I have a bit of a hard time wrapping my head around Muto in regards to natural properties.
Page 78 of the core rules states that
I assume this is to differentiate Muto from Rego, but I don't understand exactly what is considered a natural property (and conversely, what are unnatural properties). For example, many Muto guidelines mention changing color. Isn't color a property that something has naturally? The description for Rego mentions that the technique cannot change color because a single substance naturally only has one, using the example of a black dog. It seems to me then that the game regards color as a natural property of something, but one that cannot be changed by Rego because it generally does not have different states. So how can Muto affect it? Is the idea that it doesn't affect the dog's natural property of color, but adds another "color property" to mask it, as per the last line of the quote above?
The same question applies to other static properties of something, such as weight, solidity, opacity, cohesiveness etc. All seem to me to be properties that a substance has naturally. Can Muto affect them directly?
Short answer: Muto does what Creo, Perdo and Rego don't - so if you're improving in a way that could be done naturally, you should probably use Creo, if you're removing something you should probably use Perdo, and more "natural" forms of control should be Rego rather than Muto. This does mean the edges get a bit blurry and there's always argument as to whether some of the published spells are using the correct technique or not.
Longer answer: The supplement Art & Academe has a section on "Causality and Change", and the bit on p25 says that Muto "manipulates the category of quality to alter the properties of a substance"...."without an actual change in substance" and gives the example of a man turned into a big being a man with the categories of a pig. If you really like philosophical discussions about esoteric points, A&A is a great book.
A way better way to say it is that a black-furred dog cannot turn itself into a golden-furred dog, so doing that transformation is Muto, even though 'dog' can be both colors. Muto cannot, however, transform a human into a human with a broken limb; That's Perdo. Muto cannot turn a black-furred dog into a black-furred dog with +2 strength (Creo). Actually darkwing's description of 'does things other techniques can't' is pretty good.
Thank you both. I will definitely read Art & Academe. I sort of understand the 'does things other techniques can't', but it still isn't completely clear to me.
Take the different properties I mentioned above, such as weight, opactity and solidity. As far as I know, Perdo can remove weight. Does that mean that Muto can only add weight, as the removal of weight is exclusive to Perdo? Or can it do neither? Or both?
I think part of my misunderstanding stems from an unclear view of how mutually exclusive the techniques are. From what I can tell, different form and technique combinations can achieve roughly the same effect in many cases. For example, a PeIg spell could remove heat from water, causing it to freeze, and and ReAq spell could transform water into ice.
However, it seems to me that Muto very easily steps on the toes of other forms, provided it can affect static properties. Invisibility is commonly achieved with a PeIm spell, but can a Muto spell achieve the same effect, by changing for example a stone's property of opacity?
This is one of those cases where the conceptual description of the Art is more complicated than the practice. Take a look at some canonical spells in the corebook.
PRETERNATURAL GROWTH AND SHRINKING, MuCo 15, pg. 131, does just what the name implies. On the same page, CLOAK OF MIST, MuCo 40, transforms the caster into "a thick, cohesive mist of approximately human size". That'd be an pretty thick mist to weigh as much as a human body of the same size.
For truly huge shifts in weight or mass, requiring a Creo or Perdo requisite might be appropriate, but apparently not in all cases. There's usually more than one way to achieve the same result using Hermetic Magic.
Muto can definitely make you heavier or lighter.
It cannot make you weightless, though.
It also cannot make you "immovable", I would say.
Yes, Muto tends to overstep its bounds; so in our troupe we are generally stricter about Muto than about other Techniques (particularly Perdo!). That said, you can change a stone's opacity, but it will always remain somewhat opaque, I would say, possibly reflective, etc. Like glass. If you want to make it really invisible, Perdo is the only way.