Yes, if you use a momentary spell to destroy its solidity it remains non-solid. Since there is no creo spell to restore a destroyed property that is irreversible- muto can change it to solid for a little while, but its essential nature is now liquid rather than solid. What Perdo destroys tends to remain destroyed...
Only if the property destroyed is something that the thing can naturally exist without - page 78 of the main book says "Further, destroying properties that a thing cannot naturally lose falls under the Limit of Essential Nature (see page 79), and thus cannot be permanent; the destroyed properties return by themselves at the end of the spell's duration."
I'd be inclined to say that (room temperature) granite cannot naturally lose its property of solidity, so it would return to solidity at the end of the spell.
One could argue that glass is no natural material.
Though some form of natural glass may exist when lightning strike a mass of sand, or when rocks are melt in vulcanoes, I'm not convinced those things are well-known in the middle-ages.
Fine and transparent glass could require an additional +1 magnitude, because natural glass samples are not polished. I think Covenant give some guidelines for manufactured furnitures.
Then I would propose :
Aery Breach in the Castle
MuTe(Au) 20
R: Touch, D: Sun, T:Part
Change a portion of a wall into air. Note the wall or the building may collapse.
(Base 3 change earth into gas, +1 to affect rock, Duration Sun +2, Target Part +1, Prerequisite +1)
Deny the Rampart Existence
PeIm(Te) 20
R: Touch, D: Sun, T:Part
Delete the ability of a wall to emit visible species. Works on true ramparts and more simple walls too.
Base 4, Range Touch +1, Duration Sun +2, Target Part +1)
And as for "Level 5: Destroy one aspect of dirt, such as its weight or its cohesiveness" the following :
Shred the Wall's Toughness
PeTe 30
R: Touch, D: Sun, T:Part
The affected portion of a wall loses the ability to stop matter, objects and people. A character who don't know cannot guess it though she has Double Sight.
(Base 5, affect rock +1, Range Touch +1, Duration Sun +2, Target Part +1)
I don't think you need a requisite, Veil of Invisibility and that kind of spell can already affect discrete objects, with Part + magnitudes you should be covered, right? A Base Individual for Imaginem is the size of a human. Base 4, Touch +1, Sun +2, Part +1, +2 Magnitudes for a large wall? PeIm 30?
Thanks! It definitely struck me as not in accordance with the rules, and was looking for that passage.
Both seem legit. A level 20 spell with Prerequisite and a level 30 spell without prerequisite, any may be useful.
I'm looking for a spell to go through walls too, ideally lvl 15
I'd be happy to take the lvl 15 version here, but I can't see how "change stone so that it is slightly unnatural" turns it into a passageway, because of the Muto definition is "grant or remove properties something cannot naturally have" (core p78)
Being in the form of a Passageway is a shape that stone naturally can have, so wouldn't that be Rego instead?
Another Muto option would be to make it act like a curtain (like in the Net Grimoire spell) or something else highly unnatural, but that's:
Base 4, Stone+1, Touch+1, Conc+1, Part+1: MuTe 20
With Rego, I worry that it might falls into the Realm of Rego crafting (Covenants p49) and its crazy high Finesse rolls.
By my calculation, it would require Finesse roll of ~12: "simple" task for a simple archway (6+3) that takes a month's work (+3)
It's also permanent, which is not what I'm looking for. I don't see a temporary Rego option for that kind of effect.
Another option would be Perdo to remove stone + Rego to keep it in place, because the target Part of the wall is effectively destroyed, and it's only the area around it that I'm making a better example of itself.
That would be permanent too, except that I have picked the Harmless Magic flaw:
Edit: A slightly unnatural state (base 3) that would work might be some kind of gelatine state that allows you to slowly walk through (idea stolen here)
I'll make a version of Rock of Viscid Clay (Conc, Stone) that gives this kind of property instead. I might even find other uses for it 
I've also seen mentioned PeTe to remove the substantiality of stone, but that's base 5 (HoH:S p37)
Other References:
There's a Minor Lab Flaw in Covenants p119 called Impregnable that involves your lab having no easy access, for example by not having a door.
The suggested method of getting in is MuTe spells
An example MuTe(Au) 25 is in MoH p93).
MoH p.93 has MuTe(Au) 25 R: Touch, D: Mom, T: Part Short Step of the Ghost based on ArM5 p.154 box Muto Terram Guidelines level 10 "Change dirt into a highly unnatural liquid or gas (with requisites)" and +1 magnitude for affecting stone.
Cheers
Yes, I had seen that lvl 25 version and mentioned it at the end of my post.
