As most people know here the Parma Magica was originally a Rego Vim spell. Does anyone know why this was turned into a skill instead of Tech Form? Having it as a spell makes you pursuit your Rego and Vim forms instead of an otherwise pointless stat. To me it makes sense that a magus with a high parma magica knows something more about magic than just Parma Magica. Has anyone tried to use the 1st edition rules and what is good/bad about it? Why was this rule abandoned in the later editions? Too powerful? I think not as high ReVi-scores are not what you see on blaster magi. It should be possible to get about the same stat score with some modifications.
I was not part of the group that made this decision.
My guess is that the thought was that one should be able to have a character who is good at magic resistance without them needing to be good at rego and or vim. Nearly every character will want to have some competence at parma, it would force too much uniformity among characters to require nearly every character to have some competence at rego vim.
It's not like rego vim doesn't still have some use, you have spell containers, tunnel spells, magical wards, controlling spirits, and the Aegis of the Hearth. Actually in (serf's parma) second, third, and fourth editions you could still make a parma magica spell using rego vim.
The logic is that it is not tied to a specific art but crosses all the arts. If you leave it as art, then everyone is spending XP into Rego and Vim to have magic resistance and that is also wards against magical/demons and spells to move magic around and shift it from one target to another and so on. This change is so that characters that aren't rego focused or vim focused can still have magical protections of the order. This also means someone deficient in rego or Vim or incpatible arts Rego/Vim or similiar can have a parma and actually talk and discuss wtih others of the order without gifts bothering them.
The thing you consider a pro, many people consider a con. I personally disagree with the idea that the Parma should be based on particular Arts since it gives such a gigantic benefit to practitioners of one particular TeFo combination. ReVi is already one of the better TeFo combinations to specialize in, so it doesn't need any more boosts... But more importantly, having Parma be based on a skill means magi of all stripes can focus on it to the degree they please, which makes it more effective at putting magi on an equal footing and thus ensuring that the Code works properly.
Or to simplify, having the Parma as a ReVi spell rather than a skill means that the Order will no longer make sense in some fashion. Either the Parma couldn't have helped the Order form at all, or the Order would be advanced way more heavily in ReVi than anything else and the guidelines would get skewed.
Oh, and as an added bonus, having it as a Gifted-only skill rather than a Hermetic magus-only spell opens up stories about enemies of the Order stealing the secret of Parma Magica, which would be entirely a non-threat if it were a Hermetic spell.
If you're looking for an in-universe justification, keep in mind that the Parma Magica wasn't developed within the frame of Hermetic magic in the first place; Bonisagus developed it before the Founders were gathered, so it's not even a part of the overall magical system. And all of the above things about how the Order would've developed differently with a Parma spell are things the political maga Trianoma would've been able to figure out, so she could've taken intentional steps to prevent the Parma from being made a spell precisely so non-ReVi specialists could survive and increase the Order's magical productivity.
Thank you for your replies!
I think it makes totally sense that if you are good with PM you should be good with Aegis of the Hearth and similar spells as well. The other option is just gamey... I have never seen a power gamer specialize on Rego Vim. They go Ignem or Auram. Making them specialize in Vim as well will nerf them. I see no problem with that magi who specialize in ReVi actually have better PM than the rest. If PM is a skill why is Aegis of the Hearth and Ward against Demons not a skill?
Why would ReVi PM not make the Order special? It is the only Order I know that actually uses five forms and ten techniques. Other magic users has these stats for game-function only and would not use these terms "in-universe". What is the problem with having an Order that has high ReVi scores?
Gifted only skill rather than Hermetic Magus spell? In that case why did all the magi adopt the Hermetic Tradition rather than keeping their hedge traditions and just adding PM-skill. I have not seen this and to me this was the main reason other magi abandoned their original ways.
Again, I see no point in not going back to the ArM1 rules. To me the only question is how to make the spell work just as the skill stat wise. I.e the same amount of xp/training should result in the same Parma Magica-effect.
