Ars Magica

[virgil]

I have come to the decision to begin running an Ars Magica campaign in the beginning of November, and so I've been delving through the materials. I'm still likely to add a definition to Muto saying that it only adds gross physical characteristics, using official spells as guidelines & a requisite Form if the added trait is strongly associated with it (granting fire breath is MuCo(Ig)).

One annoying thing is that the equivalent of a beginner's covenant is unavailable and explicitly not going to be made by Atlas Games, apparently deciding that attracting new players is just not a priority for them. An equivalent to PACKs for grogs would've been highly appreciated; while I am aware they did it for the Grog supplement, I'm unsure of its efficacy.

For a game that is decidedly not rules light, it is certainly smaller than any other contemporary RPG core book. I mean, it's got a smaller pagecount than *-World and probably comparable wordcount, which is a testament to its ability to be concise and still give more plot hooks than most of them. It could stand better editing, because even people on the ArM forum have been surprised to learn that self-buffs anchored to familiars don't cause Warp.

[Longes]

http://www.redcap.org/page/Welcome_to_A ... t_Covenant

[virgil]

Thank you for the link, since others will likely appreciate it. I've found similar elsewhere as well. This doesn't invalidate the original complaint being levied at Atlas Games, because introductory material shouldn't require half-decade old books (at the time of 5th's release, at least) from an older edition. I mean, not even the book on covenants gives good guidelines/samples for a complete covenant, which is downright embarrassing.

[Username17]

From what I've seen, delving down the rabbit hole and finding out how many ostlers and grooms a stable sufficient for the needs of the troupe is one of the draws of Ars Magica. The rules themselves are little more than springboards for people to do amateur research into medieval economics, philosophy, and language. People don't really turn to the book when they have a question, they turn to each other and to accounts of period monastery ledgers.

This is why you should be totally unsurprised that long time hands of the game appear to not know rules or authoritatively answer questions incorrectly. Actually getting the game from the rulebook is not high on the priority list of ardent fans of the series. From edition to edition, the stuff has gotten more esoteric, not less.

-Username17

[virgil]

This is why you should be totally unsurprised that long time hands of the game appear to not know rules or authoritatively answer questions incorrectly. Actually getting the game from the rulebook is not high on the priority list of ardent fans of the series. From edition to edition, the stuff has gotten more esoteric, not less.

Well, it's unlikely that I have the skill or knowledge to make an Alternate Ars Magica, so ArM5 is what I shall use.

I shall endeavor to record my personal houserules/kludges at streamlining actual play during this campaign. Maybe down the line I or others on this forum can find it of use in fixing Ars Magica. I know I'm drawn to the multi-character group play, the core concept of the XP system, and the seeming universality of spell/item design that actually encourages the kind of game described.

[Longes]

One great thing I like in Ars Magica is its Flaw philosophy. The flaws (at least most of them) are the plot movers, rather than the gameable penalties to stuff. Penalties are boring. Like Shadowrun's "Sensitive System". No street sam will ever take it, because Essence is so precious. Every mage ever will take it, because mages don't use chrome. However flaws like "Fairy freind" and "Curse of venus" are awesome. They give plot hooks. They add to the character, rather than restrict him.

[virgil]

One great thing I like in Ars Magica is its Flaw philosophy. The flaws (at least most of them) are the plot movers, rather than the gameable penalties to stuff. Penalties are boring. Like Shadowrun's "Sensitive System". No street sam will ever take it, because Essence is so precious. Every mage ever will take it, because mages don't use chrome. However flaws like "Fairy freind" and "Curse of venus" are awesome. They give plot hooks. They add to the character, rather than restrict him.

If it's inconsistently applied, and it is, I doubt you can call it a philosophy.

[mean_liar]

One annoying thing is that the equivalent of a beginner's covenant is unavailable and explicitly not going to be made by Atlas Games, apparently deciding that attracting new players is just not a priority for them. An equivalent to PACKs for grogs would've been highly appreciated; while I am aware they did it for the Grog supplement, I'm unsure of its efficacy.

