Magic Resistance Design
The details of Magic Resistance work differently in Fifth Edition than in previous versions of the game. Under Fifth Edition rules, it is much easier (compared to previous editions) to decide whether a given effect would or would not be subject to Magic Resistance. The Fifth Edition rules are not without their problems, though. This section explains the rationale for why the rules are what they are, and offers some suggested ways to deal with their shortcomings.
 Magic Resistance in ArM5
 Why Magic Resistance Doesn't Dispel Magic
A few of [the effects of Magic Resistance] are a bit odd. (Never mind Edge of the Razor, try Edge of the Spoon, a MuTe effect that gives weapons -1 to damage, by blunting them, and still makes them bounce off Parma.) However, all the principles I could think of had odd effects, and the odd effects of this one seemed the least serious -- the odd effects of most of the others opened large holes in Magic Resistance. You can always House Rule around them, but given the interminable debates magic resistance provoked [in Fourth Edition], I decided to go with something simple.
The fundamental problem is that RPers D&D shaped instincts make no sense. Why on earth shouldn't magical weapons be resisted? They're magical effects. A set of rules that said 'magic resistance resists whatever you think it should in order to preserve game balance' would not have been much use...
--David Chart, Berkeley Ars Magica List, 26 Nov. 2004
Magic Resistance in ArM5 does not create a kind of anti-magic zone around the magus. That approach leads leads to serious problems. If Magic Resistance dispelled magic (even temporarily), a magus could transform a huge boulder into a pebble and throw it an opponent. The spell on the boulder would stop functioning when the boulder hit his opponent, and would turn back into a boulder! As another example, a magus could take something deadly, like poison, and transform it into something harmless, like wine. Then he could give another magus a glass of wine. If Magic Resistance could dispel magical effects, then the wine would turn back into poison and kill the other magus (unless it penetrated his resistance, in which case it would remain water until the spell wore off).
In short, a model of Magic Resistance that dispels magic is deeply flawed. There would be many ways to turn it against its possessor.
 The Pink Dot Loophole
On the other hand, since Magic Resistance keeps out all kinds of magical things, a magus can abuse his own magic resistance to get protection from mundane attacks. This has come to be called the Pink Dot Loophole.
Because Magic Resistance keeps out magical things, a magus can defend himself by casting a harmless spell on his opponent's weapon. The enchanted weapon would be repelled by Magic Resistance according to the rules as written. The standard example of such a spell is a Muto Imaginem effect that makes a tiny pink dot appear on the blade.
Any general Magic Resistance system is going to run into problems like this. The question is what problems are players and storyguides willing to accept. In ArM5, the second problem ("parma blocks pink dot") was considered less serious than the first problem ("parma can kill you"). There also seems to have been a strong desire to make a system that is simple and consistent so players can understand it and use it without major problems.
 No One Likes the Pink Dot Loophole
Not everyone is happy with the implications of the rules as they are written, but then, no one has come up with an alternative system that pleases everybody, either. People are welcome to try to "fix" perceived problems in the Magic Resistance rules for their own sagas.
There is nothing wrong with playing with a vague, fudged version of what Magic Resistance stops in your sagas, if you don't like the effects of the rules as written. Actually, it's positively a good idea; this is your game, play it in a way you like. But you'll find that you do have to fudge, or people will invent the Tree of Dragon Slaying spell: MuHe a mature oak tree into an arrow, and fire it at a dragon. [If magic resistance dispells the magic, t]he mundane tree gets through, and being hit by an oak tree doing over a hundred feet per second would hurt even the toughest dragon...
--David Chart, Berkeley Ars Magica List, 27 Nov. 2004
Neither do I. On balance, though, I like the alternatives less.
 Possible Fixes for the Pink Dot Loophole
A number of possible fixes for the Pink Dot Loophole have been proposed. All of these (except the first) would be considered House Rules.
