It seems to me that a lot of supernatural, and even natural, forces have the power to grant a +1 to +3 bonus to Abilities. Hedge wizards can do it. Divine and (IIRC) Infernal powers can do this. Even non-magical scholars can do this without using any magic at all. However, there are currently no Hermetic guidelines for duplicating this rather useful and evidently common power. I would like to incorporate such guidelines into Hermetic magic but I'm not entirely sure how to go about it.
The "simple" idea I have had is to just define it as a Creo Corpus guideline for "physical" Abilities and a Creo Mentam guideline for "mental" Abilities... essentially enhacing the characters natural physical or mental nature to better enable them to perform the skill.
Alternatively, I could see such spells as Rego (Form) guidelines where the spell effects the "target" of the Ability. For instance, Rego Animal to give a bonus to Abilities related to dealing with animals (Animal Handling or Ride) or Rego Mentam for Abilities related to persuading or thinking... etc.
This seems reasonable. But instead of making the guideline "+1 to Ability Score" (or +3, or whatever), I think you would be better off making the guideline "raise an Ability Score to 3" (say). This way the guideline will have no effect if the character already has the Ability Score in excess of 3.
If the guideline simply adds +x, you run the risk of a player finding a way to arrange for his character to munchkin his way to an Ability Score of 1,000,0000, or some similar disaster.
Also, you need to consider whether it will work for Arts as well as Abilities, and whether it can grant/raise Supernatural Abilities. The easiest answer will be "no" to these questions.
Think through whether you like the consequences of player characters being able to raise their Parma Magica Ability Scores (or Penetration, or Magic Theory, or Mystery Cult Lores, etc) via spells.
Edit: deleted my previous response because I realized this was the critical point of misunderstanding.
The guideline is not a +1 to an Ability score. It is a +1 to an Ability roll. Therefore it would no impact on Parma, Penetration, lab work (Magic Theory) or initations (Mystery Cult Lore) because those aren't rolled.
You do understand this isn't a new concept? I'm just trying to adopt existing hedge magic guidelines into Hermetic magic. The "absolute limit" is +3, just like with hedge magic (Learned Magician Fortunam, Gruagach Blessing), invoking Gods aid and ligatures.
I'd say the "in character" reason for getting a bonus to rolls and not totals is roughly the same as the "in character" reason you get to add between 1 and 10 (or 2 and near-infinity) when you roll and not when just calculate a total.
Basically you want to grant Puissant Ability with hermetic magic.
I think the problem is that hedge magic can grant virtues, not that hermetics can't. But that is me. I dislike that specific potential in HM since I think it is extremely open to abuse, soecially with new virtues appearing in each new supplement. No reason why hedge magic cannot grant you a firebreathing power (focus power, supernatural in ROP:M) or the sight of the Chimera, as per the rules... Anyway, enough ranting.
For hermetics you could use MuCo base 2 (grant minor ability) or go for a higher total. ReCo10 (control motion) could also work as a base guideline. It is about motion and capabilities, so that should do the trick. for mentem, InMe5 (understand the meaning behind spoken words) would be enough for puissant folk ken, and ReMe3 (make a subtle difference to the target mental state) should be enough for influencing him with Charm or Intrigue.
So, base 5 to be on the safe side should allow you to grant puissant to most abilities
No, actually I don't. While I find the ""grant virtue/flaw" guidelines from Hedge Magic interesting as a simple game mechanic for granting special abilities or curses, that's not the guideline I'm after right now. Right now, I'm just after the "provide a +1/+2/+3 bonus to single Ability roll" guideline. (Give Blessing 5/10/15, Succuro Fortunam 2/5/10, Invocation Blessing General, see also Ligatures and Invoking Gods Aid.) While it is similar mechanically to Puissant Ability, it's actually something different.
That said, I see where you're going with the guidelines, though you seem to be split. On the one hand, if I follow you, you suggest Muto or Rego Corpus to enhance the individuals capabilities at performing a physical ability, On the other hand, you suggest Rego Mentam to influence the "targets" reactions to the use of social abilities.
