Question:
Is it possible to ward against species (sound, visual species etc.)?
You can move species, but I have an opinion from another that:
Does this make sense to anyone?
MoH p30, ReIm5 Circle of Undisturbed Rest blocks outgoing species.
Also in the Jerbiton section of HoH:Soc, where it talks about "transparencies" (p 64 etc). Nothing as definitive, but I believe it lays the groundwork for such very clearly.
And I've used it as such - for a similar "privacy" spell, that keeps all outside noise out, and also as a "work quietly" spell that keeps all noise from traveling beyond a circle.
Whether such a ward could be made "one way" (that is, allow sounds to come in but none out) is saga-dependent (and may imply serious ramifications re other Forms), but is very handy for species-wards.
Also, if the above "one way" option is allowed, a Range:Personal Ward that blocks all visual species from going out would be a very creepy effect - a walking jet-black shadow. 
(If it's not allowed, it would still be creepy, but would also blind the user from using normal vision.)
That's a hard line to take. Easier on the SG, but I have to believe that there are endless "grey areas" that are simply not specified. For instance, using PeIg, can one not "slightly reduce the amount of light in an area"? I'm sure other examples can be suggested, and the supplements are constantly specifying logical expansions of various Guidelines.
More, in many Guidelines (and etc) it suggests looking to "other Forms" for inspiration or parallel Guidelines - Teleporting with Animal, Scrying via different Forms, etc.
If it doesn't break the game or come way out of left field, I don't see why not. (Some - no. I'm not sure I'd be happy with a Rego Vim spell to teleport an existing magical effect to a different location, for instance. ysmv.)
And I believe that Ward vs. Imagonem/species is one of these instances - should be perfectly valid.
That is complete and utter rubbish.
Nope.
I wouldn't say rubbish. There are some grey areas (like Glamour muto spells, for instance, which require the Glamour mystery).
However, it seems to me that Wards are a form of Rego movement spell, that is, they arrest all movement of the substance warded from passing a certain restricted boundary (a Ring, a Boundary, the skin of an Individual etc).
As such, they are part of normal Rego guidelines and anything which can be moved or displaced can also be warded without the need for a breakthrough.
Visual and audio species can be moved. Therefore they can be warded.
Does that make sense?
100%. Fits with the first sentence on page 114 under Wards, with the definition of Rego on page 78, with the Guidelines in general, and in parallel with other RegoForm Guidelines.
With Imagonem, the Form is species. That is the "stuff", the exact equiv of sand, dirt or gravel in Terram, or heat & light in Ignem. Could one use ReIg to create a ward vs. "light", as opposed to "heat"? I don't see why not. And that's certainly not in the Guidelines.
I'd wager that your friend is just a little vague on what Imagonem encompasses, and how it works - which is not at all uncommon (took me long enough to figure it out!). I'm guessing he's thinking that somehow "illusions" are diff from "species" - when in fact they're the same thing.
Not only can they be moved by Rego, they must move in order to be perceived - there are effectively no "stationary" species.
Try explaining it to him with something like this tragic yet somewhat helpful diagram:
(Otoh, if it's his Saga, he could rule "There is no Imagonem, only 9 Forms" - and he'd be right - for his Saga.) 
What effect were you hoping to get? Is this a theoretical question, or is there a specific end in mind? Is there another way to get there?
The original concept of Ars Magica was that Hermetic Magic could do anything not prohibited by the Laws of Magic (called the Limits of Magic in early editions). Just assign the appropriate Form, Technique, and Level and off you go. Since then things have gotten more ambiguous.
The AM5 rulebook still suggests that Magi invent their own ranges, durations, and targets when creating new spells. More recently, HoH:TL tells us that this requires a minor breakthrough. Even more recently the authors have introduced the concept of guidelines which do not require Virtues but which are nonetheless not generally available, such as the Magic Realm guidelines in RoP:M. Legends of Hermes informs us that certain guidelines are available in 1220 that were not available in the early years of the Order.
I'm not sure where it all ends up.
Jabir, I tend to combine all those rules like this:
For example, a player of mine wanted to invent a spell which combine: PeVi to destroy might and ReVi to take vis from creature. His idea of guideline was a guideline who would "steal" the might in form of pawns in his hands, while weakening the magical enemy.
At first, he tried with PeVi base, and said the guideline would need a mandatory rego requisite, and thus a +1 magnitude. The guideline was: "General: steal [level of base -5] might pool from the target, which appear in an object held by the caster. (rego requisite free)."
He invented 3 spells based on his putative guideline, 2 failed (they resulted in normal Perdo Vim effects), the third worked.
When, after 7 additionals spells, he invented the guideline (a major breakthrough), he discovered that the guideline was, in reality:
PeVi : "General: reduce the might of a magical ennemy by (level +5) and the caster will get [points effectively destroyed/5, round up) pawns of vis. Mandatory (free) rego requisite."
His experimental spells were using a less efficient guideline... but that was part of the breakthrough process, and the advantage was that he had innovative spells before even having the breakthrough complete.
Those experimental spells respected the Core Rules: "may create spells with new guideline/range/duration" and respected HOHTL which requires breakthrough for novelties.
As for the Ward vs species. Certianly, it should exist. however, no spell use it in core, nor does it have guideline. Conclusion IMO: you can invent it, require breakthrough.
For the level, I disagreeed with MoH, which use base 1. There is no reason : that number is only picked random and I don't like that.