Saying that Ignem or Auram are the power gamer forms suggests that you only see power gaming in terms of combat, but combat isn't the main focus of most Ars Magica sagas as far as I'm aware. (And for that matter, a combat magus being better at combat than any mundane could hope to be still doesn't qualify as power gaming, since magi are meant to be a lot better than most other character types.) Rego Vim already covers the Aegis, while also covering the controlling and summoning of supernatural creatures, supernatural wards, the control and movement of magical forces (in particular, transferring vis between containers), and various magical tricks that don't fall under Muto (spell conduits and containers, spell suppression, etc.). Despite the limited number of guidelines, especially in the core rules, ReVi's individual spells are typically so broadly useful that ReVi ends up one of the most versatile Art combinations.
And for what it's worth, Vim is a pretty darn useful Art purely for the benefits to lab activities even if you limit your foray into Vim spells themselves, so they might not get as nerfed as you expect.
The problem with an Order that makes ReVi scores such an integral part of its competition between magi is that magi aren't going to focus much on the other Arts. A whole lot of neat magical inventions from the other Arts would probably never be pursued, and players would be disincentivized from pursuing them as well since they'd probably need to focus a lot more time on building ReVi than focusing on whatever they actually want to focus on. Unless, of course, you do what you say and make it so that people can get to about their normal desired levels of Parma with the same effort, but then you run into the issue where ReVi specialists become basically untouchable, given the scaling that would have to happen.
They didn't keep to their hedge traditions after the Parma was given to them because the sorts of people who would do that typically weren't approached by Trianoma, and those who were likely realized that becoming part of the Order would be better for their groups than trying to oppose it. The Order was joined by many groups largely because of its potential as a concept... Y'know, the whole "unified magical theory" thing.
Also, addressing your main gut argument of "it makes sense that knowing more about this would make this easier," that isn't really true in the world of Ars Magica because it's a matter of doing the same thing differently. The Art of Fire (Hedge Magic Revised Edition) can't be combined with any Hermetic Techniques and doesn't make Ignem use easier precisely because, even though both traditions create fire, they do so via completely different methodology. Parma Magica is effectively like hedge magic that all the Order can access but which isn't integrated.
(Also, it's worth pointing out that there's another major benefit to non-spell Parma: longer Hermetic lifespans. Using the Parma Magica as a spell that most magi would keep up constantly would incur Warping.)
The game offers ways for you to integrate the Parma into Hermetic Magic as a Hermetic Breakthrough. If you want to pursue it in your saga, you can feel free. I, however, think the system is much more balanced, story-prone, and character-inclusive under the current rules, so I'm glad they're the default.
Wards are spells that cross all sorts of forms but more importantly, many of the wards including ward against demons actually came from integrating Ex. Misc Circle magic into hermetic theory.
Aegis of the hearth was a breakthrough on ward magic + ReVi combined. It is immobile ward though with some special elements.
Parma's special isn't just magic resistance, it also blocks the negatives of the Gift for anyone that uses it to allow order to actually deal with each other. This makes it a special tied into the nature of the gift itself, not magic theory.
As for power gaming, the NASTIEST and most vicious character that I ever made was a Cr/Re Vi character. Open intangible tunnel and throw in spells to create warping. Then move, erase or change (muto to make it like someone else) the magical traces. The perfect crime, destroy them with twilight.
fun detail:
Making Parma Magica into an ability, rather than a spell, actually made it easier to steal for non-Hermetics.
Anyone with the Gift can learn Parma Magica (the ability) and use it as is, no changes needed,
If it was a spell, significant research would be needed to integrate in into other traditions.
Thus this change would block a certain (fairly narrow/specific) group of stories.
I think you are defending a rule out of game conservative reason. If I would have purposed that PM should be a skill instead I think you would have defended PM based on ReVi even more 
I do not think that PM should be a spell. It should be based on ReVi because it IS ReVi (as given by the rules for other types of magical defence). If you are good at PM you should be good at AotH and CWaD as well.