I'm pretty certain that the Covenants 5e sourcebook includes a budget for beginning Covenants. Still, like most things in the game, it needs to be discussed communally, as support for an invisible castle on a floating island in the Mediterranean is a different feel from a tower in a swamp next to a city.

EDIT - the Story Flaws are explicitly that, and they're cool.

[Longes]

One great thing I like in Ars Magica is its Flaw philosophy. The flaws (at least most of them) are the plot movers, rather than the gameable penalties to stuff. Penalties are boring. Like Shadowrun's "Sensitive System". No street sam will ever take it, because Essence is so precious. Every mage ever will take it, because mages don't use chrome. However flaws like "Fairy freind" and "Curse of venus" are awesome. They give plot hooks. They add to the character, rather than restrict him.

If it's inconsistently applied, and it is, I doubt you can call it a philosophy.

Weeeeell, they got about half the flaws right. That's more than I expect from people who made oWoD.

[Heaven's Thunder Hammer]

The supplement "Covenants" has some premade concepts in it you may find helpful, as well as a nice expansion to the boon and hook list.

I think on atlas games website there is some stuff at the bottom. http://www.atlas-games.com/arm5/

[virgil]

I'm pretty certain that the Covenants 5e sourcebook includes a budget for beginning Covenants. Still, like most things in the game, it needs to be discussed communally, as support for an invisible castle on a floating island in the Mediterranean is a different feel from a tower in a swamp next to a city.

Point budgets are terrible at giving you a baseline, as described in Ars Magica. An 800BP covenant may sound like it's typical, but I'm certain you aren't going to call it typical if it has a library with a L15/Q16 summae for each Art, a queen of vis for each Art, and 300 pounds of silver; or a covenant without library or vis, just 50 level 40 enchanted items.

"Discuss it with your group" is a damn cop-out when designing a game.

[mean_liar]

I don't know; it isn't like there's a starting setup expected in ArM as much as there is in, say, DnD. Besides, the initial covenant status is going to determine a large arc of the game. A starting covenant with a shitload of summae is a different game than one with none but a shitload of items.

More importantly, the covenant sets up a lot of story arcs. Not enough vis to go around? Shitty labs? Not enough labs to go around? Lots of books? No books? It's as important as the characters in the game and should be approached like that, and you're basically asking here "what's a starting character?", and there's no easy answer to that question.

So, with that in mind and in an attempt to answer your question rather than just collectively bitch that Covenants doesn't do an awesome job of doing this for you (which it doesn't, and you're right that collectively deciding things in a game where the players don't know the game is a bit of blind-leading-the-blind)...

There's two streams. One is the mechanics, and the other is the story.

Why are the magi there? Are they in tune with its awesomeness? Are they better than the rotting place they're in? Worse than their majestic abode?

How many magi?

How much should they be struggling for vis sources? Did any of them have generous masters which gave them vis (or ungenerous politicos who gifted them vis in exchange for a later favor)?

Did they leave their apprenticeships with summae or lab texts? Should the character's advancement in their Arts be dependent on reading, or hustling up copies of summae/burning vis?

What makes sense for the starting dispensation of labs? Do all the magi have labs, and are they crappy, okay, or awesome?

Should they have enchanted items in stock? Why? Are they useful tools or artifacts they just ended up acquiring in the past?

Do they have staff? How many? Why are the staff there?

...

In general terms, if you want a paint-by-numbers scenario, then take a look at the base covenant resources sidebar on p72 of the main book. The Covenants book might be drinking from the firehose, I'm not sure what kind of experience you have with the system.

[Heaven's Thunder Hammer]

I'm pretty certain that the Covenants 5e sourcebook includes a budget for beginning Covenants. Still, like most things in the game, it needs to be discussed communally, as support for an invisible castle on a floating island in the Mediterranean is a different feel from a tower in a swamp next to a city.

