Creo feels quite intuitive to me too, and yet making an animal larger in RAW is clearly Muto (p. 118), so this would create some inconsistency between Forms (unless you house-rule animal growth to Creo as well)
I just realised the animal growth in RAW is unnatural, so has to be Muto, whereas a boulder can naturally be any size. So perhaps growing an animal to the natural limit for its species would be Creo, and there is not any inconsistency after all?
Wouldn't using Creo create an object, rather than create matter that is part of an existing object? Such that, without a requisite, a spike would be created, but probably naturally fall down since it's not part of the wall, and the boulder expansion would actually be two separate rocks stacked on top of each other? My guess is that you would use Creo (Rego) for something permanent, and Muto would be the simplest route for something temporary.
There are multiple ways of doing this. Creo and Muto are the most obvious.
You're changing the size of something that does not naturally change size.
That's Muto. Could someone please explain to me (using small words etc) why this could be Creo?
Mind you, if you just need a bigger boulder, CrTe certainly can solve your problem.
Totally. With just one caveat: Creo spells don't target objects to expand them, they create elements at the target location. I really think you would need a Rego requisite to connect a created rock spike with a wall, for example (this may be less relevant on the ground), and I'm not sure you could simply "expand" the rock through a creo spell.
Yes, maybe. Muto and size is what troubles me the most, in that it requires adding additional material to the world - perhaps a lot of it - that wasn't there before, whereas something like the spike could be mutating what currently exists into a spike shape. What @Jetpack proposes perhaps makes more sense to me, that making the rock larger with Muto would make it hollow or more porous, as you are spreading out the rock that is there.
Yes, it's related to a challenging part of the system where Forms that are types of matter (rock, water) exist alongside Forms that are more typically entities (animals, people). It makes some intuitive sense for Muto to make a person (an entity) larger, and then perhaps, as you say, also to make a rock (as an entity) larger, although Creo suddenly starts to seem possible because rock is a quantitative substance - and then when we get to water it seems odd to say Muto can make a body of water twice the size, right? And Creo suddenly seems very much the correct Art.
In short, the targets of Terram often exist in an awkward place between an entity and a substance.
See the base book, p. 154, Object of Increased Size and Teeth of the Earth Mother. Those seem to be similar to what you're looking for. I rapidly browsed the spell index by form, and could not find a spell that seemed relevant under Creo, on the other hand. Maybe it exists and I haven't found it, but I believe you'll at least have a way easier time under Muto as the existing spells seem to suggest.
Yes I am leaning towards Muto more now, although it doesn't quite make sense to me in the ways it is done in RAW that you cite. It just feels like creating stuff, and it's interesting that we have had opposing views in this thread about which Art(s) can do it. I may stick with Muto but with a house-rule caveat that the material is "spread thinner" as Jetpack suggested, unless you use a Creo requisite.
Another interesting example is a lake versus a frozen lake/glacier. To double the size of a lake is it Creo or Muto? RAW suggests Creo, but can it be done with either? What about doubling the size of a glacier?
Well, Aquam being a liquid, you wouldn't run into the problems with Creo Terram generating an object separate from existing objects, because whatever water you create will simply add onto the existing water for the duration of the spell, so yes, Creo Aquam would work very well to flood the ground arround a lake by adding more water to the lake, but you could also "expand" the existing water with Muto Aquam, or you could Rego Aquam to take the existing water, create a wave and flood an area - there wouldn't be more water however with Rego alone. A Glacier would be easier with Muto again, because it is solid, although you could go the Creo (Rego) route.
I see, so you resolve the conflict by allowing Muto to "expand", and therefore increase, the existing quantity of a substance, such as water. Meaning, I guess, that a magus with a handful of sand could Muto Terram a tonne of sand, or a flask of water could Muto Aquam into a lake. Carrying a small quantity of something would be a good substitute for a low Creo score... interesting, but personally it doesn't feel quite right to me. I'm still more inclined towards the "Muto alters what is there but does not add" approach.
The thing to remember is that medieval philosophae did not have a conservation of mass or matter/energy as a guideline- so simply changing the size of something is perfectly legitimate as a muto effect.
No, because typically, Muto changes something by multiplying its size, and if you increase it too much, it would add magnitudes to the spell. It's not a substitute for Creo. Yes, you could carry with you a hundred pound rock to mutate into a two or four hundred pound rock - if you think carrying a hundred pound rock is reasonable. But if you carried a rock of one pound in order to mutate it into a two hundred pound rock, the number of magnitudes behind such a spell would the equivalent creo spell to make a brand new rock seem like a cantrip in comparison.
Yes you could turn a puddle into a lake, although normally it would be more useful to convert a lake into a puddle, put it in a flask, carry it with you and then release the spell where you want a lake.
I am firmly in the "Muto alters what is there, it does not add" camp.
While you might Muto the shape of a rock to have a spike of rock jut out of it, you are not adding any new rock. The rock that makes up the spike has to come out of the material that makes up the rest of the rock. You could also argue Rego the rock and allow it to attack with "thrusting spikes" as a cosmetic effect.
If you want more water, you need Creo. You can change the shape or properties of the water with Muto, but it will not give you more water.