I never understood why Silence is an Illusion(Glammer). It really should be Transmutation, IMO.
Well, school classification is extremely arbitrary. I think everybody agrees on that. But I do try to set guidelines and boundaries.
Changing the qualities of the subject with regard to the five senses-- particularly sight (easiest), sounds (easy), smell, thermal, and even touch (the most difficult to master) is the purview of Illusion.
You can either change those sensory qualities as they are perceived by the target (via changing what he "thinks" he is sensing, in which case it is
mind-affecting) or you can actually change the sensual qualities of the target itself (usually, this will be a glamer at least, possibly even shadow-stuff if you require tactile qualities).
Illusions, in other words, can be either subjective (each viewer may view the illusion differently) or objective (the illusion "appears" the same to all viewers).
[sblock]
You could also think of the Illusion school as "hacking" the sensory information. You can either intercept the sensory information at the source, suppress it, change it, and from there pass on false information to all observers (an objective "server side" change); or you can attempt to hack the information as it arrives at each observer, and change it there (a subjective "client side" change).[/sblock]
So for example, the observers of a creature under the effects of
invisibility do not get a saving throw because the target is objectively invisible; its visual qualities have actually changed.
I don't believe that either
invisibility or
silence should be Transmutation spells, as I prefer Transmutation to involve the alteration of much more tangible matter.
Transmutation is a slippery slope: because all spells "alter reality" you can pretty much justify sliding anything into Transmutation if you don't set clear guidelines up front. (Ergo, the long stretch of design time where Transmutation spells were ubiquitous.)
I have a device I call, "the Spoon of Sufficient Size" that I use to measure the targets for various spells. Telekinetic abilities notwithstanding, Transmutation should generally not involve the manipulation of light waves, sound waves, or energy, but rather only those things that you can hold in the Spoon. (Ditto for Conjuration-- you can't conjure "fire," but you can conjure "a fiery chunk of magma.")