When you call up a spell spirit with hermetic theurgy, you need to instruct it for 1 round about casting its spell (after casting it, the spirit departs until summoned again). My question is: how complex can the instructions be?
For example, could you tell the spirit something like "cast your pilum of fire against that target if he leaves that circle"? What about "Incinerate the first person to enter this sanctum"? Could you command multiple, already summoned spirits in a single round if the same instructions could reasonably apply to all e.g. "Blast that demon with your powers NOW"?
they can be as smart as any spell you yourself can cast, but they have no knowledge or interests beyond themselves - they won't/can't talk.
When you "describe" what you want or "tell the spirit" what to do, you aren't so much talking to an intelligence, as shaping an external magic which is a reflection of your own, a magic which has been bound closely to you over a season-lon process.
I thought you could talk to a spell, with magic, just the way you could talk to, say, a stone or a lake (serf's parma - perhaps this is a throwback from ArM 4th, and inanimate stuff got dumber with 5th edition?). Now, IIRC, a lake does know, among other things, whether things have been dipped into it, and whether it's cold or warm. It also has a sense of space, and a sense of time, so that it can say "something about your size moved on top of me from the left shore to the right shore". In these terms, how smart is a spell? For example, can it perceive whether a potential target is in range?
Ok, I realized that, but I'm still not sure how sophisticated can the shaping be, both in terms of "sensing" and (to use a modern metaphor) "computing" capabilities. It does not seem obvious to me from your answer if one can tell a Pilum of Fire "incinerate the first thing that comes through that door". Can it perceive that something is coming through the door? Doing so is certainly within the "IQ" of some spells (e.g. a Waiting Spell). What about "incinerate the seventeenth thing that comes through the door"? Also, it's not clear whether this shaping can affect multiple spells at once ("All of you, blast that demon NOW").
you would have to use a second spell (InVi "talk to a spell", guideline? improvise from InTe or such?)
What I really meant was the spirit is just hovering around to be shaped into the spell-cast you want - think more like a delayed cast than an active intelligence. It's sole purpose in existence is to cast that one flavour of spell - even narrower an existence than trees or rocks!
on that scale (being interested in things other than its own spell? "very dumb". Not even interested in "self")
look at the section on enchanted devices in the Lab chapter. There are sections on triggering conditions, and for when triggering conditions need Intellego (which spells lack, unless designed in).
It can have the normal selectivity of a spell you cast, and if in your presence, things which you can sense. Hmmm - its own senses are limited (and not well described) to its own Form really.
Another way to look at it is:
you can extend the Summon Duration so the spell spirit hangs around waiting to be instructed ("used"), but once you instruct it, it will discharge its spell and depart. Only if the "spell that the spirit casts" has a further delay or extended duration will the immediately-cast spell hang around.
You can't per TMRE summon a spirit and ask it to remain behind after you to cast a spell "on the first being to enter the room". You can have it "cast a spell now (ie 'in the next round') on the first target to enter through that door" -- but if no-one enters, the spell fizzles and the spirit departs.
The Mystery can sound like a way to surround yourself with a could of elemental beings which will chat and converse with you and each other, and will rush to serve your will... But it is not...
It is meant to be that you can perform slow-cast magic avoiding botches, and gain some flexibility. (Note that Spirit of Form Magic, who spont for you, are great for avoiding Botches). The spirit can also move into a different Aura before casting, and makes no die-roll when performing the spell.
If you use extended summons to surround yourself with spell spirits, it's meant to be that you have a number of hair-trigger spells prepped, and need only speak to one and it will discharge.