Have any of you ever house-ruled Magic Theory into the equation for casting spontaneous spells?
It would seem quite logical to do so in my mind. Musicians are informed by music theory when improvising so it makes sense that Magi would also use theory as their guide in improvising spells.
Magic Theory helps you in the laboratory with your lab total, thereby helping you be better with formulaic spells - but never in casting, just by having better spells. I'm not sure why Magic Theory should apply to the casting of spells rather than the research of spells, but if for some reason it did, I probably wouldn't apply it to spontaneous spells alone. Although I suppose using it in a new mystery, maybe...
A&A p.11 provides Breakthroughs to add Artes Liberales to Lab Totals and formulaic spell casting totals.
Artes Liberales and Philosophy add to Ceremonial Casting of spontaneous spells.
But Magic Theory belongs to lab work, not to casting.
I don't think it's a terrible idea. Knowing what is possible with hermetic magic and how to do things with it seems appropriate for being a consideration as part of thinking about what to do in a pinch. I am unsure if I would vote for this as a House Rule in a game I play but it's definitely not something I would have an issue with. I might prefer adding something more like "Half of MT added to standard Spont Casting total (aka before division based on Fatiguing or not)" and if the group seemed set on wanting it to have some effect I might advocate for that. Either way though it's not going to be a huge bonus in what is possible with Sponting. in the +1 of +2 levels for Non-fatiguing Sponts or Up to about +5 levels for Fatiguing ones. I base this on you really need to be gunning for it if you want MT in the 15+ range and even then it's a late game thing.
[Edit to add:] Danger might come in with this as a Houserule if combined with something like Diedne Magic. With LLSM it might save a fatigue sometimes but the danger there is botching and in that case your whole casting total is 0 and you're burned no matter what so has no real effect on that possibility.
I could see MT being added to Ritual/Ceremonial casting, but for regular spell casting which only takes a few seconds you won't have time to go through the theory.
perhaps MT is too academically slow to apply to spontaneous magic casting?
Basically it is suited to the speed of seasonal lab work.
If you had 15 minutes you might be able to work it in.
When it can be used like a Lore to remember a fact about Hermetic Magic in a moment I don't think its "slowness" is the issue. It is mainly used for seasonal things, true but there are plenty of ways it could be useful when thinking about what you are about to do and if you just don't want to add another ability to a casting total just say that.
On further consideration, part of the reason I am uncomfortable with the idea of adding MT score to spontaneous casting total, is that Hermetic MT is responsible for you knowing how to spontaneously cast in the first place, adding to the casting total gives me the uncomfortable impression of double dipping.
ie you need MT to invent a reproducible Formulaic or Ritual spell. Once invented it is reproducible, MT has done its part.
Spontaneous spells are highly dependent on local conditions, and technically no two spontaneous spells you cast are the same.
YMMV
If you houseruled the spell improvisation virtue as standard with spontaneous magic it would feel like double dipping to me.As it is it doesn’t.
This feels like a solid Minor Virtue to me, or something a player could develop as a Breakthrough.
Others have given thematic reason. I'll add a game balance opposition to the thread.
Magic Theory is already really important for the mid to late game magi. It's the rare skill someone pushes to 7+ because of the extra pawns of vis they can work with, every +1 counts in the lab, etc.
Making an already important skill even better, I'd say no. If you were going to add a skill to spont casting, the two mentioned earlier by One Shot, Philosophy or Artes Liberales seems a better fit.
Yeah. Magic Theory 7 ... My Verditius just wrote a level 6 Summa in Magic Theory
Hi,
A rules variant for Diedne Magic that I had previously posted involves MT. (I currently prefer a different unposted version, because this is a bit too good imo.)
An existing canonical minor virtue lets you add an S&M bonus to the casting total of sponts, up to the level of your MT, but you might not be permitted to take it unless you take the whole Rusticani package.
Anyway,
Ken
Do you not feel that sponting a spell in the lab is part of the process of inventing a new Formulaic spell? I'm coming from the mindset that it is indeed a part of the process. "Let's see what happens if I add Ferdinadus's Flourish here..."
Sponting spells in the lab could possibly form part of the process of inventing formulaic spells, but it can't be a mandatory part, since even mages who can't cast spontaneous spells can still invent formulaic spells just as easily as any other mage.
Any sponting during the inventing of a formulaic spell would almost have to be non-fatiguing sponts unless you want to risk dealing with a bunch of botches during the process..
Very much not. If sponting a spell in the lab was necessary, flaws penalizing spontaneous casting would also penalize formulaic and ritual casting. They don't.
There should be a difference between fasr cast spontaneous casting and prepared spontaneous casting.
The latter being, in theory, easier to cast by taking 1 or 2 min to prepare your cast (without going full ceremonial).
Do it as you wish but if you want to make Spont cast easier in your campaign, allow your mage to add their TM or to make a TM stress roll to see if they can prepare their spell with a bonus.
The point of formulaic spells is they can only do 1 thing, but they can be a lot stronger than spont magic. Every boost to spont magic makes formulaic magic less useful in comparison. I think the balance between the versatility of spont magic and the power (and less botch risk) of formulaic is in a good place.
Me too 
But I could understand some don't
That is a point I'd overlooked.
It also contributes to the increasing feeling that I'm the only musician who plays Ars Magica, ever improvises, and also frequents this forum. Or maybe just the most theory-conscious?