing for these spells only that allow this method.
We never wrote anything regarding Fast Cast metamagic, but my feeling is I'd require Fast-casting for someone else to get the timing right to affect your spells. And I'm on the fence about using Fast -Cast MuVi to affect you own spell just as it's cast, as a reaction to it. We like fast-cast defenses to be defensive, not a way to get more spells off. "That guy is picking his nose, I cast PoF as a reaction!"
Then again, you need to spend exp on Mastery to pull this off.
I think I can safely say - or else @Tellus may correct me - that this has in fact never actually been used. I play the Pralician VIm specialist, and my interest was apparently just theoretical. Although the saga is still going on.
Actually, not I read uour entire House Rules section, we've also invented a new Mastery: Metamagic - which allows one to cast a MuVi spell at the same time you cast the target spell. So, it reauires a very specialised Mastery.
Again, don't believe it has ever been used.
Not that I can recall. Some serious ReVi, but no MuVi that I can recall.
Hey David, for what its worth, I just spent 45 minute digging through the Ars forums and the discord to try and answer this question for a player of if the special concentration roll in the MuVi rules meant the spells were special and didnt need to be fast cast to be cast in the same round, what order to cast in, etc.
even a "the standard method of casting a muvi modified spell is casting the muvi spell in one turn, holding it in your mind waiting for the next spell (thus the special concentration check), then casting the spell to be modified in the second round, with all effects occuring upon successful cast of the second spell. To cast a muvi modified spell in a single round, you must cast one of the spells regularly and fast cast the other immediately before the combined spell takes effect. MuVi is a relatively recent and novel addition to Hermetic Theory and thus operates slightly beyond standard understanding." would add SO much clarity for newer players.
It is, alas, too late to make any changes to ArMD — it's well into layout.