Point budgets are terrible at giving you a baseline, as described in Ars Magica. An 800BP covenant may sound like it's typical, but I'm certain you aren't going to call it typical if it has a library with a L15/Q16 summae for each Art, a queen of vis for each Art, and 300 pounds of silver; or a covenant without library or vis, just 50 level 40 enchanted items.

"Discuss it with your group" is a damn cop-out when designing a game.

In 4E Atlas tried stating out covenants in their 4E tribunal books and the reception was pretty poor - for the same covenant, some people would say it was under powered, and other people argue it was over powered. Or nonsensical, etc. It just created endless griping, because Atlas can't please anyone whenever they actually stat up the damn things. Recognizing that it really does come down to the group, that's why they say "talk it over". Then they devote their page count to other stuff that has a better return for the value.

See http://forum.atlas-games.com/viewtopic.php?f=4&t=30546 for the discussion of this in 5E.

Re point totals, I never bothered with them. I just put down what I wanted the PCs to have access to for the stories I wanted to tell.

I agree though, they could give written up examples of "this is what a low power game covenant would look like, that wants to tell stories about x, y and z", medium, high, super high etc.

There are GMs like me, who feels that in general, PCs should be getting MINIMUM 15xp per season, preferably 30, and I've played online PbP games where GMs felt more than 5xp per season, on average, was too high. I give minimum 10xp per adventure, up to 25 in a big one. The core book recommends much lower values.

The bigger issue with the game, is no one can actually figure out what level and quality of books "should" exist.

How many people actually copy spells for trading? The game, by default, lets any scribe with a magic theory of 3 or higher copy any hermetic book at all. So - is the order awash with high quality texts, and a near infinite supply of lab texts? Or do they hoard things avariciously? Do covenants have piles of enchanted items lying around? Depends on the vis availability or the historical amounts of vis, neither of which are canonically defined. No one knows (or can agree with each other)!

It's up to the GM to decide what the hermetic culture is, the availability of resources, and that will help define whether you have low or high powered games.

This has a big effect on the verisimilitude of the game. As written, the OoH poorly integrates in with the rest of mythic europe. You're telling me people who can spells that can kill armies and level cities with ease have never interfered in history? Oh, you say, "they have a code, and it's enforced". Basic truism of law enforcement, most of it is reactive instead of preventative.

The only way I made sense of the lack of sense of the game was to play in 780, right after the Order was formed, and let the PCs define where they interact with the hermetic history for later games. They want to give the Emperor of Rome (Byzantium) powerful magic items that let him win battles? Sure, there's hardly anyone to investigate this kind of thing anyway.

Find out a founder's daughter is an infernalist - kill her, which results in a wizard's war that kills the founder. I didn't like the portrayal of some of the founders, so got to change things to my liking.

Anyway. Frank rightly has pointed out issues with Muto and Spontaneous casting. I personally have not found them as crippling as he suggests, but my personal gripes with the game are how the rules, and the implied changes of powerful wizards don't match the setting

[virgil]

One thing I'm noticing, which I'm unsure how it compares to other stuff, is how awesome Corpus can be. Longevity Rituals are key to maintaining ultimate power. ReCo Circle wards are, if you can penetrate their Parma, absolute defenses against any spell a magus casts no matter their Penetration.

Gauging comparative power level is downright painful. There are virtues that bonus XP in differing ways and in differing values, Puissant is worth a variable amount of XP. So it's possible to be objectively better in every way with the same number of points, but outside of 1-to-1 comparisons, the lack of benchmarks makes power comparisons difficult.

[mean_liar]

Gauging comparative power level is downright painful.

I don't really think that's a useful undertaking in Ars Magica. It's not like most games where all PCs are present at the same time, all the time, and expected to take part in the same story simultaneously. The fact is that magi will be busy most of the time and chasing down stories tends to be something the PC magi really wants to do just to be involved, or is an imposition on their Pretty Princess Dressup/Progress Quest action.