I do not have a problem with magi with high ReVi scores at all. It is fun isn't it? If ReVi makes you nasty – well go have fun with it. Getting a high PM score as a skill will steal time from other studies just as much as pursuing high art forms. Just that a skill in PM is boring compared with
Why would it be easier for hedgies to steal a ReVi ritual rather than a Skill-based parma? How hard is is to say *no" in the rules?
I, in turn, think you already know what you want to do in your game. I encourage you to try it. You won't find the confirmation that you seek here, but in the spirit of House Tytalus, you must exercise your own true nature.
As others here have said, moving Parma into a skill allows magi to pick specialized Arts other than ReVi. My argument for a Parma skill has nothing to do with in-world logic or setting -- which I find quite malleable -- and everything to do with encouraging diverse player characters. When we, as game designers, make one Art required for a successful magus, that means every magus has to learn it and PC magi all start to look the same. You encourage an arms race of Vim, as characters become defined by their Parma in relation to other PCs and NPCs. The player who wants to make, say, an elementalist is now screwed because in addition to his four elements, he now also has to learn Vim. And this is true of every magus concept. They now must also learn Vim, will be less good at everything else, unless they specialize in Vim, and soon your Vim specialist is running the table and all non-Vim specialists are losers.
(Editorial note: Longevity Rituals are a good example of exactly this. Once, you had to make your own ritual. As a result, every magus had to learn Corpus. Player character concepts suffered, and everyone began to look the same. Corpus specialists won twice: they could dump all their XP into Corpus, have plenty of cool magic and also get to play their characters more than all those losers who were dying first. Now anyone can make your Ritual, so other concepts are free to flourish and specialists in CrCo have a reliable income stream.)
You can disregard these defenses of the current system. But they are not invalid. That doesn't mean you have to keep the system the way it is. Others have noted here that, by your mention of Auram and Ignem as the Arts most popular among your players, your table is mostly made up of fighting magi who want to throw fireballs and lightning bolts. Your rule change will force these players to diversify their Arts, and this may be a net positive for you. Indeed, you may see no down side to it. But there are other ways to encourage diversity, if that is what you really want. And perhaps we should ask ourselves: if our players are having fun with Auram and Ignem, why is it important that they diversify at all? Who does this rule change truly serve? The players or the GM?
It's really too easy to steal, under current rules. It's very hard to accept that the secret hasn't leaked from the order in 450 years.
I agree Parma is better as a skill than as ReVi, for the reasons people cite about avoiding the need to be Vim specialists. I do kind of miss the way in which Parma blocked emotions in one of the early editions, angsty as it was.
Penetration and MR have always been a core issue of Ars Magica.
In 5th edition, Parma is an Ability, and Penetration = Casting Score - Spell Level. Magical Foci and Arcane Connections permit high Penetration.
In 4th, Parma was an Ability, but so was a Magical Affinity, and Penetration = Casting Score. Magic Resistance also got a roll. Arcane Connections offered no bonus Penetration.
These differences made large changes in how things played out. I'd expect that Parma as a ReVi spell would have the same kinds of ramifications.
As a Tytalus at heart I don't look for confirmation. It would be interesting if to hear if someone else is using this original rule and how it compares arithmetically to a skill based parma.
I still think you have not have explained why AotH has not been copied/stolen by hedge wizards in 450 years. I think this was explained in the original setting... once you go hedge magic you can not turn back and become Hermetic as you are too much tainted by your own inert magic. I think you are wrong about the original intention of making PM a skill. It was to make it possible for hedgies to copy it that it was turned into a skill. The art-specific parma made no sense to wizards who did not use Art + Form in their magic. The AotH is a rudiment from ArM1. It really should be performed with the Parma-magica skill too if you actually like the idea of having PM as a skill that is.
IMS, Parma magica can't be written. It just don't work.
So you could force a magus to explain, but if you were powerful enough to force a magus to explain, you will be or are already, in trouble.
Parma Magica as written in 5e, a supernatural ability that can be learned without penalty by anyone with the Gift, isn't particularly consistent with the ruleset.