I already had several functional lvl 20 versions and I was looking for a lvl 15, which I found an acceptable (if not amazing) version of.
My main question was about the interpretation of Muto that Erik used: by my reading, it doesn't seem to work, but I might not be understanding Muto correctly?
I don't think that rego is appropriate. You're not moving the stone around or working it, you're transforming it in to something different. I see it as akin to changing a Walrus into a zebra being a muto spell rather than a rego one.
Would you instead use the base 2 guideline; "change dirt in to another sort of natural earth" ? That seems ok. I wrote this spell a dozen years or so ago and just copied it to this thread last summer. I'm afraid I can't give you a good explanation for why I chose the guideline I did. I mean it's not natural for a wall to turn in to a passageway, but that's probably not what I was thinking.
My best guess is that I thought that the spell altered both shape and substance so it needed a bit more than the level 2 guideline which only changed substance.
My doubt about Muto came from this:
Stone naturally can be arranged into a wall or a passageway, so I thing this requires Rego. It would actually be difficult because it's a crafted item: it's Rego Crafting.
But I think that a magus can sidestep this by targeting a Wall instead of an amount of Stone (Yay, Natural Philosophy):
A Wall can't naturally be a Passageway, and therefore must be changed with Muto
I think I'll use base 2 "change dirt to another type of natural earth" to turn a Wall into a Passageway, yes.
Crafted items have been left out of the MuTe guideline (as compared to the MuHe guidelines) so I'll add +1 for complexity (shape and substance sounds like a reasonable explanation)
Guidelines referring to dirt are repeatedly used to affect crafted items in the Core spells, but that's more for manipulating them than completely transforming them...
Yea, I think the base 2 is appropriate to turn a section of wall into an archway, maybe with the added magnitude of complexity. It's like targetting a human with Muto to transform his visage. You could also Rego craft the wall into the door, as you mentioned, but more permanent than you want. You're also transforming (roughly) 15 cubic feet of stone into roughly 8 feet of stone, so Muto definitely feels appropriate; Rego would leave a pile of dust as part of it.
I mean, my online magus has Harmless magic, so my temporary doorway is an easy Perdo Terram spell. (Presented as alternate solution, if you have the right virtues and flaws)
Thanks for the detailed answer.
Actually, I took Harmless Magic too but destroy is base 3 so that's already as hard as Muto base 2+1 complexity mag, and the way I wrote it I needed an additional mag for a Rego Req to avoid collapse.
I should think so.
I don't believe this works in ArM5.
Your argument misunderstands the phrase from ArM5 p.78 Muto (Mu) "I transform" "Muto cannot affect the properties that something has naturally". A wall can have a passageway "naturally", and a Part of a wall can be a passageway "naturally".
Of course, "naturally" can mean a lot of things not sufficiently specified in either ArM5 p.78 Muto (Mu) "I transform" or ArM5 p.78 Rego (Re) "I control". But it is a reasonable assumption, that both mean the same by it: hence making a passageway in a wall requires Rego (which clearly can reshape and rearrange stones), not Muto.
Cheers
My argument was that "Naturally" meant the same thing in both description, but that what is Natural to Stone is different from what is Natural to a Wall.
Stone is stone, and is still stoce whatrever its arrangement.
A Wall is not a Passageway. There can be a Passageway in a Wall, but that part of the wall is not a Wall anymore.
They're different Platonician concept, which I understand are the basic units of Magic in Ars Magica
So you choose whatever 'concept' you apply "naturally" to in any way you like? Ask your troupe!
No, you wish to elaborate on the universal Form of wall among Platonic universals? Go ahead: please with quotes about whether the universal Form of wall allows windows, doors and thoroughfares or not!
For the relationship between Hermetic magic and Platonic universals see:
Reading ArM5 rulebooks assuming Platonic or Aristotelian terminology isn't helpful, no matter what your troupe decided here. But understanding especially Scholastic natural philosophy and psychology, as explained in A&A, does help a lot.
Cheers
You keep saying "Ask your troupe" in a way that sounds like an insult.
Basically, I set the troupe interpretation. Any help to make that interpretation better is welcome. Dismissiveness and veiled aggressivity aren't.
The result of this theorycrafting matches the design decisions of Erik to use base 3 Muto to turn a wall into a passageway.
It might be arguable, but it isn't laughably off-base.
Thank you for the reference to A&A anyway, I'll check it out.
An object can be an example of several forms: Table, Wood, Brown... Wall, Stone, Grey...
Emphasis mine. I'll look into A&A for more info.
