Again, Muto doesn't create matter, it changes matter into unnatural matter. Bear in mind that, according to ars magica physicks, atoms of water and atoms of olive oil are the same Aquam building blocks of the universe, but in different sizes - the olive oil atoms are smaller, which is why it floats on water even though it's still an aquam atom. So you would effectively be turning your water into unnatural water atoms are merely larger, spacier / heavier - not creating more water. That's the theory behind it. Since you're probably not stopping to weigh the water you muto'd and analyze it under a microscope, it merely looks like the lake expanded. Also, bear in mind the most important restriction of all for such effects - the resulting effect would be temporary.
Ultimately you don't create more water, you just alter the space and weight used by the existing water. The lake will not quench thirst or water crops any more than the original puddle would, and it would not put out any larger of a fire. The essential amount of water is unchanged, it just has greater amount of volume and weight.
This is probably what I am going with, but the fact that people's views differ so much is very interesting, as are the associated arguments.
By RAW it is quite clear that you can use Muto to change the size of something, including increasing or decreasing its mass a lot.
Changing a human into a bear increases his weight by a few hundred pounds that come out of nowhere, yet that is a (normally) uncontroversial use of Muto. No different in principle from changing a small rock into a huge rock.
Preservation of mass is simply not an issuse with magic in Mythic Europe - at most it makes it harder to work the magic.
Yes, I have no problem with that when it comes to changing one distinct entity into another entity, such as a man to a bear. I only have a problem with quantitative substances and changing "some of substance x" into "more of substance x", when to me that intuitively feels like Creo.
There is certainly some overlap between what can be done with Creo (or Rego) and Muto, but also some important differences.
The biggest difference is of course that with Creo it is possible to make permanent creations, while things that have been changed with Muto always return to their original form once the spell ends.
Creo is not so much about creating things as it is about improving them, making them more perfect. An existing boulder is in some sense more perfect than a non-existing boulder, thus one can use Creo to make a boulder come into existance.
Changing a small rock into a huge rock doesn't make it better, it just makes it bigger. Hence that is not something suitable for Creo magic, but it fits perfectly into Muto magic which is all about changes.
Yet rock is not only an entity, it is also a substance, and, as you say, an amount of substance that exists is more perfect than an amount of substance that does not exist. Hence, more water in a lake is Creo according to RAW, even though the lake itself does not become more perfect - instead, the creation of additional material (the water) creates something more perfect than "no additional material", so logically, the same should apply to adding more rock to something.
I'm curious, why?
Turn a cow to a mouse and vice-cersa. It's all mamallian flesh. Why the distinction between mamallian flesh growing and shrinking as opposed to rock?
If it makes you feel better we can add a rego requisite and make the rock spike sculpted like a narwhal.
Nope. It is the lake which is the target of your spell, and it is thus its perfection which matters as far as Creo magic is concerned.
You can add water to a lake through Creo by creating a whole lot of water just above the existing lake, which will then splash down into the lake and mix with the existing water there, but you can't use Creo to transform the existing water into more water.
If you look through the spells in the core rules you will find spells like Object of Increased Size or Beast of Outlandish Size that increase the size and weight of their targets. These are Muto spells, not Creo and they don't have a Creo requisite. So RAW it is Muto that is used for such things.
You will have to look long and hard to find any canon Creo spells which does that (one specific exception are spells that mature an immature living being.)
If targeting a lake, Creo Aquam would be excellent to purify stagnant water, for example.
Well yes, that's the point. Of course you can't use Creo to transform water into more water. I'm not sure I really get the idea of transforming water into more water at all - that's the problem. To me, it sounds like creating water, hence it is Creo, as shown in "Deluge of Rushing and Dashing" on page 121 of ArsM5.
I know, I brought up this exact point a few posts back.
Well the first thing is that it feels very intuitive that changing a man into a bear is an act of transformation, whereas adding more water to a pool or making a rock grow to 4 times its size feels more like an act of creation.
Yes, in this instance, it makes perfect sense to me that you could transform a small piece of rock into a narwhal tusk with Muto. But to make that rock into a much larger piece of rock feels very different.
What is your solution to the fact that the Muto Aquam spells to cause lakes to flood, etc., are conspicuously missing from ArsM5? Is your approach that you can use Muto if you want but it just isn't included in the base book? Or is it that water is an exception to the general rule about Muto being able to cause an increase in a mass of a particular substance, such as rock? (I feel like that final one may be the thought process behind RAW, but I'm not sure I can justify it very well).
Magic in ars magica does not work on substances, only entities. Entities can be made of a substance, and a discrete quantity of a substance is in essence an entity, but the divisions are things like individual, part, and group, not "70 KG, 140 KG" etc. A human who is changed into a bear increases the quantity of the substance which is muscle, but that is simply not the framework that hermetic magic utilizes.
You can do either - create more water with Creo, or transform the existing water into a larger body of water with Muto. For substances like water or air where new water will easily mix with old water the end result will be pretty much the same.
For more solid substances like stone the differences become more noticable. You can use Muto to change a small rock into a large rock, or you can use Creo to create a large rock. But when using Creo to create a rock, the substance created will not be added to an existing object but will be a separate object.
The difference here is simply due to the substances involved behaving differently by nature - two rocks will not naturally merge into one larger rock, while two pools of water can easily be merged into one pool.
It is worth noting that strange things can happen when you divide the material in question- a puddle transformed into a lake and then stored in a variety of containers remains a divided lake until some point at which the various waters are rejoined, at which point it reverts to a puddle. One can imagine a variety of odd effects from this... for example using the water from an enlarged puddle to displace the water in a lake or pool so that it overflows, and the original puddle reforms (emptying the pool) when the last container is added.
So, would you rule that "Deluge of Rushing and Dashing" could be created as a Muto version?
I just tried to work out what that would be based on the guidelines, but there are no guidelines for enlarging/increasing water under Muto Aquam. That doesn't mean it's not possible of course, because as I'm sure you know, there are missing guidelines for some things in all the Forms. The normal method is to take them from another form, so we could take the ones from Terram, which say base 4 for an 8-fold increase in volume, that seems like it's probably a lot more than "Deluge of Rushing and Dashing" produces, and is 2 mags lower, so would be a MuAq 30 for a potentially larger effect. So maybe the Terram ones aren't right for Aquam...? What base would you use for Muto Aquam to increase water?
I'm wondering if "Deluge of Rushing and Dashing" has really been put under the wrong Technique. It would seem more consistent as Muto, based on the rest of the Forms. Not sure that makes intuitive sense to me, but it would be more consistent.
I think, if I wanted to design a spell to flood an area, I would rather use ReAq to control a wave which would rely on a target: ind and therefore would be a relatively simple spell than to use a MuAq with a boundary target to affect the entire lake, therefore probably needing a ritual costing vis and leaving the local topology to decide what gets flooded, unless we're talking about a pond, which could be affected by a spell with target individual to flood.
Deluge of Rushing and Dashing (which floods a stream) as discussed above.
An 8-fold increase in volume is not actually nearly enough to create a "Deluge of Rushing and Dashing" effect, unless you started with an already large volume of water. You would need something like a whole base individual of water (5 pace diameter by 2 paces deep) as a starting point to end up with something 10 paces in diameter and 4 paces deep. Then it would be sufficient, but you are required to start with a very large volume of water to use the spell.