Does the Aegis effect corpus or mentem or both?

Perhaps under those circumstances the demon asserts control and the possessed person - Corpus included - bounces off the Aegis. I suppose that would happen if one tried to carry a trussed up Faerie or Magical Creature through an Aegis. Or else the demon abandons the body and fights another day.

This does make Aegis act as a demon detector but, as has been discussed in other threads, that's really trivial to do using Muto or Perdo under the RAW anyway.

In this model sigil is a complex thing with sufficient detail that aegis remembers, and the aegis remembers a lot of that detail. It becomes a matter along the lines of how close a replicate key needs to be made to fit into a lock

True Lineages p. 71 tells us this (emphasis mine)

This doesn't mean thespian's model where the Aegis recognizes the sigil can't work. The Aegis is looking at active magic, not traces, and if the aegis can identify sigils better than any hermetic magic, does that really cause any problems for us? I'd still hope for an intellectually tidier model.

Thinking more along these lines there is a lot of intellego magic that is cast at range personal and is difficult to explain without magic bringing information back to the caster from outside of the parma. There are analogous issues elsewhere in the system.

This is the irrelevant part of HoH:TL p.31f Shroud Magic: to fake out an Aegis it doesn't matter how well you can conceal a sigil, but how well you can mimic a specific existing one. And there HoH:TL p.31f Shroud Magic provides a complex set of rules, which compares knowledge of the sigil to mimic, spells, Abilities, Acute Sense Virtue and rolls on both sides.

'Thespian's model' - with the Aegis just storing all the spell traces volunteered by a caster as he helps to set it up, a data trove otherwise not available and thereby beating any attempt to mimic his sigil - as a rule is more tidy, but less dramatic than that part of HoH:TL p.31f Shroud Magic you didn't quote. And tmk players and authors like their Aegis tidy.


Your approach implies that the Aegis spell has some sort of Intellego-related ability to detect and analyze magics and act based on that result. I don't think this is the best way to look at an Aegis. I prefer it to be a passive protection.

The Aegis spell creates an ongoing magical effect with Year duration. Ongoing effects often retain some sort of connection to their casters - not big-A Arcane Connections but certainly something arcane. See for example Phantasm of the Human Form (CrIm), which allows the caster to control the effect throughout the Sun duration. In the case of Aegis, all magi involved in casting the ritual retain this link, with a basic control of "don't suppress my magic". That may mean that the sigils of all the participants are visible to Intellego magic, but that's not the key to the workings of the protection and doesn't allow spoofing.

Its magic, not computer coding. It does what it does.