By comparison with ReCo: ward against human is level 15 as is move a traget up to 50 paces which, in Imaginem, doesn't exist, but we have move a target to up to 100 paces (base 5) or 15 paces (base 4). ReCo base 10 moves up to 5 paces, while base 20 is up to 500.
Consequence, by analogy with ReCo, the base should be base 5, for one sense.
(Now I understand why this is base 1 in MoH. They went from the opinion that it was ReCo -4 magnitude since there are 5 senses and guidelines are made for 1. That makes a little more sense. yeah, base 1 would finally be okay (but MoH authors should explain where they pick their guidelines ^^). Also, there is nothing in the guidelines file on atlas website ^^.)
My belief is that both of these are true. When a magus invents his own RDT, these tend to be inefficient. A Range of 'Reach' introduced into a spell -- something you could touch but don't actually need to -- is equivalent to Range Voice. A Duration of 'Week' is equivalent to Duration Moon.
When a magus has a Virtue that grants new RDT (whether from a Mystery or not), or makes a breakthrough, then these new RDT become more efficient. D: Sabbath (obtained from the Holy Magic Virtue) offers a 2 magnitude shift, but can last up to a week. Under normal Hermetic Magic this would incur a 3 magnitude shift (to make it equivalent to Moon).
There may be some RDT out there that break this, but in general it seems to be true.
Mark
@ Mark Shirley
However, your point rather supports my own.
If you're introducing a novel range or duration then, yes, you require a breakthrough.
Wards, however, are not a novel effect. Rather they are a normal part of rego movement guidelines.
One might note that the guidelines are extrapolated upon all the time.
Using Exar's reasoning, we might suppose that before HoH: Societas there was no means by which to use Muto to change one form of species to another form of species; clearly absurd.
We might also have assumed that there was no guideline to teleport non-corpus objects, likewise ridiculous.
New "guidelines" are introduced all the time, so long as they are extrapolations upon existing effects. Only spells which introduce a novel effect not yet seen ought to require a breakthrough.
According to medieval logic, if you can move it, you can ward it using NORMAL Rego magic. Warding is simply stoping motion. To say that power exists to move images in any way imaginable EXCEPT for stopping them at a certain point is.... well, its a stretch.
We might also note the introduction of the spell section in ArM5 which explicitly states that spells which
are ALL available without breakthroughs. Its part of the fun of ars magica; a creative magus can use all manner of interesting and innovative spells without resorting to saga-dragging banality of research.
Research is for breaking the Limits and making new R/D/T effects, not for making any spell not part of the introductory grimoire.
At least, that's what ArM5 says.
I don't think medieval logic would even know what Rego magic is.
Thus, you mean "medieval mythic logic", and we fall in the "who knows" field.
Someone can move magic. Can we ward against magic?
Which limit does the aegis break? Which new RDT?
Your argument fails there because the aegis IS a breakthrough result.
I am with Vulcano here. No need to require a breakthrough for everything. 
We have had rego-ed sheep, rabbits and cows being thrown against besiegers in the past for example, and sheets of eternal ice covering windows. No explicit guidelines for that even if we all know it can be done. 
Cheers,
Xavi
That looks like a good workable system and very in tune with the core rules. There are counterexamples out there, such as the astrological durations from TMRE, a book with I like very much in general. By the core rules one ought to be able to use Days as a duration equivalent to Moon (the next level of difficulty) without any special breakthrough but that book requires both a Virtue and a difficult die roll.
This is where I would draw the line too. I can accept the more restrictive R/D/T requirement but restricting and delimiting guidelines within the Forms and Techniques is a bridge too far for me. Imagining a historical order with more limited Vim is all well and good in theory but in practice I'm sticking to the concept that Hermetic Magic can do anything not prohibited by the limits of magic, not requiring custom R/D/T, and not breaking the basic rules of the game.
The reason Aegis was a breakthrough is explained in the spell description.
The Aegis does not require a Perdo requisite, nor does it require complexity requisites for limiting who is permitted to cast within it and who not.
These things are beyond the scope of normal Hermetic Magic. Think of the equivalent for other forms:
Example: A ritual where only fires created by rubbing wood together can ignite, and only fire carried on wooden torches may enter and all others are dampened to extinguished, with no Perdo requisite;
See? Definately beyond the normal mechanics of Hermetic Magic.
Wards against natural phenomena are all part of normal magic. Complex wards which distinguish based on human identification rather than natural type, and ward and surpress together, without Perdo and Intellego requisites, are not.
Not true.
They are a very specific sub-category, with their own set of rules. They don't "move" or even "stop" anything (in the sense of "holding it in place"), just bar it from going across a barrier. And the Targets, the thing(s) that the Rego magic works upon, need not be perceived, not at the time of casting, not at the time of affect. Nothing normal about them at all.
(For those who argue that any Creo Ignem effect can target things not perceived - sorry, wrong. That's target, not Target - the Target of Ignem is always light or heat.)
And species are natural phenomena.
The type of "complex wards" you're talking about are often not Wards at all, but more standard ongoing magical effects that surround a mage or operate in a Circle. The only "non-standard" aspect would be if a mage tried to make them work like a Ward - which would be a major breakthrough imo, if possible at all. (Because, typically, the magic requires a target that is not there at the time of the casting.)
I think that really Rego couldn't do this, but Perdo do ir...
The only limitation on Circle/Ring is taht must be touch and that can't create anything, oan make possible any other thing with a couple of limits, like only on Touchh, but...