It seems clear that Perdo can snap things or break them, as per "Fist of Shattering" ArsM5 p.155 "an object of stone breaks apart".
But can Rego also do this? The simplest example is to consider a stick - can Rego snap it? It would seem reasonable to say Rego can move the stick and simply bend each end until it snaps, so yes. But then can Rego Terram break a rock apart? Can Rego Corpus break a person's limb, or their neck?
Since rego can be used to craft wood and stone, which involves cutting it I would say that is explicitly affirmative, where the object can be broken by mundane means.
ArM5 p.78 Rego (Re) "I control":
Mundane craftsmen can only change something’s state to another state that the thing can naturally have. Thus, any change that a mundane craftsman can make can also be made by Rego magic. In this case, a Finesse roll is required to determine the quality of the outcome.
An executioner, a surgeon (and a cabinet maker) are craftsmen: so cutting necks, wrists (and sticks) requires a Finesse roll, but can be done with Rego.
Without Finesse, one can just ReCo somebody up up up, so that he might break his neck after falling ... or he might not.
And I imagine it would be quite a high Finesse EF to break a neck... Seems to make sense.
Not necessarily, but the weird part is looking at is as a craft- ignoring the executioner (who is wielding a weapon) it becomes a question of what would be the surgery roll to break a neck... YSMV and it might be disallowed as this is not a regular use of surgery...
Indeed. I have always viewed craft magic as more like the magus as craftsman, sitting down with time and concentration to do a job; just using magic to do it instead of mundane means. The idea of craft magic principles applied on-the-fly in the field isn't something I'd really considered before, so I'm not entirely convinced by it... but it gives something of a framework to work from.
A perdo requisite would make this unquestionable.
Let me take your question to an extreme: If you intend to destroy by creating a disease in someones body it's still perdo instead of creo right?
Only if you wanted to break it in a particular way. Just wrenching and twisting with sufficient force should break any neck, without any Finesse needed.
If you just want to kill someone there are likely much easier ways of doing it however.
Hmm, well, on that line: what is the easiest way to kill a mundane person with magic?
Depends on what you mean by easy. I mean you can literally lift the ground up under their feet a hundred feet in the air flip it like a pancake and slam it into the ground, burying them if you have the right arts... and really parma won't matter (it's the magically neutral ground you are being slammed into and the normal dirt suffocating you that actually kill you)
... or a gallows ...
If you mean with canon spells, I'd say Sulphurous Membrane (MuAu(Co) 10) in Hermetic Projects. Cast it right after sunset, come back at sunrise and your victim will probably be dead.
To the original question, yes of course you can break/snap things with Rego. The difference between breaking things with Perdo and Rego is in the how they do it. Perdo is done through the controlled weakening or destruction of the target. Rego is done through the application of force against the target.
Perdo can be as precise or broad as you want when it comes to breaking and destroying things. Anything from a small chisel to a sledge hammer to just erasing it.
Rego on the other hand can only do it through manipulation, which means it is always some form of hammer. In addition that forceful manipulation from Rego is much less precise and actually requires a Finesse roll for anything other than the most powerful hammer blow allowed by the spell.
Breaking apart a bolder with Perdo, you have a wide range of choices. You can weaken the bolder to the point that it is so fragile any strike with destroy it. You can crack it, anything from a single split to cracking it into little pieces. You can just erase it as if it never was, in whole or part.
Breaking the same bolder with Rego, you have much less choice. You are limited to the application of raw force. So you could slam another bolder into the one you want to break as hard as possible. You can forcefully squeeze it with magic as hard as possible until it breaks. You can forcefully pull it in every direction as hard as possible until it breaks. If you can figure out where a weak point is, you can apply as much force as possible to that point until it breaks (with a Finesse roll).
When it comes to spell Magnitudes required, Perdo will always be cheaper for the same level of destruction in addition to having the much greater potential of just erasing the target.
And at the same time, Rego can chisel at the rock precisely, and accurately (with a high Finesse), that you could carve the boulder into a statue. But Perdo would not be able to ever achieve that.
True but working a rock into a statue is not destruction, at least not in the conventional sense of the word. In use it is manipulation of the rock, which is what Rego is best at. Perdo is only destruction.
Each has their uses. Rego is much more broad, but requires skill (Finesse rolls) to do many things. Perdo is much more limited but better (both in efficiency and maximum application) in the areas the two arts overlap.
You could probably do that with Perdo too, by destroying every part of the boulder that should not be part of the statue. But doing that kind of precise work with Perdo would also require a Finesse roll and not be any easier than doing it with Rego.
Also, if you want a really smooth surface on the statue you probably can't do that with Perdo.
(Destroying parts of a boulder is clearly within the realm of Perdo, but once the boulder has been made into a statue then polishing the surface smooth involves making it a better statue, and that is not what Perdo is for.)
Personally I wouldn't let Perdo get any closer than a vaguely humanoid shape being left behind. What perdo does really well however is clean up the mess, since you can not just break something but reduce it to either very fine dust or nothing at all. Remember conservation of mass is not part of the medieval paradigm.
Adding a perdo requisit to a rego spell is, in my opinion, where you really get effective sculpting.
Perdo is destruction.
Yes, Rego can move things. For example, Dangling Puppet. When you drop the person it's all bad, however, it's the dropping, not the direct application of magic causing the destruction.
I think the moment the spell is purely destructive, ie snapping a neck, it's Perdo.
Otherwise I can make a rego corpus spell to give a compound fracture. I'm moving the bone. Rego Aurum, a choking spell to move all the air out of one's body. Rego Aquam, make someone a dessicated husk by moving the water out of them.
The mage i'm playing in the current saga is a Rego specialist, and I'm saying this is a step too far.
Yes, I kind of like this approach. It means snapping a stick would be Perdo and could not be done with Rego, and is perhaps a cleaner divide between the Techniques (although I'm now trying to think of blurry edge-cases it leads to - I'm sure there'll be some). It does clash a bit with craft magic, as has been discussed above, in that you have to break/trim things to craft something, but I tend to see craft magic as its own separate thing anyway, so could live with that.
I would probably rule towards casting requisites.
If I had a ReCo spell to control a person's body violently, and tried to use it to twist their head around in a full circle, that spell would gain a Perdo requisite on casting. If you cast the spell and made them dance, and then decided you wanted to snap their neck, that would fail.
Just like if I summoned a horse and later decided I wanted a black horse.