Is there a statement anywhere in the books that tells us that a creature with might can suppress their magic resistance? Or is that just a thing that I presumed without clear evidence?
I think it's actually a thing you're presuming despite (obscure) evidence to the contrary.
There's a PeVi spell "Revoke the Protection of (Realm)" on page 62 of The Contested Isle which cancels the magic resistance of a creature with Might. The write-up notes that "This spell is typically used when the caster wants to affect a supernatural creature with a beneficial spell..."
That's not completely clear cut, as one could argue that even if a creature can suppress its own magic resistance the spell is still useful for those without the intelligence to do so, but there's no distinction there in the write-up.
I also note that magi not having to penetrate their familiar's magic resistance is noted as a (implicitly useful) thing - again, you can argue that it's useful in situations where it suppressing its magic resistance is dangerous, but I think the evidence definitely leans towards such suppression not being possible, even if not absolutely conclusively.
There is nothing stating this explicitly, but following the wording in the core books I think you should be able to.
In the core rules: p85 "A magus can suppress his Parma by concentrating, and this also suppresses his Form resistance" and goes on to detail more about suppressing parma.
p184 "Magic resistance based on might functions like Hermetic resistance, resisting the powers al all realms, and all types of mystical power" - if it functions like Hermetic resistance, there's nothing explicitly stopping you from applying the suppressing rules from Parma Magica to general magic resistance.
To be able to suppress its Magic Resistance, a being with Might needs to know how to do this. At least tmk there is no example in the books, which allows to do this by instinct or inborn capability. Many beings with Might do not even know that they have Magic Resistance - so are most unlikely to know how to suppress it.
There is also no reason to allow, that an apprentice can simply suppress the Parma she has received by her master: she need not even know what she has got, but certainly doesn't know how and why it works. She can leave her master's sight and thus lose the Parma's protection even unintentionally, though.
EDIT: Even Stellatus the Dragon (ArM5 p.194), with Int +4, Magic Lore 8 and Order of Hermes Lore 1,
So he has no idea of the real working of his Magic Resistance. Why should he then know how to suppress it? He will be rather: "Sorry, little friend, but didn't you know that I'm immune to your magic? No, you can't heal my sore tooth with it."
Well, it's speculative, a little convoluted and may just be a mistake/oversight, but how about this - Aeolus (RoP:M, page 35) has a Magic Might of 8 and is said to be affected by an amulet Victor the Redcap (from Semita Errabunda on the webpage) owns and said amulet doesn't seem to include a modifier to provide it with Penetration for a Touch range effect.
I think it's the latter.
If there is an hermetic spell for it, then there might be an identical magical power for any magical being you could design. It even may be a constant power.
It seems a far more reasonable presumption than the opposite. If creatures of might cannot suppress magic resistance than they are cut off from all the minor magics around them. That doesn't feel at all right for Faeries or other creatures from stories.
I've never been able to find a statement to that effect, so I'd say the latter.
I looked for it back when the above-mentioned spell was in playtest.
Simply because I'm not even sure the option you're suggesting (or the spell mentioned) make sense.
Should we compare it to a mundane turning off breathing (easily doable, temporarily) or a mundane turning off being corporeal (less doable, I've tried)?
I don't think ordinary MR can be suppressed.
PM provides MR, when it is active, but is not itself MR. It has specific, explicit rules that differ from the rules for MR:
It needs to be (re-)activated Fiddler on the Roof style. (I don't like this rule, but it's the rule.)
It can be actively suppressed.
Implicitly, PM differs from MR in another way:
- It can be dispelled, like any other magic effect. (I suppose it can also be turned into a wolf with MuVi(An) .)
If ordinary MR could be suppressed, there would be no need for PM to have the second rule. And if there were doubt, the PM rule would belong with MR.
Most of those magics would be targeted at things around the faerie rather than the faerie themselves, and I would expect powers meant to be targeted directly at faeries to include penetration in their design