Most magi don't have to share the spotlight with more than one other magi and given the wide swath of capabilities there isn't much overlap, generally. Not only that, but Rocket Launcher Tag is very, very common regardless of comparative power levels unless you've managed a tricky way to jack up your Parma Magica.

The ReCo ward is a good defense, but its main problem is that you're probably warding against yourself as well, and won't be able to act or move across that boundary either.

[virgil]

Warded things cannot act across
the circle, no matter which side they are on, nor can they damage the circle, directly or indirectly.

And so begins the attempt to understand what this means. This is actually something that is equally questionable in D&D, because I'm not sure what the definition of indirect covers in this context.

Is asking a friend to break the circle permitted, or even tricking? What about pushing over a stone pillar to break the circle, and if not, how about pushing over the pillar so that it in turn knocks over something that breaks the circle? Is inaction permitted, such as stepping out of the way of a falling pillar so it lands uninterrupted onto the circle? What if the creature catches the pillar, realizes that it will break the circle, and releases its grasp so it continues on its the original trajectory? If said creature can't jump over the circle, can it be thrown over the circle? What about setting up a see-saw system to launch itself over the circle, cartoon style; or it sets it up and asks someone else to thrown down the rock to launch it over?

[mean_liar]

Yeah, it's not well-defined. Or defined.

Personally I'd just rule that the ward is only against direct actions and first-order indirect actions and be done with it. Like, you couldn't summon a beast to attack, but you could call to someone who could summon a beast to attack.

Even that's obviously an imperfect definition, but there you go.

EDIT - for example, my definition doesn't cover the "allowing the circle to break by shrugging off the falling pillar" question you had. Or if a falling entity otherwise landing on the circle is obligated to fly away if they can.

I suppose to cover that the answer is that the ward would actually obligate you to carry the pillar until you couldn't, and falling is inaction and so you could.

But... yeah. Not really all that awesome to adjudicate, check with your table, etc.

[virgil]

One thing that is of concern is that, by the looks of it, it's easier to kill monsters than to ward them; killing them requires, essentially, a casting total of spell level + their Might. If you do it right, especially with something like multicasting on Demon's Eternal Oblivion, can be spell level 10. Wards require your casting total to be at least double their Might. For the low-end beasties, this is fine. But the bigger they are, the easier it gets to kill them, which strategically already has a strong incentive. I can see where many would be disappointed in the narrative consequences; and have seen mentions of house-rules removing the Spell Level = Might restriction on Wards (base it on warding against mundane versions) and only requiring getting past their Magic Resistance.

An abuse I'm noticing, which I'm not sure is good or bad yet, is being able to create your own vis. It's base 50 to create a magical creature, and all magical creatures have vis within their bodies once you include the Vim requisite. Increasing the target to Group, cast as a ritual for the Momentary duration, you can get yourself ten creatures with Might equal to the level of the spell, which you then harvest for the vis at a 1000% profit.

[Windjammer]

So because Ars Fan is being extremely thick, let's open up the actual page 78 definitions:

By using Muto magic a magus can grant or remove properties something cannot naturally have. Thus, Muto can give a person wings or turn her skin green, or turn a person into a wolf. The difficulty of the magic depends on the extent of the change, so that turning someone’s skin green is easy, but turning someone into a golden statue is difficult.

Muto magic cannot affect the properties that something has naturally, although it can add other properties to them to mask their effects. Thus Muto magic can neither injure nor kill someone directly, although it could render her immobile, by turning her to stone, or kill her indirectly, by turning her into a fish on dry land so that she suffocates.

Now, those of you who aren't mouth breathers will notice that that description contradicts itself repeatedly despite its brevity. Also you'll note that since it has everything to do with "properties" (undefined) and whether or not they "can be natural" (also undefined), that there is essentially no chance whatsoever of finding two storyguides who agree on what you can and cannot do with Muto. No two people agree on what is and is not a predicate, nor can you find two people who agree on what properties are and are not things that can occur.