Just inventing yourself some 'concepts' and applying guidelines to them doesn't cut it, shouldn't you think so? What am I supposed to think of that kind of argument? The best I can say about it is indeed: if your troupe is happy with it, play as you like.
On the forum I certainly can expect an argument, why these 'concepts' shall be appropriate. If you refer to Plato, this requires at least, that you show what Plato or one of his many followers wrote on that subject, and how it relates to ArM5 walls.
Did you find out what the New Aristotle and newly found works of Plato you underlined refer to? The former is a main driver in the development of Thomist and Scotist Scholastics, the latter are a base of Renaissance Humanism.
For the New Aristotle you can go straight to A&A p.9f The New Aristotle. It is a 13th century term you will find rarely in our time's summaries of Aristotle's teaching, and which refers to those central works of Aristotle becoming accessible to the world reading Latin in the first third of the 13th century.
The newly found works of Plato are TMK not explained in A&A. They refer to the problem, that of the Platonic dialogues only the Timaios was in the 13th century accessible in Latin: see here. Urbane magi reading Classical Greek (Attic) might discover Byzantine manuscripts of most of Plato in Constantinople, though: see here.
Cheers
More to the point: your initial disagreement was that I wanted to apply magic to the concept of "Wall".
I quoted Wikipedia's explanation of Plato on this, but the Core rulebook already lays out the kind of things magic works on:
Also:
A Wall in an artificial thing, and for example Wall of Protecting Stone (ArM5 p153) draws from the realm of Forms the Idea of Wall rather than Stone, in order to produce a well-constructed Wall rather than a pile of rocks.
I cast my spell on the exact section of Wall that I mean to turn into a Passageway, drawing from the Idea of Passageway rather than the Idea of Stone.
There is no trace of a Wall in that section once I've cast, only a Passageway. That's the whole point of targeting a Part.
This is effectively turning something that is "Wall" into something that isn't, which is Muto.
You haven't actually argued against that, only dismissed it, which I feel entirely justified in ignoring.
As for Aristotelician Vs Platonician concepts:
The passage that you quoted was about how you could get more in-depth understanding of magic by studying the masters (adding AL to your casting and Lab scores), with different methods depending on whether Plato or Aristotle (or both, or none) was "the logical model for magic theory".
It doesn't change how Magic works in the first place, which is close enough to the concepts of both authors that a bunch of magi have been arguing about it for decades without either side getting a clear advantage.
You are arguing now! Congrats! So let's sort out at this problem, which part of Hermetic Magic Theory is Platonic and which is Aristotelian!
First, substance is basically an Aristotelian concept. Especially:
So you need to decide, whether you wish to argue following Plato or Aristotle here. ArM5 follows the latter here even with Creo. You also skipped the rest of the text you quoted, namely:
As an artificial thing by ArM5 p.77 a Wall has a complex form: a combination of several natural forms put together in a particular way.
Why is this Muto?
A Wall is not natural but artificial and hence has a composite, complex form. By turning a wall into a passageway no natural form gets changed, but only their composite. Just like Covenants p.51 ReTe 4 The Invisible Glass-Worker transforms a piece of glass into e. g. a chalice. Yes, the complex form of the chalice is different from the complex form of the piece of glass, but Muto is not needed, because
The same with the wall: it can naturally, namely by mason's work, get the form of a pathway.
So it appears, that your argument combining forms and the term "naturally" does not match ArM5 examples and rules. Maybe in this aspect Magic Theory is more Aristotelian.
Indeed: Hermetic Magic Theory takes from both Aristotle and Plato. This does not mean, that for reading a part of ArM5 you can just arbitrarily choose one over the other. You rather have to look carefully, which one is currently applied.
TMK most people who play ArM5 just do not care about Platonic or Aristotelian parts of Hermetic Magic. Playing it by the book generally works and avoids trips to the library or to the Stanford Encyclopedia of Philosophy.
Cheers
You seem to find joy in dismissing others regardless of the reality of what you imply.
Knowing a lot about this game doesn't excuse your attitude.
Players just as qualified as you have agreed that Muto works.
I've already argued why I think it does, and I'm unconvinced by your arguments.
Without resorting to fundamental theory: Muto transforms, Rego rearranges. I'm transforming.
I could argue about simple and complex forms, go into symbology and different aspects of a thing having different Ideal Forms, and it could be fun.
But you're a jerk and it wouldn't.
It's a pity.
Please refrain from answering my future posts and keep your entirely hypocritical "Cheers" to yourself.
Edit: If I was an admin here, I'd add "of Tytalus" to your screen name, as a warning to others