One possible change / clarification is to consider Parma to be a Supernatural Ability which was integrated into Hermetic Magic by Bonisagus. Any Hermetic Magus can learn the ability freely, as per the current rules. Other Gifted wizards would need to initiate a Mystery to begin learning the Parma. This goes some distance to explain why the ability hasn't leaked far outside the Order by now.
One difficulty with this is the evident fact that most Hermetic do not practice Mysteries and lack the lore to initiate hedge wizards. I'm not sure that's really a problem though as it's never been clear to me why the Order would be particularly interested in teaching Parma to non-Hermetics. Even the Pralixians, who explicitly want to bring hedge wizards into the Order, accomplish this by focusing on opening them to the Hermetic Arts rather than signing them up as is.
Another possibility to consider is that the Parma has probably already leaked outside the Order, as has Hermetic Magic in general. Guardians of the Forests has a renegade covenant right in the heart of Germany. It's not hard to imagine other renegades fleeing from the Schism War or other divisive events to more peripheral areas of the world and settling in comfortably.
Perhaps the Initiation into the Parma Mystery is part of the opening the arts or it is part of the gauntlet ceremony that bestows it with the 15 year apprenticeship counted as ordeal?
Wouldn't still be possible to create personal wards against magic with Rego or Perdo spells? I'm thinking of stuff like Repel the Wooden Shafts or Ward Against Heat and Flames.
What would be the difference between repelling a mundane arrow and a magic-based arrow using that sort of spell? I assume that the difference between casting totals would be the main issue here, and according to the rules for Fast Casting you can defend against magic attacks if your casting total reaches at least half of the offensive spell's total, being able to completely nullify it if you get the same total. I would assume from these guidelines that for the most part, you can target the effects of any spell as long as yours is at least half its level. Another example that wouldn't even require to compare levels at all, would be spells that change the nature of something to make it impervious to certain magics. If you turn into mist, no physical attack should affect you, for instance, and turning yourself into anything at all, should protect you against Corpus spells (Is it possible to use a spell to turn your mind into something else? I'm thinking of MuMe/An, which would transform your mind into an animal's which would protect it from Mentem based spells).
Personally, I understand that the Parma Magica is a convenient plot device to avoid characters overdeveloping their defenses, but I do prefer the idea of creativity being the best ward. Take Eragon for instance. Not my favourite series, but the one thing I loved about how magic was portrayed there, was that you may be the strongest wizard who ever lived, you may have come up with a thousand wards against a thousand ways to attack you, still, there will always be the possibility of somebody figuring out some weird way to attack you that you never saw coming.
In any case, that's not too far from the reality of this game. Magic defenses don't protect you against mundane attacks, so you will always find yourself thinking about the best "loophole" to use something mundane against a magus. The Parma Magica simply narrows the possibilities, but the fact that there's still a way to breach it with creative use of mundane resources kinda defeats its purpose to me.
From a meta-game perspective I think there is a contradiction when Parma can be torn down by a hermetic spell but a similar defence cannot be established with a hermetic spell.
I follow some of the reasons, but dislike the spells which remove Parma because there are no cannon spells which replace it. I'd prefer if the Parma stripping spells were excluded, or both were added.
Anyway,
I think there is not a contradiction. You can kill someone but you can give back real life. You can desrroy a Lot of magic you can not cast. Parma as a skill gives much more play than as a re vi effect and this, as the location of the game in mythic Europe is arbitrary, but works well. Another possibility to drop the skill of Parma is to rule that magic resistance is a total of form + technique involved in the magic attack. Or form + rego. Adding technique you will got a better defense total, lower than Parma, but you will focus more in arts.
Spells to end other spells (even as they are being cast, if you master them) do exist, however. And the Parma stripping spells are generally fairly difficult to pull off - if you can strip someone's Parma with PeVi, you can probably do something else really nasty with a different Perdo or Vim spell with the same or greater Penetration. IOW it's easier to RUIN someone with magic than it is to remove their magical defenses, at least for the majority of magi (Dispelling specialists being the major exception).