- You could simply ask everyone in the Troupe to agree not to exploit the loophole. Doing so may be intellectually unsatisfying, but it does allow you to get on with the game. Erik Tyrell's suggestion from the official Ars Magica message board.
- Change Parma Magica so it doesn't protect a magus against his own spells. There are two consequences to this. First, the Pink Dot Loophole is made harder to exploit but it is not completely eliminated: two magi working together could still make one another invulnerable to weapons. Second, a magus will not need to lower his Parma to cast beneficial spells on himself, so this could be seen as increasing the power of a magus.
- Rule that Parma Magica only protects against effects that target the magus or include the magus in the target area. Magic swords would then slide right through Parma, but so would a lot of other things. In particular, under this variant, magic resistance would no longer offer any protection against nonmagical water magically transformed into poison. It would open a lot of vulnerabilities in magic resistance, allowing it to be bypassed by clever (read, devious) players.
- Make some kind of "intelligent Parma" rule that magic resistance can distinguish between effects that are potentially harmful and those that aren't. This also makes it easy to cast spells on yourself and it also calls for a lot of rulings by the troupe on what exactly can be resisted and what can't - the very situation the Fifth Edition rules seem de
 Hedge Wizards and Magic Resistance
In ArM5, the only general-purpose Magic Resistance available to humans is Parma Magica. That means only Hermetic Magi have magic resistance. Hedge Wizards don't. David Chart explained the reasons for this:
Given that we are talking about game design, a meta-game reason is sufficient. After all, why are fermions subject to the exclusion principle, apart from the meta-game reason that it allows solid bodies to exist?
Ars Magica is a game about Hermetic magi. They are, and should remain, the focus. However, other traditions of magic are fun, and people like them. Thus, I wanted to create some space for them.
One possibility, that seems to have a number of supporters [on the BerkList ], is that Hermetic magic is wonderfully general, but that hedge traditions are more powerful in their specialities. I don't think this option is viable.
Why? Because people only play one magus-type character (in general), and very, very few people want to play pure generalists. If you want to play a particular kind of specialist, which will you pick: the Hermetic, who could , in principle, learn to do things in which you have no interest, or the hedgie, who is better at what you want to do?
Of course, if Hermetic magic is just better in every respect than every other tradition, no-one will really want to play the hedgies, either, so that's not much fun either.
The path I'm taking is this. Hermetic magic is the most powerful form of magic available to humans. It is the most flexible, and while other traditions might match it in raw power, in a limited area, nothing obviously exceeds it. However, there are things that Hermetic magic cannot do; the limits of magic. Some hedge traditions will be able to break one of these limits.
However, to be fair, there should be something that only Hermetic magic can do. That something is general magic resistance. It's probably the most useful unique power, which is right, since Hermetic magic is supposed to be the best. If you like, the 'limits of magic' are those listed in ArM5, plus 'The Limit of Magic Resistance'. Major traditions break one limit each; Hermetic magic breaks the limit of magic resistance.
Now, a specialised tradition will have defences that work against specific magic. It might even have something like Form resistance, against a limited range of magic. But if a Hermetic magus goes outside that range, the other tradition has no defence.
Not having general magic resistance is a major weakness. Giving it to all hedge traditions gives us more flexibility to make their active powers interesting without risking making them more attractive, in raw power terms, than Hermetics.
If magical and faerie creatures can grant general resistance, then traditions that work by summoning and binding, or bargaining, would logically have it. Ruling out traditions that summon and bind creatures does much greater violence to my sense of the fitness of things than disallowing the power to magical and faerie creatures. Hence, in order to allow spirit masters, Titania can't grant general magic resistance.
Pure meta-game considerations. The in-game justification is 'Magic and Faerie don't work like that'. They are, after all, realms of power with no real-world, or even, to be honest, real-myth equivalent.
--David Chart, Berkeley Ars Magica List, 10 Jan. 2005
 Legacy Page
The history of this page before August 6, 2010 is archived at Legacy:magic_resistance_design