I was looking at an old post on Spell Mastery, specifically
The mention of "wards" has left me wondering what the effect is if a magus learns the Magic Resistance Mastery for "Aegis of the Hearth" (apart for the general effect you get for learning a level of spell mastery).
Would this mastery ability allow you to alleviate any of the penalties a foreign Aegis might impose on your magus?
No, it would not.
Why not?
The Magic Resistance Spell Mastery ability just gives you increased Magic Resistance versus certain spells. Your Magic Resistance normally has no effect on a foreign Aegis, so increasing it will not change that.
(One could argue that the Aegis needs to Penetrate the MR of a magus to have any effect on his spell-casting while within a foreign Aegis, in which case any increase in MR would help to resist the Aegis, but I do not think this is the intended reading.)
That's exactly how we read it, and play it 
Ultimately, the Aegis is not really effective against magi of the Order. Its main use is against minor hedge wizards and supernatural creatures. Basically, it keeps pests out of the house.
Same here!
Though not many people in our sagas know the Aegis spell and when they master it it’s mostly to make the casting easier.
Do you require the Aegis to penetrate to block a spell cast from outside of it towards within it? If not, but you require it to penetrate to disrupt magic inside of it then you are nerfing it in a strange way.
You are treating the giant magic suppression field (which only does not effect people who were present when it was created or are given a "key" in the form of a token) as being ineffective against someone inside the field but at full power while they are outside of it.
AotH is Parma Magica for a place. If you require it to penetrate then you must also require Parma Magica to penetrate.
Nope. Remember, they are two very different effects. And this makes sense.
In one case it's the external spell that "attacks" (i.e. attempts to penetrate) the Aegis. The Aegis passively defends.
In the other case it's the Aegis that "attacks" (i.e. hampers the spellcasting of) a caster. The caster passively defends with MR, if he has any.
Note that the mechanics are quite different!
No. In both cases an attack tries to bypass a passive defense. It's just that when something tries to get into the Aegis, it's that something that's attacking the Aegis. When the Aegis is trying to smother something already inside it, the attacker is the Aegis itself.
No, the mechanics are very different. Parma need not deal with "internally generated" hostile effects.
Except when this 'something' is a creature with Might, it is the Aegis which needs to penetrate the creatures MR. Which is intuitively backwards.
I guess you could say that the Aegis "attacks" creatures (unless they were involved in the casting, were invited etc.), and "defends" against supernatural effects. Parma performs the latter function (in a slightly different way), but not former.
I think that ultimately this comes, rather than from some "philosophical" issue, from a simple mechanic design issue: for effects it's much easier to identify an "attack score" (the penetration) and for creatures a "defense score" (MR).
One could say that, and the rules pretty much do say that, but it still feels intuitively wrong that it is the passive defense which needs to penetrate to stop the active intruder - which is probably the underlying reason for all the controversy about wards/Aegis needing to penetrate.
I agree... though only in terms of preventing entry, not in terms of smothering spellcasting - including R:Per spellcasting!
I've had characters who mastered the aegis and took the spell resistance mastery ability.
No. The Aegis is defending in all cases here.
Nope. The Aegis is defending from something trying to be done within it.
No no no no. While the mechanics are different, it is clearly stated multiple times throughout the line that the Aegis is the result of OR to allow the Parma Magica to affect an area.
And back on the ignore list you go once again.
I agree with you and @ezzelino that requiring a defensive effect to penetrate vs. an attacker seems backward. So did the rest of my troupe.
Since we couldn't find any where in 4th ed. where this was clearly addressed we ruled IOS that Effects must penetrate vs. Wards. For context we are broadly defining Effects as spells, creature powers, and creature Might, and defining Wards as any passive defensive magic (e.g. Parma, Aegis, various Circular Ward...spells).
So IOS a creature with might has to penetrate an Aegis to physically enter or magically attack a warded covenant.
But since I've always been a fan of "rules cutting both ways" we gave creatures a Penetration roll (Might + Stress die) to balance the Arts + Aura+ Stress+ Penetration of magi casting spells.
I could agree with you if R:Personal wasn't blocked by the Aegis. I have difficulties calling that a defensive move.
It could be compared to aura effects, but there's nothing in the description that implies Notatus research was based on them. Still, that might be the best way explain Penetration away.
To me, Aegis is a high pressure zone. First effect is low momentum spells "bounce" off the surface (fizzle), that's a defense. Second effect is anyone inside feels the pressure crushing their magic, that's an attack.
I'm sure you agree this opinion is aesthetically pleasing, even if it's not the only nice way to skin this cat.
Presumptous ... but I have to agree
Let me say that I would be perfectly ok with an Aegis that did not need to penetrate to affect "local" spellcasting. But it would have to be written differently. And crucially, the "keep the critters out" effect ... I just can't see that avoiding the penetration requirement.
Well I'm poking people a little here. I hope others will rise to the challenge and present other "aesthetically pleasing" ways to skin this cat. 
After all, we're only brainstorming ideas from which David will shape the cleaned up errata.
I feel our saga's solution is the best, but since it conflicts with (apparently) clear canon in 5th, I'll suggest the alternative of rewriting AotH to exempt spells cast at R: Personal within its boundary. That could be a simple addition to an Errata entry if the change were made in 5th ed., or put right in at the start if the mythic 6th ed. ever manifests.
It attempts to block all magic performed by "invaders" within its area.
Part of my job for decades was designing and implementing facility/base defense in combat zones. Engaging hostiles "within the wire" is a defensive rather than offensive action.
You also seem to believe that spells/effects with Personal range should be viewed differently from other spells. An invisibility spell can be cast with Range: Personal. Combat boosters, transportation, sensory spells, and several other effects which are actively or passively dangerous to the locals are possible. Preventing "invaders" within your area from turning invisible, boosting themselves, flying, and collecting information on internal activities are all defensive in nature.
Since the intent behind all actions by outside "invaders" can not be known, preventing all of them is the only valid defense.
I guess that clears that up.
This entire thread has been illuminating.
I had not thought about needing to penetrate the magi's magical resistance (Parma plus Vim Form resistance). I had always thought that a magus could always enter a AotH, since they had no Might to block, but then automatically suppressed.
So the subset of Spell Mastery abilities applicable to Aegis casting would be Penetration, Rebuttal in general, and Quiet and Still if you wish to cast an Aegis unnoticed.
A magus can always enter a Aegis of the Hearth, but the question then is if the Aegis needs to penetrate the magus' Magic Resistance in order to give him a penalty to spellcasting while within it.
Quiet and Still casting would be of very limited use for Aegis of the Hearth since it is a ritual. Casting without words or gestures is only an option for Formulaic or Spontaneous spells, not for Rituals.
Of the Spell Mastery abilities in the core rules, it is really only Magic Resistance and Penetration that are useful for Aegis of the Hearth. There are several options in the rest of the books that could also be useful though. (Rebuttal, Stalwart Casting, Adaptive Casting all come to mind)