We could get into deep philosophy of time dependent traits or traits of number to get really weird arguments ("What happens if I give myself the trait of 'only surviving member of House Anjou?'"), but it actually falls apart way earlier than that because of the whole "naturally" thing. Any trait you don't have is one which in this particular time line is one that you can't have had at this moment.

I think it's a tall order for any roleplaying game to try to address these issues, but I'd agree that, absent a proper treatment of defining those terms in a remotely satisfactory way, the system is better off avoiding them.

If, then, Ars Magica wants to stick with the approach in its 5th edition, I recommend they pay someone versed in modern Aristotelian essentialism. What you need is someone with a graduate training in analytic metaphysics who's willing to work through the following three sources (and by telling you these I'm giving the dev team a head start):

1. David Charles, Aristotle on Meaning and Essence, Oxford University Press 2000. Spells out conditions for "F is an essential property of x if and only if..." that respect Aristotelian scholarship and contemporary metaphysics.
2. Kit Fine, "Essence and Necessity". Key paper, around 1994, that helps us move beyond a purely modal or temporal understanding of essential properties.
3. Several papers, another one by Fine, in this collection: Toumas E. Tahko (ed.), Contemporary Aristotelian Metaphysics (Oxford University Press 2012). Those pressed for time and money can read a servicable review here.

If, otoh, the dev team can't do that, then I'd think they have to rethink their approach and substitute non-technical, non-philosophical terms - in short, ordinary language - in place of the philosophical terms Frank singles out. Either that, or 'fess up that the book is designed to provide an outlet for sophomoric wanderings in Thomist speculation, just as WoD was an exercise in goth-story-wankery.

[Username17]

It would be interesting to go all-in on Aristotelian philosophy and have magic that interacts with Essential Traits and forms and stuff. But I think Muto is still a dead end. You're giving things traits that they can't have, right? So you're starting from a contradiction, and once you have a contradiction in your premises you can logically derive the set of all possible conclusions and their antitheses.

If you messed with traits they do have, or traits they could have, I think you could create an internally consistent logic for how the magic worked. It would probably look pretty weird, but I think it could be done. Since Muto works by adding traits to things that cannot take them, I don't think there's anything you can do to tease open the puzzle. It's turtles all the way down.

-Username17

[virgil]

It would be interesting to go all-in on Aristotelian philosophy and have magic that interacts with Essential Traits and forms and stuff. But I think Muto is still a dead end. You're giving things traits that they can't have, right? So you're starting from a contradiction, and once you have a contradiction in your premises you can logically derive the set of all possible conclusions and their antitheses.

If you messed with traits they do have, or traits they could have, I think you could create an internally consistent logic for how the magic worked. It would probably look pretty weird, but I think it could be done. Since Muto works by adding traits to things that cannot take them, I don't think there's anything you can do to tease open the puzzle. It's turtles all the way down.

Do you have advice for handling Muto in Ars Magica? Does it differ from how I've most recently described handling it?

[virgil]

It would be interesting to go all-in on Aristotelian philosophy and have magic that interacts with Essential Traits and forms and stuff. But I think Muto is still a dead end. You're giving things traits that they can't have, right? So you're starting from a contradiction, and once you have a contradiction in your premises you can logically derive the set of all possible conclusions and their antitheses.

If you messed with traits they do have, or traits they could have, I think you could create an internally consistent logic for how the magic worked. It would probably look pretty weird, but I think it could be done. Since Muto works by adding traits to things that cannot take them, I don't think there's anything you can do to tease open the puzzle. It's turtles all the way down.

Do you have advice for handling Muto in Ars Magica? Does it differ from how I've most recently described handling it?

[fectin]

It would be interesting to go all-in on Aristotelian philosophy and have magic that interacts with Essential Traits and forms and stuff.

How?