Ultimately, I think the issue is that there are very few specific guidelines for how the Arts interact with Abilities and the Arts all have very specific, fairly narrow, effects. For example, the Gruagach spells Blessing of the Swordsmans Expertise grants a +2 bonus to Single Weapon. Exactly how that manifests is left up to the imagination.... the target of the blessing is simply better in battle. In contrast, Hermetic magic would take a much more specific approach... Muto Terram to make his sword sharper, Creo Ignem to briefly blind his foe, Creo Imaginem to distract his foe, Creo or Muto Corpus to enhance the swordsmans phsyical prowess, perhaps even Intelligo Mentam to aniticipate his foes tactics. Every one of those Hermetic effects and more are encompassed in a single Gruagach spell... but only with respect to the use the Single Weapon Ability.
As far as I can tell, there is no way to add an amount to an ability.
However, you can add a bonus to a roll under certain circumstances. ie when in forests. (in he can't recall offhand, I'm 70 odd miles from my books.)
would this be sufficient to your purpose?
ArM5 rolls are generally Characteristic + Ability, and Hermetic Magic can already up your characteristics.
So, if you want to improve sword-play, Hermetic Magic can improve Dexterity, Quickness and Strength.
If you want to improve singing, it can improve Communication and Presence.
The only place I've seen Hermetics be able to improve abilities is in sections on Mystery Magic, like Items of Quality for Verditius or Inner Heartbeast Refinements from Bjornaer. Perhaps Theurgy would work as well.
But IMO Hermetic Magic couldn't give improvements to abilities without a breakthrough.
The trouble, I am coming to realize, is that the Ability system is fairly abstract... as most rpg "skill systems" tend to be. A whole vast array of different activities are covered under even a fairly specific abilities like Ride, to say nothing of broad general abilities like Charm. However, Hermetic magic is very specific and defined. As in my earlier example, there are a lot of specific spells you could cast that would alter circumstances and which could be ruled as giving you a bonus to certain uses of certain Abilities... but a single spell that gives you a flat generic bonus to a general ability is simply too board under the Hermetic magic guidelines.
Essentially, I'm just now realizing that the magic system and the skill system do not interact smoothly in terms of game mechanics and require a bit of GM-fiat to really function.... which is fine, except that the hedge magic rules take a very different approach making them hard to incorporate.
I don't think it would be a vanilla effect - it would be a new mechanic, like Talismans or Familiars. I think it would be the inclusion of Theurgic principles or the inclusion of the powers of Items of Quality into the power of Talismans.
For example, it might become possible that a magus, when instilling powes in his talisman, could choose to bring out a Quality rather than providing a magic bonus. So, if investing a sword with a power, you could have it provide a +4 bonus to damaging human or animal bodies in combat, as opposed to +4 to PeCo or PeAn spells.
Or, maybe a Mercurian group has reforged the connections between magi and the Roman Pantheon, allowing reliable and universal theurgic invocations, which allows, for example, one to call upon the Grace of Apollo to improve one's singing. These would almost certainly involve rituals, or at least ceremonial casting.
Or maybe it could be powers drawn out of Familiar bonds - you can invest a power that gives (Silver Chord score) to mental-based or analytical skills (Folk Ken, Intrigue, Concentration etc), (Bronze Chord score) to physcial abilities (Athletics, Brawl etc.) or (Gold Chord score) to artistic or expressive abilities (Craft, Music, Charm etc.)
One ability can be improved by each effect. 1 pawn vis/ +1 bonus. Bonus is set at time of investing, can only be improved by reforging the chain. Each of these bonuses trakes up (bonus * 3,4 or 5) points of space.
I would NOT allow ability bonuses to be given by simple formulaic or spontaneous magic.
... and yet that appears to be a staple of Hedge Magic. Do you feel that those guidelines are too powerful for hedge magic? Or do you simply feel that they aren't appropriate for the "feel" of Hermetic magic?
The question I would have is not of balance but of HOW you'd be increasing an Ability with Hermetic Magic. The rules on ME establish (sensibly) that the Art cannot grant the sort of knowledge that constitutes an "Ability". You can grant increased physical or mental characteristics, as others have mentioned, that will affect the Attribute + Ability total. You could probably grant a limited version of a characteristic, such as quick fingers, for lower level. The practical problem here is that unless you grant a superhuman level of attribute the effect on the roll is going to be fairly small. You might also grant such things as superhuman senses that help in specific situations. Did you have some other sort of mechanism in mind?
In general Hermetic Magic is extremely rational and mechanistic, which requires us to explain this sort of effect. Hedge magic by contrast is usually storybook magic, which doesn't. I agree the combination can sometimes be jarring.