I was under the impression that a magus must be able to percieve a target to target him with a spell with all spell ranges except arcane and perhaps circle, but now I cannot seem to find that in the RAW.
Serf's parma: Can someone help me find that rule, or clarify that it does not exist?
You're looking for the limit of arcane connections. It is under hermetic limits near the front of the magic chapter.
(However, it's still quite possible to put someone/thing within an area of effect of an area spell by accident, luck or guesswork, or a combination of those, even if that isn't being "targeted" per se.)
The situation I was thinking of was more like this...
You have a voice range spell and know the bad guy is in a house but can't see him, should you be able to target him simply because he can hear your voice as you cast the spell?
Sorta kinda....maybe.
Under the Blind flaw: "Blind magi can detect targets by other senses, and thus are less limited than people trying to use missile weapons. However blind magi cannot aim spells without magical aid."
The Limit of Arcane Connection:
Hermetic magic cannot affect an unsensed target without an Arcane Connection.
Voice Range: Anything to which the magus's voice carries.
Sight Range: Anything the magus can see. A blind magus can only affect himself.
If the Limit is the absolute law, then a caster cannot affect things behind them with Voice range. Unless they are causing noise or stink, etc.
Does a caster's relation to a spell make it "sensed" by him? Thus can he create something out of his direct sight with Voice, if not Perdo it?
As to the question of targeting someone within a house, the Limit of Arcane Connections specifically says "....Intelligo magic is much less tightly bound by this limit than other kinds. Intellego can determine whether, for example, there are any people behind a wall the magus can see, but Perdo Corpus magic cannot affect those people until the magus is aware of them.
Thus once your Intellego magic sensed them, you could Perdo them, but not before.
The way I read the rules originally (Serf's parma) was that unless you have a way to sense them, whether it is sight or hearing, you would need an arcane connection. I would think you would have to have a good enough idea of their location to say "That guy!" even if this is not accurate enough to fire an arrow reliably. So a blind magus could hear someone moving and cast a spell on them, but he could not simply shout and hope to zap anyone who might be there.
Maybe I should make a blind magus, that would be interesting. I suppose I would need a servant to read to me so I could study from books. A literate servant with some magic theory. Or maybe a clever intellego spell....
Furion has been talking about a blind magus, I bet you could find some illuminating ideas in a search for "Furion Blind"
Urgh.
This is exactly how the rules read (p 80) - does this imply that a fireball, cast somehow where an invisible person was standing, would then not affect them??? That the same fireball, cast into a room, would only burn/ignite what the magus could see? RAW, so it would seem, however there's no way I'd allow that interpretation in any game I ran, nor be very happy with it in any I played in! 
I want to believe that this refers to spells that have to be targetted, not aimed, like Perdo spells and the like, and not Creo Element and such, but don't have the time at the moment to check.
I certainly think you could target a spell at a spot where you think someone is. I think it is just a matter of knowing there is somthing there. ou can't just say "Just in case there is something around here, I am going to cast Demon's Eternal Oblivion over and over in the dark in case one shows up."
Ah ha! But the work around for this is that the target for a Creo creation spell is the item created. So large creo spells care not a whit for whether you can sense anything within their area. Or you could cast a small Creo spell over and over hoping to hit the invisible person...assuming the spell itself didn't require you to target a person. That is, fire burns anything within it, but you can't randomly cast a spell to age a target to maturity at random around a room, THAT requires a standard target.
I prefer to think of it that the target in the case of a fireball shot into the room, is that the room is the target, and anything caught in the blast is collateral damage and doesn't need to be sensed. Likewise if you cannot see someone hiding in bushes but want to blast them anyway, you target the bushes.
The problem here is that all spells that target individuals would be assumed to be too small to hit more than the target that you aimed at. In the case of shooting at someone in the bushes with pilum of fire, you hit the bush but the hidden person within gets out in time.
Up the target to group/room level and it should justify a blast radius.
The fire is the target. When you do a BoAF, you CrIg, so the target is the fire you create, not the room or the bushes or the space you create the fire within.
In such a case, it doesn't matter if you don't see the people in the space, they're going to get the fiery effects (assuming penetration).
-Ben.
A couple of cents:
First of all BoAF is not a fireball as such. It only hits one target and it is not an earea of effect spell. Secondly even if the fire is the target, you still have to hit the thing you want to be affected with the spell, which requires you to either sense it or have a damn good sense of where it is.
A magus has to sense a target by some means, if not using a AR and an AR-range spell, whether by touch, sound or smell (I'll disregard taste for this one 
). Then the peculiar problem of your spellcasting arises, your voice or gesture might very well rob you of the very way of sensing the target (speaking latin in bold makes it harder to sense the target) - in this instance I have house rules that you can still target someone even if your own spell casting might disturb the sensing of the target.
Finally there is the option of using other spells to sense your surroundings and thereby target whom you want to target. This however makes you dependent on such spells and their ability to sense a specific target (MR might foil some such spells). Being dependent on such spells might also very easily bring you in conflict with other magi, as they might argue this as scrying - which is exactly what it is - but to remedy this one could take to 'forceless' casting of the spell, but this would make it har for your to realise certain persons with MR (using some spells at least).
Just to clarify, when using Creo magic to actually create something the target is the thing created, not the location it is placed. You cannot use "room" target to create a room full of flame, only increase the individual size via magnitudes (10x size per) or the "Group" Target.
An important point! The RAW specifically disallow Creo-spells with Room Target, and using a group will not help you much if you have trouble sensing the targets in the first place. A smart fix, if you want to do area damage in a room is to use another technique than Creo - e.g. a PeIg spell with target Room to freeze people.
Or a CrIg spell with a big size modifier... A big enough fireball could fill any room - but wouldn't care if it was being used in a room or not...
Remember, target:room only allows for a room size equal to 100 individuals, so a target indiviual, with a size modifier of +2 (allowing for 100 times the size) should fill the room in question just fine - and be the same level spell too...
I find the possibilities in CrTe spells involving sand fun.
With base individual being 10 cubic paces ,
it is relatively easy to fill rooms or structures with sand.
But even if the magnitude is equivalent, it is still not a Target:Room spell. And that has bearing on more than just the size - take range for one.
Actually, the only thing I can see it affecting is adjustability (a set size vs automaticly the size of the targeted room), and that can be fixed by adding a magintude or two for complexity (making the size modifiable).
After that, all I can see is limits on target: room (must be used indoors etc), vs options on large target:ind.
Can't really imagine how you get a this to affect range... You must be able to reach the target area with your range, and at that it doesn't matter if you're reaching the room with target: room, or the area (same room) with target:ind...
Concerning this you will find elsewhere on the forum a splendid calculation of area of effect of a fireball, and it actually does require several magnitudes to get sufficient cubics of fire (which is more than just adding a few size multipliers). That a Target Room spell equal so and so many individuals is not entirely the same as volume in the sense of cubic feet. The RAW explicitly forbids creo spells with Target Room - so you have to do the math to get the size of the fireball starting of from a Individual target and sizing it up.
Not entirely true. That only goes for any Technique but Creo.
RAW p. 113: "A Creo spell with target Room cannot be used to fill a room with something"
With target Room you can simply touch the outer wall of the room. With an Individual target - even an area of effect that would otherwise fill out a room - you would have to be able to sense the inside of the room. This makes for a huge difference in many situations. Especially if you want to stir fry anything within said room 
, and even moreso if the caster is blind!