Would that just be designing a real system that happened to resemble Aristotle's descriptions, or are you thinking of a way to bulk import his descriptions?

[Heaven's Thunder Hammer]

One thing that is of concern is that, by the looks of it, it's easier to kill monsters than to ward them; killing them requires, essentially, a casting total of spell level + their Might. If you do it right, especially with something like multicasting on Demon's Eternal Oblivion, can be spell level 10. Wards require your casting total to be at least double their Might. For the low-end beasties, this is fine. But the bigger they are, the easier it gets to kill them, which strategically already has a strong incentive. I can see where many would be disappointed in the narrative consequences; and have seen mentions of house-rules removing the Spell Level = Might restriction on Wards (base it on warding against mundane versions) and only requiring getting past their Magic Resistance.

If you search the Atlas Games Ars Magica board, you will have that people have two fixes to this problem:

1.) Get rid of "X's eternal oblivion" line of spells. It's an artifact/problem point - magi need (or least, their is a perceived need by game designers) that Magi need some way of getting rid of demons, ghosts and the like. This isn't so bad, but then it becomes weird when applied to more general magical creatures.

2.) One rule I've instituted for mastery - is that copies can only be made for each level of mastery explicitly purchased. I.e. someone with "Mastery - Pilum of Fire 4" with Penetration x2, Multi-Cast and Subtle, can only cast a second copy of the spell, not 1+4=5.

An abuse I'm noticing, which I'm not sure is good or bad yet, is being able to create your own vis. It's base 50 to create a magical creature, and all magical creatures have vis within their bodies once you include the Vim requisite. Increasing the target to Group, cast as a ritual for the Momentary duration, you can get yourself ten creatures with Might equal to the level of the spell, which you then harvest for the vis at a 1000% profit.

My understanding is that these creatures always require a ritual to create, and do not have more vis than the vis put into the ritual spell in the first place. At group level, yeah... That part should have been clarified a lot better in the core. As GM I'd just flatout disallow any shenanigans like that and be upfront with my players. Rituals can be pretty potent once players get more powerful and can use that 10^n multiplier effect.

It would be interesting to go all-in on Aristotelian philosophy and have magic that interacts with Essential Traits and forms and stuff. But I think Muto is still a dead end. You're giving things traits that they can't have, right? So you're starting from a contradiction, and once you have a contradiction in your premises you can logically derive the set of all possible conclusions and their antitheses.

If you messed with traits they do have, or traits they could have, I think you could create an internally consistent logic for how the magic worked. It would probably look pretty weird, but I think it could be done. Since Muto works by adding traits to things that cannot take them, I don't think there's anything you can do to tease open the puzzle. It's turtles all the way down.

Do you have advice for handling Muto in Ars Magica? Does it differ from how I've most recently described handling it?

Honestly, in my games, my PC's rarely use Muto for much of anything other than self buff spells using MuCo, or turn into a cat or something. Frank has pointed out that you can get into stupid arguments easily, but it's just something i've never experienced. I'd say play it straight with "normal words" - and be willing to set limits with your players so they know what they can do with it, should they even be interested in the first place.

[Windjammer]

It would be interesting to go all-in on Aristotelian philosophy and have magic that interacts with Essential Traits and forms and stuff. But I think Muto is still a dead end. You're giving things traits that they can't have, right? So you're starting from a contradiction, and once you have a contradiction in your premises you can logically derive the set of all possible conclusions and their antitheses.

If you messed with traits they do have, or traits they could have, I think you could create an internally consistent logic for how the magic worked. It would probably look pretty weird, but I think it could be done. Since Muto works by adding traits to things that cannot take them, I don't think there's anything you can do to tease open the puzzle. It's turtles all the way down.

-Username17

The confusion here results from the different modal strength one can assign to "can", "cannot", and "could", and these are precisely studied by modern Aristotelian essentialists. Since this is not my area of expertise, I can only give a very imperfect summary of their findings, but this is roughly how it works.

