After this discussion I can only say "glad we only play "traditional" imaginem, with MuIm manipulating species, not turning them physical at all". A big can of worms. Or a big wayto create the nuke. I doubt Mr Tweet ever thought of imaginem as the equivalent of a nuke. Make Paris (or a mountain range) go "boom" with a boundary MuIm? oh yeah baby 
Cheers,
Xavi
You can, yes, but since ideas are discrete items, all you really do is render them immaterial and inactive for the duration. Or, I suppose, temporarily turn them into a spirit of said thought, which is more fun. Either way, the fish argument still holds. Would you agree that the easiest way to kill a man is a touch range CrIg spell? You only need a level 15 spell to do enough damage to instantly cripple or kill (depending on rolls) a normal human.
Ars is loaded with anachronisms, though. The setting is, explicitly, "Medieval Europe but where the Divine and Infernal are real, there really are faeries at the bottom of the garden, magic exists and where Plato and Aristotle are broadly correct." This introduces all sorts of headaches, but I find them entertaining ones.
Totally agreed...
Actually, Imagonem, as it was called a long while ago, was a lot my powerful than my tricks with species. Have a look at some of the old "Sense of pain" spells in 3rd edition, which don't work in the new edition (HoH:S says you can't do species e of pain because you can't get species to hit the targets while they are encased in flesh). Have a look at how they did "Sense of solidity" stuff. In previous editions, Im let you do -everything- you could do with everything else, by basically making Ilusionary things you were forced to treat as real. Arrows and swords and fireballs which you really believed hurt you, and you felt pain as if they'd hurt you, and passed out if you suffered too much pain (at which point a grog could hit you on the head with a rock.) Illusionary cold and heat which were as good as real cold and heat, because if one kept you alive through a freezing night you didn't die retrospectively (illusionary fire with real warmth, +% levels) and if the other froze you, you weren't able to act as if you weren't frozen.
Don't pretend Mr Tweet had less powerful wizards and I've stretched things: look at what he did, and you'll see David (mostly) and I scaled it back to far saner limits. Perceptions of pain are now Mentem or Corpus, depending on how you do things...and that seriously cuts down on the offensive capacity of Imaginem, and illusionists are no longer able to do virtually everything which everyone was able to do, but at a lower level. The whole species thing may seem a cludge to people, but it really is the rope leading out of the mire that was how illusions originally worked in Ars.
Most certainly and absolutely agreed on that.
Thats also why at least i personally dont like this spell, it drifts off into the "bad old days" style and potentially or at least arguably sets precedents i REALLY dont like at all.
It IS a bit of a "cludge", but its still a clear improvement, and a cludge is also alot better than "totally broken overall" and "open goal for munchkins".
If its stupid but it works, it isnt stupid. And MOST of the time it does work(which again is an improvement).
I lost my 3rd edition book (lent it to a guy that proceeded to disappear into the Twilight void), but I do not recall Imaginem being this powerful in 3rd. Maybe 2nd edition? In 3rd we had things that confused you, but not things that actually harmed you IIRC.
I stand corrected if that is not the case, but I still think this is a really bad precedent in 5th edition. Shinning footsteps? Yeah, sure. Exploding mountain ranges? No way.
I've kind of worn myself out thinking about this spell.
I don't have a problem with a system of IM based on species, even if I do think it's anachronistic. In general it seem to work to explain perception with the context of the magic system at least.
I do think current AM goes a little too far in the direction of turning the magic system into a physics lab, where we posit weird laws of nature and then find ways to twist them with hermetic magic. I don't want my spells to be expressley manipulating elemental particles, be they species or photons. A little bit of a move back towards storytelling magic would moderate a lot of abuses.
I have no idea what spells may exist in the supplements, because we got by in 3rd edition (and still do) with nothing but the main rulebook, but there is Visions of the Infernal Terrors (MuIm 30 - Me requisite). This spell kills on a Stamina + Personality roll. I assume that it's a pure Mentem spell in 5th edition.
ReMe exactly 
Well, I'd it's a problem here. I suggest that since MuIm-affected objects don't affect Magic Resistance, the species emitted under them are perfectly natural. Hence, MuIm spells actually don't change the species at all (if they were changed, they would be magical, and resisted). Instead, MuIm spells affect the generation of species. So it isn't possible to change earth into species, since Hermetic Muto magic doesn't really affect species.
That's a very good point, too.