Take a green apple. It has the colour it has. But it might have been differently coloured, or colour-propertied. So there is a range of colour properties it may have, but which it (currently) does not have. This is a very weak modal "can". Not least because, at this point, we only assume that a certain range of colour properties is entirely natural for an apple (or, to be more precise, for the apple species of this specimen).

Next we ask what the range of natural colour properties of things are. It's perfectly possible for my shin to have a different colour it currently has - for instance, if you kick it, and it turns black (purple, blue, whatever). That too is a possible property for my shin to have, but is not a natural one. How can we tell? Because, given time, my shin is going to resume its natural colour. The hue, post kicking, is not a natural hue because the organism of which the shin is a part of will not maintain that colour property. So being able to have a property and having that property naturally is not the same.

Having the property essentially is not identical to either, if we demand that essential properties enjoy very strong modal force - such that (as Aristotle had it) the thing could not possibly lose that property.

So we have temporal or diachronic indicators for which properties of a thing are natural for it to have - those it will sustain having over a given time after some or other causal external impact.

The more interesting cases of property change that really mess with things´natures, then, have to impact them on a more profound level. For one, property changes that are a permanent alteration, which the organism cannot revert. For instance, it would be quite radical magic if the pigmentation of my skin (in the shin area) were to change permanently. That´s a causal intervention at a level of magnitude and depth well beyond those little everyday changes.

What´s even more radical is if you combine such radical, permanent changes with altering things to have properties that they or anything else like them don't ever have (had thus far). This again comes in degrees. One, you alter the skin colour of a black person to that of a white one or vice versa. Alter other properties we associate with ethnic identity. (One of the Brosnan Bond movies illustrates this rather drastically with one of its villains.) More radical: endow the organism with a property that nothing in its species range has. Yet more radical: endow the organism with a property that nothing in the natural world has. An example of the latter would be to permanently alter the colour complexion of a human with the colour that Lovecraft describes in "The Colour out of Space". This is a radical intervention in the organism´s own nature, and in the natural world more widely. In philosophical parlance, such an intervention would require a momentary or lasting suspension of nomological localized necessity - that is, property statements about which features things can and do have owing to the laws of nature, that is, the nature of this world. The final, and most radical step of all, is to endow things with properties they cannot have on pain of self-contradiction. It is literally impossible, in the strongest sense of "cannot", for a thing to be red and green all blue over. No amount of bending the laws of nature - global laws or those pertaining to individual organisms or species - can implement a change to that effect that results in a thing now having the property of being red and green all over.

How to bring this in game terms? That is easy. What you have to codify is the degree of severity to which an alteration in a target thing's property upsets the laws of nature, and the impact range. Do you require to alter just one organism's nature? Would you change the laws of nature more profoundly? For more than one species? And so on. Observe how Ars Magica appears to only think of a target in isolation, so the impact range appears to be limited by its spell formating. But it would make sense to still codify the power of the spell by the degree of alteration of natural laws.

The irony I see is that such codification would considerably alter the magic system, since it changes it from a wobbly 'spontaneous spell system' where players come up with whatever to a system that at least proscribes parameters of change - say, the range of possibilities of transmutation available to a PC at a given time and power level. And that, of course, is D&D 3rd edition, with its very precise (if confusingly written) polymorph system. That system explains that the degree to which a caster can alter his own nature or that of anyone else will progress along very specific parameters depending on the power level. In the beginning you can go barely beyond altering features of appearance. As you grow in power, you can absorb ever more potent features of other creatures, and features of ever more potent creatures. That is neat.

What would be genuinely interesting is to come up with a magic system dabbling in alteration and transfiguration that would operate between these two extremes - between Ars Magica's utterly open natured and hence unworkable basis, and 3rd edition's extremely codified way of proscribing possibilities. Both give you a range of what features things can and cannot have, both avoid contradiction, but neither is particularly amenable to accomodate players wanting to creatively engage in a make-believe world and altering things in it in novel ways.